# Work/Energy Kinetic Problem

1. Dec 12, 2011

### Wa1337

1. The problem statement, all variables and given/known data
A body of mass 8 kg moves in a straight line on a horizontal frictionless surface. At one point in its path its speed is 4 m/s, and after it has traveled 3 m, its speed is 5 m/s in the same direction. Use work-energy relationships to find the force acting on the mass.

2. Relevant equations
PE = mgh
KE = 1/2mv2
ME = PE + KE

3. The attempt at a solution
Have no idea, do I calculate KE at first then at the second spot and get ΔKE or what? How do i get to finding the force with work-energy relationships???

2. Dec 12, 2011

### genericusrnme

okay, you know that work = change in energy
so all you need to do is find what the change in energy is
you know how to calculate the kinetic energy KE=1/2mv^2, and you know the two values of v
so at worst you could be a factor of -1 off

once you have this value, which I'll call W, we can get to work trying to find the force (/bad pun)
you may or may not know (you should though) another way of defining work, that is

$$W=\int_{x_1}^{x_2} F dx$$

and we'll assume it's a constant force since nothing is mentioned about it varying, so solving this integral just becomes

$$W=F(x_2 - x_1)$$

which is force times the distance traveled, which you know
you should be able to work the force out from that

do you understand what I've just sad?

3. Dec 12, 2011

### Wa1337

So Work = Force (Δdistance), where work is KE?