Physics 11 question - its probs rly easy

[roxxyroxx]

Physics 11 question -- its probs rly easy !

Homework Statement

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?

Homework Equations

p = mv

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??

 

[PhanthomJay]

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?

 

[rl.bhat]

Check the calculation of the final velocity.

 

[roxxyroxx]

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?

how do you find final velocity then?

 

[Ithryndil]

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

 

[rl.bhat]

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

 

[roxxyroxx]

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

 

[Ithryndil]

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