1st year problem. Rolling Motion.

[assman]

I am haveing trouble solving a problem that should be fiarly easy. I believe that i have all the components of the question togethor but i can't solve it.

1. If you have a cylinder and a sphere roll down a 3m long board at angle theta, and the time for the cylnder to reach the bottem is 2.4s slower then the sphere. WHat is theta?

THanks

 

[allistair]

are you sure that that is the question? if they start out at the same height they should allways reach the bottem together i think...

 

[assman]

Ya that's it. and its a hollow thin walled cylinder.

 

[arildno]

What have you done so far; in particular:
What is the physical reason for the two objects not arriving at the same time at the end of the slope?

 

[assman]

i have no clue for hte arrival time. its a question out of a textbbook. I tried relating the t1 = t2 in the v = v0 + at with t2 = t1 + 2.4 and it never works.

I'm stumped.

 

[arildno]

Again:
Have you no clue whatsoever why the ball arrives first?

 

[assman]

The question from the text is very straight forward. One shpere one hollow cylinder roll down an incline at angle theta. THe incline is 3m long and the sphere gets to the bottem 2.4s faster. What is theta.

The answer sould not be that they land at the same time because of their differing moments of inertia. RIght?

 

[arildno]

That's true, it's because of their different moments of inertia that they don't arrive at the same time:

Hence:
You need to find out how the moment of inertia enters your equations, right?

1.Question:
If you are to calculate the torque of external forces about the center of mass, what external force provides the net torque?

Set up Newton's 2.law for the object, plus the moment-of-momentum equation with respect to the center of mass (the torque equation, that is).

2. The object ROLLS:
What relation does this give between the acceleration of the center of mass and the angular acceleration?