# Homework Help: A ball rolling down a slope, find its velocity.

1. Nov 20, 2013

### Milsomonk

Hi guy's, I've got an interesting problem in my coursework, I have to say I am stumped, so here it is.

A hollow sphere of mass 56 grams, starts at rest and is allowed to roll without slipping 2 metres down a slope at an angle of 25 degrees. Gravity is assumed as 10 ms^2. clculate the velocity after 2 metres.

I=2/3*wr^2 is given as a relevant equation.

I'm not even sure where to start without being given a measurement for r or any dimensions. I've calculated the gravitational potential energy in the hope i can find the proportions of KE(rotational) and KE(linear), But im at a dead end there.

Any help would be hugely appreciated as I'm baffled.

Cheers

2. Nov 20, 2013

### Staff: Mentor

What might be conserved, since there is no slipping?

Tip: When not given a quantity, such as r, just assume you may not need it. Just solve it symbolically and see what happens.

3. Nov 20, 2013

### Milsomonk

Will it still accelerate at 10 ms^2 straight down? despite the slope? so then we have a time component for the distance.

4. Nov 20, 2013

### Staff: Mentor

No. 10 m/s^2 would be the acceleration of an object in free fall, not something in contact with a slope.

To solve for the acceleration, if you would like to do that, you'll need to consider all the forces acting and apply Newton's 2nd law for translation and rotation.

But there's an easier way. Answer the question I raised in earlier post.

5. Nov 20, 2013

### Milsomonk

Sorry, I was trying to understand your initial question, (Im not good at this), So the energy is conserved? MGH? ive worked that out but thats where im stuck.

6. Nov 20, 2013

### Staff: Mentor

Yes, the energy is conserved.

At the top of the ramp, the energy is all potential. But what about at the bottom? How can you express the total energy of the rolling ball at the bottom?

7. Nov 20, 2013

### Milsomonk

I get that, some will be rotational and some will be linear. Its just a case of working out the proportion of each, and then finding a velocity from that that im stuck with. Thanks fo the help by the way, I am getting there i think.

8. Nov 20, 2013

### Staff: Mentor

That's exactly it. Hint: The two energies are connected by the fact that the ball rolls without slipping. How do you express that mathematically?

9. Nov 20, 2013

### Milsomonk

PE=MGH so MGH=KE(rotational)+KE(linear). so if there is no slip, does that make the split 50/50?

10. Nov 20, 2013

### Tanya Sharma

The relation is correct but they are not in the ratio 1:1 .

Express the relation symbolically in terms of the given parameters like 'm' ,'v', 'R' ....

Use the rolling without slipping constraint.

Last edited: Nov 20, 2013
11. Nov 21, 2013

### Milsomonk

Ok so I think I might be getting somewhere, just one smal hurdle. Heres what ive got.

mgh=1/2(2/3mr^2)(V/r)^2+1/2mv^2

Then I cancelled the m from both sides gh=1/2(2/3r^2)(V/r)^2+1/2v^2

Where I'm stuck is how to get rid of r.

12. Nov 21, 2013

### Tanya Sharma

There is an r2 term in both the numerator and denominator of the first term on the Right Hand Side .

You missed cancelling them

Last edited: Nov 21, 2013
13. Nov 21, 2013

### Milsomonk

AHA silly me, awesome thankyou all for your help I have now done it!! only took me three days haha :)