# Physics 11 question - its probs rly easy

1. Dec 9, 2008

### roxxyroxx

Physics 11 question -- its probs rly easy !!

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

A horizontal force applied to a 7.0 kg object initially at rest causes it to accelerate uniformly and travel a distance of 21m in 4.2s. What is the change in momentum of the object?

2. Relevant equations

p = mv

3. The attempt at a solution

Heres what i did but got it wrong:

p=mv
=(7.0kg)(0 m/s)
= 0
p=mv
=(7.0kg)(5m/s)
= 35

so the answer should be 35 ya?
but the answer in the back of the book is 70 kg x m/s
help??

2. Dec 9, 2008

### PhanthomJay

Re: Physics 11 question -- its probs rly easy !!

You mistakenly considered its final velocity to be 5m/s. That is it's average velocity computed over the 21 meter distance in 4.2 seconds. What is its final velocity after 4.2 seconds?

3. Dec 9, 2008

### rl.bhat

Re: Physics 11 question -- its probs rly easy !!

Check the calculation of the final velocity.

4. Dec 9, 2008

### roxxyroxx

Re: Physics 11 question -- its probs rly easy !!

how do you find final velocity then?

5. Dec 9, 2008

### Ithryndil

Re: Physics 11 question -- its probs rly easy !!

You will need to use a kinematic equation to find the final velocity.

6. Dec 9, 2008

### rl.bhat

Re: Physics 11 question -- its probs rly easy !!

Distance = Average velocity * time
Average velocity = (u + v)/2

7. Dec 9, 2008

### roxxyroxx

Re: Physics 11 question -- its probs rly easy !!

so once i have the average velocity how do i find th final velocity?

8. Dec 9, 2008

### Ithryndil

Re: Physics 11 question -- its probs rly easy !!

Think about what the average velocity actually is. It is an average of two velocities together. What are those two velocities?