# Homework Help: Moving block Work Problem

1. Oct 14, 2009

### ddn87

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

a 3kg block moves initially at 4 m/s at t=0 and slides on a horizontal surface... Find the magnitude of word that must be done on the block to bring it to rest

2. Relevant equations
I believe we must use the equation w=Fs

3. The attempt at a solution

I drew the free body diagram and figured that the we have to find the opposing force and the relative displacement. im having a hard time getting started with this problem. i know that [F]=[N][m] and =[m]

2. Oct 14, 2009

### rl.bhat

Work done is equal to the change in kinetic energy.
What is the initial KE?
What is the final KE?

3. Oct 14, 2009

### ddn87

okay so i figured that the change in KE=.5 x m x (vf^2-vi^2)
for this problem it would equal 24J

4. Oct 14, 2009

### rl.bhat

Yes. It is correct.

5. Oct 14, 2009

### ddn87

There is an extention to the problem:

A constant friction force of 2 Newtons is exerted on the block by the surface find the magnitude of acceleration of the block

So I used F=ma where the force i used was 2 and the mass was 3 so id id

2/3 to find the acceleration which is .66666 is this the right method to answering this question?

6. Oct 14, 2009

### ddn87

nm i got the right answer thankyou!