# A 1-lb ball A is traveling horizontally at 20 ft/s when it strikes a 10-lb block B

1. Jul 15, 2012

### danyalasdf

1. The problem statement, all variables and given/known data

A 1-lb ball A is traveling horizontally at 20 ft/s when it strikes a 10-lb block B that is at rest. If the coefficient of restitution between A and B is e = 0.6, and the coefficient of kinetic friction between the plane and the block is µk = 0.4, determine the time for the block B to stop sliding.

3. The attempt at a solution

M1V1=M2V2

(1/32.2)(20) + 0 = (1/32.2)(Va2) + (10/32.2)(Vb2)

(Va2) + 10(Vb2) = 20

e = (Vb2-Va2)/(Va1-Vb1)

0.6 = (Vb2-Va2)/(20-0)

(Vb2 - Va2) = 12

From here I get that

Vb2 = 2.909 ft/s to the right

Va2 = -9.091 ft/s to the left

Block B

T1 + U(1→2) = T2

this where I am stuck ?

any help?

2. Jul 15, 2012

### TSny

Re: A 1-lb ball A is traveling horizontally at 20 ft/s when it strikes a 10-lb block

So, you need to find the distance B slides. You know the initial speed of B at the beginning of the slide and you know it will slide until it comes to rest.

You can either use energy concepts to get the distance of slide, or you can use Newton's second law to find the deceleration during the slide and then use kinematic equations to find the distance.

If you use energy concepts then you would think about the relationship between the work done by the force of friction and the change in kinetic energy.

3. Jul 15, 2012

### danyalasdf

Re: A 1-lb ball A is traveling horizontally at 20 ft/s when it strikes a 10-lb block

no, that i need help to determine the time for the block B to stop sliding.

4. Jul 15, 2012

### TSny

Re: A 1-lb ball A is traveling horizontally at 20 ft/s when it strikes a 10-lb block

Sorry. I misread the question. I would suggest finding the deceleration and using kinematic equations.