Throwing Balloons

Mischievous Sam is sitting in a moving bus. At the moment he spots his best friend Jim standing on the footpath, Sam throws a water balloon directly at Jim.

Will the balloon hit Jim?


Note: For simplicity, treat Jim and the balloon as if they are point particles.

Yes, it will hit Jim No, it will miss Jim Yes, it will hit him only if the bus is moving at a constant speed

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14 solutions

Arjen Vreugdenhil
Aug 20, 2017

Relative to Sam, Jim is moving to the backward. When the balloon arrives at Jim's original position (relative to the moving bus), Jim is no longer there. The balloon will miss.

Having said that, the answer "It will hit Jim" could be defended. If the balloon has a very high speed, its sideways motion due to the speed of the bus will be so little that it may hit Jim only a little to the side. Also, drag forces will slow down the balloon.

If w w is half of Jim's width, d d is the distance of Jim to the balloon, u u is the speed of the bus, then one could launch the balloon at speed v = d w u v = \frac{d}{w} u and still hit Jim. Realistic values would be d = 5 d = 5 m, w = 0.3 w = 0.3 m, u = 6 u = 6 m/s, so that v = 100 v = 100 m/s or 225 mph-- a very fast balloon.

But the balloon must have a decent speed, otherwise it splashes on the ground before reaching Jim. If the balloon is tossed from height h h , we need a minimum speed of v = d g / 2 h v = d\sqrt{g/2h} . With h = 1.5 h = 1.5 m and g = 10 g = 10 m/s/s, this minimum speed is nearly 10 m/s, or 23 mph-- still pretty fast.

Arjen Vreugdenhil - 3 years, 9 months ago

And how big is jim...

Jeannine Myer - 3 years, 9 months ago

This problem is only difficult insofar as the language used. Colloquially if you throw an object at someone that doesn't mean you are throwing the object at the stationary point they currently occupy in your reference frame.

Otherwise none would ever catch a pass in football. This isn't even including gravity which will further throw you off

Matt W - 3 years, 9 months ago

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In football, the players are trained to throw balls after taking care of their relative motions.

Rohit Gupta - 3 years, 9 months ago

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Where does the puzzle say that Jim didn't take care of their relative motions? If one player is stationary, and the one throwing the ball is moving, he is going to throw the ball "direct at" the non-moving player. Because he knows how to do that.

Heidi Cortelyou - 3 years, 9 months ago

The qualifier is that they are point particles - otherwise the question is not really solvable for the real world without more data. Although of course in the real world the human brain will allow for speed angle etc.

As also the fact Sam will probably be friendless!

Brian M - 3 years, 9 months ago

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Human brain allowing for speed angle etc is ruled out because problem says "throws ... directly at ..." Read the problem carefully, please.

Ajoy Bhatia - 3 years, 9 months ago

Actually, there is also another reason the balloon will miss Jim. Being a bus, the window will be up. No balloon will be thrown out.

Dennis Rodman - 2 years, 3 months ago

no. excses throw a bus throwthly in the days off ouer,s.thank uou!

Ahmad.M.A Banhani - 3 years, 9 months ago

While I understand that we're looking at the problem from a maths/physics perspective, in reality, the balloon is very likely to hit Jim because the human brain is quite good at correcting speed differentials and inertia on the go. A simple way of corroborating this is to walk briskly while tossing a ball high into the air. By all means, the ball should always fall behind you, more or less onto the spot you were when you toss it, but you'll find out that with the slightest application of intentional direction, you can always make the ball fall back into your hand, your brain effortlessly calculating and executing what on a paper would be a complex parabolic trajectory.

It bears mentioning, though, that if the bus is moving at a very high speed, the balloon is likely to miss if only because the thrower's arm is not capable of moving fast enough to compensate the distance traversed by the bus in that short lapse.

Edgar Lemos - 3 years, 9 months ago

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The "compensation", by definition, is not "directly at Jim".

Jason Dyer Staff - 3 years, 9 months ago

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@Edgar Lemos - Also, a ball tossed high straight up while walking briskly will NOT (and should not) fall behind you for precisely the same reason that the balloon shot straight at Jim will miss him. This is because the ball has the same horizontal velocity as the walker and the horizontal velocity is maintained (ignoring negligible wind drag).

Ajoy Bhatia - 3 years, 9 months ago

A ball tossed high straight up while walking briskly will NOT (and should not) fall behind you for precisely the same reason that the balloon shot straight at Jim will miss him. This is because the ball has the same horizontal velocity as the walker and the horizontal velocity is maintained (ignoring negligible wind drag). The point is that the brain is not doing any "correcting speed differentials and inertia on the go" to make the ball fall back to the walker. It is simple physics. I suggest you refresh your high school physics once again and not overanalyze the problem.

Ajoy Bhatia - 3 years, 9 months ago

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Fair enough, I provided a poor example. That doesn't take away from the point that the brain does compensate and correct on the go. Also, I wouldn't say it is over analyzing when I'm simply referring to empiric results in a real life scenario. Sports wouldn't be a thing if you couldn't toss or kick a ball and have it land where you want regardless of your horizontal speed.

Nevertheless, I regret to bring it up. The problem is clearly a hypothetical case constructed for exact responses using exclusively math constants.

Edgar Lemos - 3 years, 9 months ago

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@Edgar Lemos An athlete running with a ball learns to aim behind the target!

Arjen Vreugdenhil - 3 years, 9 months ago

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@Arjen Vreugdenhil If the ball was not thrown "directly at" the non-moving target, it will miss. Where you "aim" and where you "throw" are two different things.

Heidi Cortelyou - 3 years, 9 months ago
Richard Desper
Aug 21, 2017

Tautologically, if the balloon misses Jim, then Sam didn't throw it "directly at" him.

Another possibility: the moving bus is moving incredibly slow, so its velocity is negligible compared to Jim's width.

The right answer. "Directly at Jim" means "in the direction such that it will hit Jim". What the question setter seems to have meant is "in the direction of the vector that is the sum of the velocity vector of the bus and the displacement vector between Jim and the thrower."

Spicy Brigadoon - 3 years, 9 months ago

This question makes no sense. How can you say the balloon will either definitely hit or miss the target? Also, the speed the bus is travelling at is not mentioned so it is impossible to take that into account without the relevant information. As for the answer being that it will miss the target, I bet if you actually carried out this theory in real life, Jim could quite easily end up getting a water balloon in the face (considering it was thrown "directly" at him).

Phil Roy - 3 years, 9 months ago

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A clarification has been mentioned which state to consider the balloon and the target as point particles. This should satisfy your above concern.

Rohit Gupta - 3 years, 9 months ago

If it were I, he'd have a face drenched with water.

Zzurek Jerron - 3 years, 9 months ago

compared with Jim's width

Hugo Burton - 3 years, 9 months ago
Kenny O.
Aug 17, 2017

When Sam throws the balloon, the balloon has the inertial equal to the bus. The inertia of the balloon causes it to move towards the right and miss Jim.
Another scenario is that balloon does not even reach Jim (without the inertia)

You mean velocity, not inertia.

Erin Sutherland - 3 years, 9 months ago
Srijan Rit
Aug 22, 2017

When the baloon is in Sam's hand, it moves with the same velocity as him which is the velocity of the bus. When the baloon is thrown it has a velocity (the velocity at the instant of launch) in the direction of motion of bus. So, it will hit some distance apart in direction of initial motion from Jim due to that forward component of velocity.

When the balloon is thrown it has a velocity (the velocity at the instant of launch) in the direction of motion of the bus.

Why does the balloon have a velocity to the direction of the bus instead of hitting Jim?

Christopher Boo - 3 years, 9 months ago
Munem Shahriar
Aug 24, 2017

Due to constant speed of the bus, it will miss Jim. The bus is keep moving and Jim is just standing on the his position, he is not walking. As a result it changes the direction of the water balloon and misses jim. Constant speed of the bus is the reason here.

What changes the direction of the water balloon, exactly?

Christopher Boo - 3 years, 9 months ago
Annie Li
Aug 21, 2017

When he is throwing it, the bus is moving so the balloon 🎈 may just miss Jim. Also, you shouldn't throw water 💦 balloons 🎈 at people as this may get you banned from using buses, sme serious trouble from your parents and if the balloon had hit Jim, Sam will be in really, really big trouble.

wTF you appear lk my mother or the police "you will be in really, really BIG t~r~oubl~e!" Uhhh~hh

Gabriel Isla - 3 years, 9 months ago

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Haha, you make me laugh...yes...I agree...

William Huang - 3 years, 9 months ago

I love this solution 😂😂😂

Kevin Tran - 3 years, 9 months ago

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Thanks for the feedback

Annie Li - 3 years, 9 months ago

Just let Sam do what he wants.

Kenny O. - 3 years, 9 months ago

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👌 ok I get the meaning

Annie Li - 3 years, 9 months ago

the best solution!!!

Mohammad Khaza - 3 years, 9 months ago

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Thanks! I enjoyed writing it as well

Annie Li - 3 years, 9 months ago

Why is the kid even attacking Jim? Violence is never the answer, so for the sake of religion, no matter how hard you hope it'll hit, it won't

Alex Alex - 3 years, 9 months ago

haha😂. you made me laugh.

ARYAN POLY - 3 years, 9 months ago

But Jim threw one at him first.

A W - 3 years, 9 months ago

Lol! This solution went from 23 upvotes to 0.

A Former Brilliant Member - 3 years, 9 months ago

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Ha, that's sad...oh well...

William Huang - 3 years, 9 months ago

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Not just sad but really really sad 😭

Annie Li - 3 years, 9 months ago

When did it have 23? What happened?

Annie Li - 3 years, 9 months ago

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Computer glitch? Or maybe everyone that upvoted just thought about it and pressed un-upvote...at least I upvoted...

William Huang - 3 years, 9 months ago

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@William Huang Thanks for up voting it!

Annie Li - 3 years, 9 months ago
John Oshman
Aug 22, 2017

Two scenarios exist where the balloon can hit Jim:

  • Sam throws it fast enough so its forward velocity gets there before the sideways motion has any effect.
  • The bus is moving fast enough to provide enough sideways motion and the balloon travels a complete (full) global circumnavigation and then hits Jim on it's next trip around the globe...

Then what do you think is the answer, actually?

Christopher Boo - 3 years, 9 months ago
Alex Alex
Aug 26, 2017

Basically, busses have things called windows, and that shield people from projectiles such as water balloons

Quantum Zain
Aug 26, 2017

Note: this problemo actually happened to me when I had to throw the garbage (no I wasn't​ throwing a water balloon at my friend)

As the car approached the bin, I threw the garbage bag directly towards the bin, But it landed besides it!

And it makes sense, because the bag was moving towards the bin AND also in the same direction of the car ; so it missed and that's what happened to poor Sam and lucky Jim.😊

Sam had to throw the balloon earlier; and I should have thrown the bag earlier too... Which actually worked!

Simon Dancose
Aug 25, 2017

If Sam throws the balloon at Jim immediately the moment he sees Jim, the balloon will miss since it also keeps moving sideways at the same non-zero speed that the bus was moving when thrown. Sam could hit Jim if he properly calculated the angle to the right of Jim (therefore Not throwing at Jim) when he sees him to compensate for the distance travelled by the balloon sideways.

Ojasvi Sharma
Aug 25, 2017

As Jim is stationary and Sam along with balloon is moving, therefore Jim is moving backward relative to Sam. At the instant when they both are near each other, when Sam will throw balloon on Jim, the balloon will also move with the same velocity as of the bus. Therefore the balloon will remain ahead of Jim and could never catch him.

Atul Rai
Aug 23, 2017

The direction of movement of the balloon is directly out of the bus, and the direction of movement of the bus is perpendicular to that of the balloon. So adding these two vectors, the resultant motion of the balloon would actually be at some angle (can be calculated based on ratio of speeds of the balloon and the bus) from the direction the balloon was thrown in.

Rupert Bryce
Aug 23, 2017

It's relative velocity. Everyone knows you have to lead a moving target.

What do you mean by relative velocity? From which object's perspective?

Christopher Boo - 3 years, 9 months ago
Makisi Palu
Aug 22, 2017

Throwing the balloon gives the ball on a second velocity component making it move diagonally right?

Yes, that's right. It is because of the second velocity with the same direction as the bus caused the balloon to move slightly in that direction.

Christopher Boo - 3 years, 9 months ago

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