Friends Stella and Robert each have a ball of the same size, except Stella's ball is made of steel, while Robert's is rubber. They are having a competition to see who can launch their ball farthest. If they each have a cannon capable of accelerating their ball to 5 0 m/s at the same angle of projection, who will win?
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I have a doubt! In the Question, they mentioned that there is no wind, thus we need to take air drag out of the scenario right?
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"The balls are shot through air , but no wind is blowing."
Emphasis added.
Air resistance is important, the no wind condition is probably just to say that the balls aren't going to do something crazy and unintuitive like catch an updraft and go further.
I thought that if the rubber ball was lighter it could go faster. Why does a heavier ball go faster? I would rate this problem a 6 because it was kind of confusing. I
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The question states that they are launched at the same speed. You would have a point if the question had talked about the force applied and the time it was applied for (i.e. the change in momentum of the balls) instead.
That is why we have two different cannons with different capabilities.
The motion of a projectile (neglecting air resistance) is independent of mass
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The air resistance is not neglected only the wind is absent. The balls still move through the still air and experience the air drag.
Your logic seems wrong- firstly you are deriving that both balls will lose equal momentum (how?)
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According to Newton's law the momentum change is Δ p = F Δ t . If the force F is the same, the momentum change will be also the same.
Selection of ball material is problematic. Rubber is elastic material. Thus, subjected to external force, rubber ball will change its shape which consequently will have an impact on friction force.
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I doubt that the change in the shape will be significant to impact the answer to this problem. What do you think?
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On the second thought, I agree. 180 kph is not really 'high' speed after all. However, by replacing rubber with a nonelastic 'light' material (or even 'hollow steel ball') would avoid any confusion.
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@Mirek Baudys – Isn't the explosion used to drive the balls essentially the same as if you were to drop the two balls onto a hard surface and then the distance travelled would be essentially the height bounced. (Acceleration =gravity instead of propellant explosion. Air friction equal for both balls . So both receive the same acceleration due to the wave front of the explosion in the example as they both do due to the hitting of the same surface with the same force doesn't the rubber ball's elastic properties cause a higher bounce which translates into a longer shot? If not then wouldn't they make tennis balls out of heavier (hence greater initial momentum) out of steel.
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@John Lawler – The shots don't use the same force to propel the balls. The shots apply the same resulting speed. The two materials result in identical velocity, and thus differing momentum.
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@Brian Egedy – you are correct. the problem states that the cannon are capable of producing 50m\s not each cannon having equal propellant.
Wouldn't any variation, rhythmic or otherwise, in the shape of the projectile, serve to increase air resistance, overall?
Hey, I don't really understand Momentum. Would you mind explaining it to me?
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Momentum is the product of mass and velocity. The balls have different masses and equal speeds, therefore, they will have different momentum.
Momentum is the inertia of a body in motion, i.e., its tendency to resist outside forces as it moves in a particular direction. If two objects of different masses are traveling at the same velocity, then a force applied equally to both of them would affect the path of the lighter of the two more than the denser of the two.
Momentum and mass have nothing to do with it. The question is about how far it will go, not how hard it will hit the ground. The range only depends on the initial horizontal velocity Ux, which is the same since they are launched at the same speed and at the same angle, and the time it stays up in the air, which is independent of the mass t=(Uy + sqrt(Uy^2 + g 2 h0))/g. Since the balls are launched from the same height (h0), with the same vertical velocity, have the same shape and cross area meaning that air resistance is the same and therefore irrelevant, and the acceleration of gravity is constant (g), the two balls will cover exactly the same distance. This is high school physics. The default answer is wrong.
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Where does it say that you're allowed to neglect air resistance? There is no external wind blowing, but the medium it's flying through does matter...
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The balls suffer an air resistance even though there is no wind. If there is a wind blowing, it can deviate the path of the balls and it would become difficult to reach the conclusion. To avoid this complexity, the wind is assumed to be absent.
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@Rohit Gupta – This is what I meant. If the two balls were flying through vacuum, then the "air" resistance (the medium the balls are moving through), could be neglected assuming Newton's laws to be approximately true.
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@Julia Seidel – Ahh, I misread your comment. We are on the same page, great.
You have correctly said that both the balls will suffer equal air resistance, however, due to different masses, the same force will cause greater retardation in the lighter ball. Therefore, the rubber ball will land shorter than the steel ball.
Nikon is absolutely correct. The answer is wrong. Simple projectile motion is independent of mass
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Yes, but it is not a simple projectile. It suffers an air resistance which retards rubber ball more than the steel ball as acceleration is inversely related to mass.
As others have pointed it out, this is not a simple projectile motion.
High school physics generally ignores air resistance, and does so by phrasing that most projectile motion events occur "in a vacuum". The setup for this problem specifically states that the shots are fired through the air.
Ok I agree that since the balls are shot through air, the drag coefficient does affect the rubber ball more than the heavier steel ball, so if we take that into account the steel ball will go further. I stand corrected.
I think that even if air resistance is equal, it will slow down the rubber ball faster than steel ball, F=ma (force of friction due to air resistance) is the same for both but weight of rubber ball is smaller so acceleration in opposite direction is higher, for me it is the only reason why steel ball can travel slightly further. But practically I think if you shot from the same canon rubber and steel ball, rubber ball would have much higher speed, but this assume that speed is the same.
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I totally agree with you.
The two balls aren't fired from the same cannon; they're fired at the same speed, so the cannons are necessarily applying different initial force in order to achieve identical speed for the two different materials.
The essential reason why simple projectile motion is independent of mass is straightforward. Only the vertical component of motion needs to be considered (for time to reach zero height after being set in motion) and the vertical force is mass multiplied by acceleration. The vertical force acting on the ball is gravitation ie. mass times acceleration due to gravity. Hence mass cancels in both sides of the equation and motion is therefore independent of mass
I understood from the question 'no wind is blowing' that there was no air resistance to be considered.
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"The balls are shot through the air..." indicates that there must be air resistance. "...But no wind is blowing" means that the air resistance will be uniform throughout the shot.
Then the question is ambiguous. It should explicitly state that "whilst no wind is blowing, air resistance cannot be neglected"
Many Applied Maths books are written for GCE A level (and similar exams) simple projectile problems and Air resistance is normally ignored. If air resistance is to be taken into account (proportional to velocity squared), then the question will explicitly say so (as the solution to the problem becomes non-linear) requiring some sort of iterative method or the use of a non-linear solver for ordinary differential equations such as Matlab Simulink
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"The balls are shot through the air, but no wind is blowing." This is an explicit statement that the air exists in this problem, and should be taken into account. Please stop repeatedly posting that the air doesn't exist, or does not apply. This is not a GCE A level problem.
ok then we'll include the Magnus effect on the spinning balls and then we really do have an interesting analytical problem - not A level stuff!
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Magnus effect on a round ball with no crosswind? Negligible, relative to air resistance. You also have no way of defining the spin applied, since we don't know if the cannons are smoothbore or rifled.
The problem is pretty clear to me, I say this as someone with no experience with air resistance problems. "The ball is shot through the air" therefore there is air resistance. "No wind is blowing" does not mean "no air resistance," it simply implies that the air only originates friction, not force.
Can someone explain to me why we didn't say that gravity will act more on the iron ball that the rubber ball due to the difference in masses
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The fact that they have equivalent speeds at their different masses means that the ballistic motion, ignoring air resistance, would be identical. The lighter ball has a lower initial upward force applied to bring it to 50 m/s, and the denser ball has a greater initial force applied to bring it to the same velocity. If we could continue to ignore air resistance, then their travel would be identical, because the effect of gravity would cause the same ballistic track. Gravity does act "more" on the steel ball due to its greater density, but its density causes it to resist the effect of gravity more, which equalizes when the two balls are in motion in a vacuum. The only force that would affect the path of the projectiles differently, once in motion, is air resistance.
The problem is not well stated: the rubber ball can be a high density rubber that weights more than the steel ball, making most of the explanations the other way, and there’s no evidence on the finish of the steel ball (it can have a higher Drag coefficient than the rubber ball)
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The density of rubber is about the same as water. The density of iron is 7 times higher. There is no way to make a rubber that is more dense than iron. For your other point, the surface quality of the material is not a dominant factor when you consider the drag in air. Air drag is very different from the friction between two solid surfaces; it is primarily determined by the cross section and the speed. There is a small variation with the surface quality that can be important only in very special situation, for example in baseball.
The "same size" implies same not different masses.
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Same size means same volume. Differing materials could have differing densities; steel is more dense than rubber in the same volume, thus the objects would have differing mass.
Isn't the equation of horizontal distance covered by a projectile free of mass? Why will mass have any effect on the distance?
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There is air resistance which acts opposite to the motion of the ball. This drag force only depends on the geometry of the object and not the mass, so the acceleration depends on the mass of the ball.
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What if the rubber ball is incredibly smooth but the steel one is rough? The question shouldn't rely on assumptions of friction, without having explicitly mentioned it.
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@Shashwat Black – When you look at the resistance by the air, the surface quality is secondary to shape and cross section area. Just think about the air resistance on a car: A shiny Toyota Prius and a dirty one has the same resistance.
But if both of the balls have the same acceleration. Why will mass have any effect on the distance? Same acceleration no same velocity
As range R=u^2sin^Φ/2g Clearly it can be seen it depents on intial velocity and angle theta. Air resistance is neglected then they must cover same distance
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I also considered the same case but later on I found that momentum of a ball of steel is decreased less by air so that's why it will travel longer distance
Via Newton's penetration approximation / impact depth derived via momentum equation: Depth ~ Characteristic length (rho projectile/rho_air)
The ratio of air penetrated by steel (7700 kg/m^3) and rubber (930 kg/m^3) of the same geometry and velocity would be about 8.6 times as far. Thus steel travels further.
The relative lost in energy(energy absorbed) will be more for plastic ball at the time of impact...can be one of the reason!
it did not state that the iron ball has more mass than the rubber ball
the balls are the same size, but since steel is more condense material i would assume its heavier, wouldn't gravity affect the steel ball more then the rubber one making the steel ball to slow down and drop before the rubber one?
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The effect of gravity on both balls has been equalized by accelerating both to the same initial velocity. Ignoring wind resistance, both balls would fly the same path, due to having the same initial velocity. Once we add wind resistance back to the problem, it becomes the only variable that has to be accounted for, and the more massive ball has greater inertia to fight the effects of wind resistance.
Why do you assume the ball has a greater mass? It doesn’t say it’s solid.
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It doesn't have to be solid to be more massive than a rubber ball "of the same size". It's a safe assumption that they're the same construction. Ball, instead of sphere, might even indicate that they're both hollow. At any rate, the point of the materials is that there is a difference in mass between the two objects.
It is natural to assume that a canon ball is solid. Otherwise it may collapse or deform due to the explosion that pushes it forward.
R = ( u^2) * sin 2θ / g
Both balls are accelerated to the same rate by the cannons, but since the steel is more massive, it decellerates by air resistance slower and thus travels farther
The essential reason why simple projectile motion is independent of mass is straightforward. Only the vertical component of motion needs to be considered (for time to reach zero height after being set in motion) and the vertical force is mass multiplied by acceleration. The vertical force acting on the ball is gravitation ie. mass times acceleration due to gravity. Hence mass cancels in both sides of the equation and motion is therefore independent of mass
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The frequency with which you ignore multiple users telling you that air resistance can't be ignored in this problem is, itself, a problem. Please, stop commenting that mass cancels out of this problem. It's not occurring in a vacuum, it occurs in the air, and air resistance must be included in the solution.
Not exactly, it takes more force to accelerate the steel ball. However, if there is a constant force acting on both balls (say, drag from the air), then the more massive object will take longer to slow down than the lighter object.
but, rubber is more elastic and will bounce back after hitting the ground. Totally resulting in travelling farther distance.
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The challenge says the ball is measured at the point it first hits the ground
How do you know that the steel has a greater mass? It could be hollow. Also, the irregular surface of the rubber could add more wind resistance than a smooth steel ball.
Given initial speed is u = 50 m/s . Since both the balls come to rest finally so final velocity for both the balls v = 0 . Now, mass of iron ball is more than rubber ball . Let the mass of iron ball be x and that of rubber ball be y such that x > y . Force produced on both the balls by the cannon is same . Let the force be F . And both the balls have different accelerations a 1 , a 2 because they are of different masses .
F = m 1 x a 1 ⇒ a 1 = x F
F = m 2 x a 2 ⇒ a 2 = y F
Now it is clear that acceleration of rubber ball ( a 2 ) is greater than acceleration of steel ball ( a 1 ) . But the balls are retarding so the acceleration will be negative , that is , a = -a .
From Kinematics : 2aS = v 2 + u 2
So now S = 2 x ( − a ) 0 − 2 5 0 0 = a 1 2 5 0
Hence , S 1 = a 1 1 2 5 0 = F 1 2 5 0 ∗ x
And , S 2 = a 2 1 2 5 0 = F 1 2 5 0 ∗ y
Now , we will have the value of S 2 greater than S 2 (The reason is that the value of numerator is greater in S 1 than in S 2 because x is grater than y) . Therefore , the ball made of steel will travel to a longer distance .
the motion of a projectile (neglecting air resistance) is independent of mass
This reasoning is incorrect. The final speed is zero because the balls hit the ground. You reasoning is valid to the forces and displacements that happen during the short time when the ball hits the ground.
Work applied to both balls is not the same...velocity attained by both is same...means force applied to the iron ball is more because it has more mass
How do we know? We assume in this instance. And that assumption seems correct. But if we're going to assume Mass why don't we assume the compressibility of the rubber versus the steel also and take that in consideration as a variable in the outcome. There are too many variables and not enough data given allow for an accurate answer.
Both balls start with the same speed. And because of the higher mass of the steel ball gravitation is stronger and drags the steel ball down faster.
This is incorrect. It is not stated explicitly that the mass of the rubber ball is lower than the mass of steel.
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But in the question it is given that both the balls are of same size . So it is obvious that steel ball is heavier .
Answer: Stella (the iron ball).
We know that the formula to find momentum is:
p = m v
where p is the momentum
m is mass k g
And v is the current velocity m / s .
It is also handy to note that momentum p can be measured in k g x m / s . A l s o , ( k g ) ( m / s ) (which is the format I'm using) (they're the same thing).
(This basically means the distance the mass travels every second).
So, momentum is the measure of how far mass(e.g. (kg)) travels(e.g. meters) per a given time(e.g. seconds). We can take mass for an object, like a ball. Also, these are the most common units used for momentum, mass and velocity by far.
N o w t o t h e q u e s t i o n :
let the cannons shoot the 2 balls, each with a velocity of 50m/s.
First of all, we can instantly see that both balls are travelling the same angle (theta ( θ ) ) , so we don't have to worry about magnitude (direction).
Plus, the balls are being shot above ground, so we don't have to worry about impacts, until they hit the ground.
Furthermore, they are both the same size, so it isn't unfair in any way.
Now, from general knowledge (and science) we know the mass of the metal ball is greater than that of the rubber ball: as they are both the same size, and metal weighs more than rubber, with the same gravitational field strength(you could just say the gravitational field strength of earth) (By the way, this is because:
w e i g h t = m a s s x t h e g r a v i t a t i o n a l f i e l d s t r e n g t h ).
Therefore, the metal ball is heavier than the rubber ball (on earth), and therefore the metal ball has more mass than the rubber ball.
N o w , t o t h e a c t u a l l , a c t u a l l q u e s t i o n :
Now we know that the mass of the iron ball is greater than the mass of the rubber ball. ( m a s s o f i r o n b a l l > m a s s o f r u b b e r b a l l )
And they are both travelling at a velocity of 5 0 m / s
Using the formula: p = m v , we can calculate the momentum
. . .
let the mass of the iron ball = i , and let the mass of the rubber ball = r
the momentum of the iron ball = i ( k g ) x 5 0 ( m / s )
Which can be written as: 5 0 i ( k g ) ( m / s )
. . .
the momentum of the rubber ball = r ( k g ) x 5 0 ( m / s )
Which can be written as: 5 0 r ( k g ) ( m / s )
i > r , so we can easily see that:
5 0 i ( k g ) ( m / s ) > 5 0 r ( k g ) ( m / s )
Therefore, t h e m o m e n t u m o f t h e i r o n b a l l > t h e m o m e n t u m o f t h e r u b b e r b a l l
Now, remember... momentum measures how far mass(an object) travels per a given time. In this case, it is how far the mass travels (in meters) per second.
because the momentum of the iron ball is greater than that of the rubber ball, it will travel further per second (than the rubber ball), with it's first hit to the ground, further than the rubber ball.
Stella wins the competition, as she launched the iron ball.
Extra: The rubber ball may travel further overall, but ONLY after the first bounce. This is because rubber has lots of elastic potential energy (and therefore upthrust), when hitting the ground, to propel the ball even further. Basically meaning, it is bouncy.
Whereas, the rubber ball still generates more friction than the iron ball, so the iron ball may still have a moderate chance (if it is presumed not to be rough, which it usually isn't).
Furthermore, the metal ball may make up the distance the rubber ball used for propelling itself, by rolling.
Answer: Stella (the iron ball).
But what about the elastic property of the rubber balls when it is initially shot. Think about an iron baseball vs a rubber one. If both are hit with the same force ( the same wave front from the explosion in the cannon) which one will go farther. I think the rubber one. That's why they don't make baseball's or golf balls or any balls out of iron.
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Sure, but it's a different problem. In the baseball scenario, you're applying the same external force (via the bat) to the projectile, and examining the ability of the material to transfer the elasticity of the collision into flight. In the cannonball scenario, the cannon are specifically designed to apply the same initial speed (thus different forces applied) to the projectiles, and the question applies only to the ballistic motion after the initial force is applied.
The way i did it is that kinetic energy is equal to the mass of the object multiplied by the velocity squared (KE= 2 1 MV^2) so since we know the velocity is 50 meters per second, plus with the understanding that the balls are the same size, we know that the steel ball will have more mass due to its density being larger than the rubber ball. Knowing that the mass will be larger than the rubber ball, we know that the kinetic energy is larger therefore stating that it will go further.
First to answer using K.E, winner.
Since Force=Mass x Acceleration and the force is the amount of power the ball will have, when the acceleration is constant, as in the example, the higher the mass, the more power the ball will have.
I imagine the scenario and I think that the rubber ball will be easier to be deformed and flattened out due to air resistance. But if that's not the case than the one with more mass will experience less air resistance and will win. This case it's the steel ball.
My answer was based on practical experience, I've experienced to bowl faster with a hard bowl than a rubber or cosco ball, with same effort. And that is how I deduced, the one which is heavier will travel farther.
If the balls are the same size then it's safe to state that the steel ball weighs more than the rubber ball. The steel ball will then travel further than the rubber ball because it generates more force.
The steel ball doesn't "generate force" in that sense. It has more inertia, and thus more momentum at the same speed.
E=mc^2. As per this equation the object having more mass travels at high speed.
Nope, not related at all... this equation is for mass-energy conversion and not applicable to objects of considerable mass and size... irrelevant. Also, if you see it other way in this equation E=mc^2 energy and is dependent on velocity of balls and there is no relation with distance covered.
I do not think this is what the equation stands for.
But in the equation E = m × c 2 the speed c is constant for all . It;s value will be equal to that of light .
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I think the better way to reason in this method is that :
Steel has more mass so the value of m for steel will be greater than than that of rubber . So the energy produced by steel ball will be grater than rubber ball . And hence , steel will travel for a longer distance .
Heavy objects travel further than rubber because of the force excreted by source and the weight of the object.
This can be proven if canon is shooted towards water, rubber ball will travel further than steel because of the weight of the object.
I had reasoned that the flexibility of the rubber ball would absorb more of the ballistic force than the rigid steel, and thus would travel less far. Of course, at 45°, they would each travel as far as 50 m/s could launch them.
Iron ball will have more momentum so it will go a bit further
Many of the solutions are irrelevant because one of the given conditions is that the cannon fires at 50 m/s, therefore the power/work/force are adjustable for mass/density of projectiles. Since both objects are spheres and of the same size, air friction shouldn't affect them in any different way, at least at those speeds. At higher speeds, the effect of skin friction becomes more noticeable. The atomic surface of rubber is more prone to exchanging momentum with air molecules when compared to the smooth surface of a polished steel ball. The larger the projectile, the more measurable a difference in range.
My initial thought was that the rubber ball has less density therefore more of the cannon's work would go into internal energy rather than kinetic energy, therefore it would leave with less speed, however the initial speed is fixed. Also, yes the steel ball has more kinetic energy due to a higher mass, but that does not give it more range; projectile motion does not depend on mass. The air friction, however, does depend on those masses.
Let the density of steel ball be α and that of rubber ball be β .... We know that α > β !!!! Let the density of air be γ ... Therefore the buoyant force due to air on steel ball and rubber ball is proportional to γ !!!!! And frictional force will be proportional to α - γ and β - γ respectively for steel and rubber ball [see I have arrived at this by seeing frictional force as viscous force ] let V be the volume of both the balls then mass of Steel ball will be α V and mass of rubber ball will be β V and retardation on steel ball will be proportional to [ α - γ ] / α V and retardation on rubber ball will be proportional to [ β - γ ] / β V by seeing the expressions one may mistakingly predict that retardation of Steel ball is more therefore Steel ball goes to a lesser distance but actually this viscous force decreases the maximum height of a projectile but increases the time of flight !!!! [Time of flight for steel ball will be more increased]For horizontal the frictional force acting on both of them will be same therefore less retardation of heavier Steel ball so our both results favours STELLA and STELLA wins !!!!!
Here is the catch. They are of same size (i.e. volume). Since steel has higher density over rubber it will have more mass which means initial momentum of steel ball is higher. Both balls are same size, hence we can assume the drag on both balls are same (air resistance). Therefore as steel ball has higher momentum it will travel further.
At first, i thought it will be a tie because it has to do with the shape of the ball and how air dinamic it is. It also has to do with the force the balls have. Since the steel ball has more mass, it has a lot more kinetic energy ( m*v^2/2) and thus travels further.
Steel Ball is heavier so when launched it will have greater momentum than the rubber ball. The friction and initial velocity will be approximately the same so the steel ball having greater momentum will go farther
At same speed moment of inertia of steel ball is larger than rubber ball.
Presuming the steel ball is heavier than the rubber ball, the fact that they've both been accelerated to the same velocity suggests that more force must have been applied to the steel ball to get it up to that velocity. Where and when it lands would then depend on the momentum, which common sense suggests would be higher in the steel ball, thus greater distance reached.
Even though the balls are same size, there's no assumption that the balls are completely solid. If the middle of the steel ball is hollow, the Steel ball can weight less than the Rubber ball and therefore Stella can loose!
While the math answers are of course true, the situation is kind of overlooking the fact that you are shooting a rubber ball out of a cannon. Is it unreasonable to consider the rubber ball would be damaged by the blast and would contribute to a lack of flight? Maybe the picture is adding to my assumption.
We could pretend that the cannon is designed in such a way as to minimize deformation of the rubber ball; possibly an air cannon or similar mechanism to accelerate the ball to the required speed.
The iron ball has more mass so it therefore has greater inertia: and will travel farther.Gravity doesn't matter since both balls will still fall at the same rate;Unless of course there is air resistance or the rubber ball bounces.
The diameter of both projectiles is equal, the exit velocity has been equalized, obviously more energy given to Stellas gun to produce the equalized exit velocity as rubber is accelerated easier than heavy metal...aerodynamic drag would be equal due to projectile congruity... so the extra energy provided to accelerate Stellas projectile to match the rubber counterpart upon muzzle exit will carry the projectile farther since there are no other appreciable differences in flight dynamics between the two projectiles. That extra energy put into the metal ball to equalize muzzle exit velocity will have to go somewhere... and that being distance traveled. If distance were limited or equalized by say -a wall placed at equal range to both guns, then the extra energy afforded to Stella would be dissipated as impact force.
More projectiles are made of iron than out of rubber. If the projectile were able to cause more turbulence, it would lose horizontal velocity faster and therefore gravity (in vertical downward direction) would win?
Distance= mass x acceleration. The steel ball has more mass, so even though it takes more force to get it going, it will take more force to stop it.
I think you meant Force = mass * acceleration
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The iron ball has more mass and therefore more initial momentum, since the initial velocity is the same. The friction due to air will slow down both of the balls and reduces the momentum. The friction force is (approximately) the same for both balls, because it depends on the velocity and the size of the balls only. Therefore the change in the momentum, Δ p will be also the same. The corresponding change in the velocity will be Δ v = Δ p / m that is larger when the mass is smaller. The rubber ball will have slightly less velocity and will travel to a shorter distance.
In reality, the friction forces are not exactly equal, because the rubber ball's velocity is slightly less than the iron ball's. This will not change the conclusion.
If there is no air, the two balls will travel exactly the same distance.