# Rotation Question. Is there sufficient data/values? Pondered on question 4 sumtime..

 P: 13 1. The problem statement, all variables and given/known data Okay..here's d question: A sphere, a cylinder and a hoop start from rest and roll down the same incline. Determine which body reaches the bottom first. 2. Relevant equations Sphere: I=2/5 mr2 Cylinder: I=1/2 mr2 Hoop: I= mr2 F=ma 3. The attempt at a solution To find out which of the three reach first I suspect that their acceleration should b found 1st..but..how do I go about it? I also feel as if there is not much data to work on..is tis question complete? If it is solvable..a lil clue or idea would b much appreciated... =)
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P: 26,148
 Quote by brainracked 1. The problem statement, all variables and given/known data Okay..here's d question: A sphere, a cylinder and a hoop start from rest and roll down the same incline. Determine which body reaches the bottom first.
Hi brainracked!

Hint: "roll" means without slipping …

so the instantaneous velocity of the point of contact is zero …

so the speed of the centre of mass must always be cancelled by the rotational speed of the rim (relative to the centre of mass).

That gives you a relationship between velocity and angular velocity
 P: 13 Hiyya tiny-tim..we meet again..haha.. So sorry..but..i dont quite understand..the instantaneous velocity of point of contact? Relationship? w=v^2/r? So sorry for the trouble and thank you so much for ur time and help..
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P: 26,148
Rotation Question. Is there sufficient data/values? Pondered on question 4 sumtime..

 Quote by brainracked So sorry..but..i dont quite understand..the instantaneous velocity of point of contact? Relationship?
Hi brainracked!

The point of contact of a wheel with the ground is always stationary, just for an instant.

If it wasn't, the wheel would skid.

So if the centre is moving horizontally at speed v, and the wheel is rotating with angular speed ω, the point at the top of the rim has speed v + rω, and the point at the bottom of the rim (the point of contact) has speed v - rω.

So rolling requires v - rω = 0. (that is the "relationship")

This is geometry, not physics.

Now that the geometry has told you the relationship between v and ω, plug that into KE + PE = constant to get the acceleration.
 P: 13 Haha..okay..i get it..thanks!
 P: 13 Wait wait wait..I ended having two variables..h and v PE + KE = mgh +1/2mv^2 + 1/5 mv^2 =gh + 7/10v^2 I dont see how tis can solve the answer..
 P: 13 *solve the question
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P: 26,148
 Quote by brainracked Wait wait wait..I ended having two variables..h and v
v = (dh/dt)/sinθ
 P: 13 and to get the acceleration? I'm lost..
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P: 5,341
 Quote by brainracked Wait wait wait..I ended having two variables..h and v PE + KE = mgh +1/2mv^2 + 1/5 mv^2 =gh + 7/10v^2 I dont see how tis can solve the answer..
$$\Delta PE = \Delta KE = mgh = \frac{mv^2}{2} + \frac{I\omega^2}{2} = \frac{mv^2}{2} + \frac{I}{2}*\frac{v^2}{r^2}$$

Determine this equation for each of the objects and compare them by inspection. All you are asked is which is fastest.

You can always solve V for each.

As to acceleration you were asking about, lest you forget dv/dt is acceleration.
 P: 13 the h is still the problem though..i still dont understand what tiny tim means by v=(dh/dt)/sin $$\theta$$ I can only manage to simplify the equations down to: Sphere: mgh=$$\frac{7}{10}$$mv2 Cylinder: mgh=$$\frac{3}{4}$$mv2 Hoop: mgh=mv2
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P: 5,341
 Quote by brainracked the h is still the problem though..i still dont understand what tiny tim means by v=(dh/dt)/sin $$\theta$$ I can only manage to simplify the equations down to: Sphere: mgh=$$\frac{7}{10}$$mv2 Cylinder: mgh=$$\frac{3}{4}$$mv2 Hoop: mgh=mv2
For a given drop in height which has the greater V?

Won't the fastest object at the bottom necessarily be the one there first, if acceleration is uniform?
P: 13
 Quote by LowlyPion For a given drop in height which has the greater V? Won't the fastest object at the bottom necessarily be the one there first, if acceleration is uniform?
For a given drop in height, the hoop has a greater V? It is possible to find the acceleration of each of the objects right?
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P: 5,341
 Quote by brainracked For a given drop in height, the hoop has a greater V? It is possible to find the acceleration of each of the objects right?
You might want to check again which has the greater speed for a given h.

Of course you can figure acceleration if you wish. But if you know that one object is faster at one point of h below the top in a uniformly accelerated field then you know from induction that it is faster at all points on the incline don't you?
 HW Helper P: 5,341 Forgot to add that if it is faster at every point on the incline then it would be fastest to the bottom wouldn't it?
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Hi brainracked!
 Quote by brainracked the h is still the problem though..i still dont understand what tiny tim means by v=(dh/dt)/sin $$\theta$$
v = dx/dt.

It's a slope, with angle θ, say …

so x (along the slope) = h/sinθ.

This isn't physics … this is geometry.
P: 13
 Quote by LowlyPion You might want to check again which has the greater speed for a given h. Of course you can figure acceleration if you wish. But if you know that one object is faster at one point of h below the top in a uniformly accelerated field then you know from induction that it is faster at all points on the incline don't you?
The equations mentioned at top, are they right?
I assume the sphere has a greater speed for a given h. Followed by the cylinder and the hoop. This is known by looking at the equations? Is that right?

 Quote by LowlyPion Forgot to add that if it is faster at every point on the incline then it would be fastest to the bottom wouldn't it?
Yea..it would..

 Quote by tiny-tim Hi brainracked! v = dx/dt. It's a slope, with angle θ, say … so x (along the slope) = h/sinθ. This isn't physics … this is geometry.

I understand that..it's just that I dont understand how there could be values for x and h..
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P: 5,341
 Quote by brainracked The equations mentioned at top, are they right? I assume the sphere has a greater speed for a given h. Followed by the cylinder and the hoop. This is known by looking at the equations? Is that right?
Yes, that is correct.

Here is a demonstration you might enjoy:

http://hyperphysics.phy-astr.gsu.edu...oocyl.html#hc1

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