# Work done - problem set without ans. given

1. Sep 13, 2010

### Hemingway

Hello
I am doing a series of exercises and the answers are not in the back of the book. Can someone see if I have the correct answer for the following problem? Many thanks :)

1. The problem statement, all variables and given/known data
The brakes of a truck cause it to slow down is done by applying a force of 3.0 x 103 N to the truck over a distance of 850m. What is the work done by the force on the truck? Is the work positive or negative - why?

2. Relevant equations
netF=ma
W = Fs
W = F cos (theta) s

3. The attempt at a solution

F = 3.0 x 10^3
s = 850m

use 180 degrees as the direction of force is opposite to the direction of motion. - or is it? (if a decelerating force it's - and the vector is -... but i assumed it's opposite as we're looking at displacement..

W = 3.0 x 10^3 cos 180 x 65
= -3.0 x 10 ^ 6 Joules

I don't think it is correct but have no way of knowing as there is no solution in the text - many thanks
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Sep 13, 2010

### housemartin

Work = Force*distance, you got it right. But I have no idea why after you wrote: "W = F cos (theta) s" you write: "W = 3.0 x 10^3 cos 180 x 65". s = 850m, not 65 m, isn't?

3. Sep 13, 2010

### Hemingway

Sorry I was already working on another problem... sorry that would have been very confusing and random. Thank you or your confirmation :)

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