A cyclist is riding a bicycle of wheel radius r along the edge of a rotating disk of radius R ( > r ) in such a way that he appears to be stationary to a person standing on the ground.
If the tires of the bicycle don't slip on the disk (that is, the contact points of the wheel and the disk have equal velocities), then which will have a greater rotating speed?
Clarification: The "rotating speed" is the angular speed (or the magnitude of the angular velocity) given by the rate of change of θ with respect to the axis about which it is rotating:
ω = d t d θ
This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try
refreshing the page, (b) enabling javascript if it is disabled on your browser and,
finally, (c)
loading the
non-javascript version of this page
. We're sorry about the hassle.
Well, when you put it THAT way it just makes sense!
Such a concise simple explanation. Well done.
Relevant wiki: uniform circular motion
Since the tires don't slip, the velocity of the wheel where it makes contact with the disk and the velocity of the disk at the same point will be the same.
Let's call this velocity v .
⟹ v = ω t i r e r = ω d i s k R
⟹ ω t i r e = r R ω d i s k
So, since R > r the tire of the bicycle will have the greater rotating (angular) speed.
I tried answering this without realizing that by "rotating speed" what was meant was a scalar analog of angular velocity, which is measured in radians per second, not in meters per second. So I was thinking about the velocity you called "v", and not about the variables that were actually in question.
Log in to reply
I agree, the question's phrasing is sloppy
Log in to reply
Any suggestions on a better phrasing besides changing "rotating speed" to "angular velocity"?
Log in to reply
@Geoff Pilling – Or "the magnitude of the angular velocity" to be more precise?
I thought that the rotating speed of an object means it is the rate at which it rotates, and would not be confused with its linear speed. Nevertheless, I see that a note has been added to the problem statement to clarify what rotating speed means.
with no slippage between the tire and disk the speed of the two would be the same the tire, having a smaller circumference would just make more rotations.
First of all velocity(v) = (radius (r) )(angular velocity ( ω ))
Bicycle tire speed = r ω 2
Disk speed at distance R = R ω 1
Both the tire and the disk need to be moving at the same speed so,
r ω 2 = R ω 1
solving for r
ω 2 r ω 2 = ω 2 R ω 1
r = ω 2 R ω 1
Using substitution and R > r
R > ω 2 R ω 1
Dividing by R
R R > ω 2 R R ω 1
1 > ω 2 ω 1
Multiplying by ω 2
1 ω 2 > ω 2 ω 1 ω 2
Yields
ω 2 > ω 1
So the tire rotation speed is greater than the disc.
I think you are adding an extra factor of 2 π .
ω = 2 π f .
(Where f is the frequency and ω is the angular velocity)
So, v = r ω
Log in to reply
It doesn't matter. He factored it on both sides so it's still an equivalent statement. Just like 3x = 6 is the same as 6x = 12 :D
Correct. I will change my solution to reflect that.
Do you know what mqth concepts are requured to understand this problem because I don't get any of this. Little help if you may.
Small gears spin faster than big gears.
Tangential speed is the same at disc and bicycle's tire ⇒ ω 1 R = ω 2 r and we can see that R > r ⇒ ω 2 > ω 1
The cyclist travels tangentially along the perimeter of the disc. The radius of the disc is larger than the bike's wheel radius. Therefore, the perimeter of the disc is larger than the perimeter of either of the bike's wheels. So, proportionally, it would take more "wheel perimeter" distance to catch up to the perimeter of the disc.
In a way, consider if you took off the tire of one of the bike's wheels, chopped it in one place, and unrolled it along the perimeter of the disc -- you'd need more than 2 tires to cover the entirety of the disc's perimeter. Well, speaking proportionally, if Distance = Rate * Time, and the bike's wheel's distance [unrolled tire length] is decreased compared to shorter than] the disc's, then the wheel's rate would need to increase to match the disc's rate.
Right, since the perimeter of the bicycle wheel is smaller, it has to make more revolutions than the disc to remain stationary.
My analogy was similar: think if tire was coated with wet white paint except in one spot with black paint to mark a complete revolution of the tire. As the disc turns, the tire will paint several revolutions around the disc.
Log in to reply
bicycle speed: ω × ω tire speed: x² × 2 human speed: 5mph ω + 2 + 5 = 389.0
Problem Loading...
Note Loading...
Set Loading...
Relevant wiki: uniform circular motion
Consider the motion of the cyclist during one revolution of the disk.
If the cyclist didn't pedal at all he would be carried right around the perimeter of the big disk.
So in order to stay stationary, he must actually cycle exactly once (in the opposite direction to the disk's rotation) around the disk.
Since the circumference of his wheels is less than the circumference of the disk, the wheels must rotate more than once on this journey, while the disk has rotated exactly once.
And so the wheels have a greater angular velocity than the disk.