# Homework Help: Potential Energy And Force

1. Sep 26, 2010

### zorro

1. The problem statement, all variables and given/known data
A particle of mass 5 kg in the x-y plane has its potential energy given by U=(-7x + 24y ) J where x and y are in metre. The particle is initially at origin and has a velocity u=(14.4i + 4.2j)m/s. Find the magnitude of force on the particle and speed of the particle at t=4s.

2. Relevant equations

3. The attempt at a solution

dU/dx=Fx
dU/dy=Fy
Therefore, Fx= -7N and Fy= 24N
Fnet = 25N

magnitude of u=15m/s
magnitude of acceleration=25/5=5m/s^2
Hence v=15+5x4=35m/s
But the answer is 25m/s

2. Sep 27, 2010

### Staff: Mentor

Careful. You're missing a minus sign:
Fx = -dU/dx
Fy = -dU/dy

That doesn't affect this answer but will the next.

The velocity and acceleration are not in the same direction. Analyze each velocity component separately, then combine to find the speed.

3. Sep 27, 2010

### zorro

Thanks alot!

4. Sep 27, 2010

### fizzynoob

Wouldn't -dU/dx give you the x component of the electric field? I think you need to multiply by a factor of charge to find the actual electric force. And dont forget that -dU/dx is actually a partial derivative.

5. Sep 27, 2010

### Staff: Mentor

Who said anything about electric fields here? U is the potential energy function, not electric potential.

6. Sep 27, 2010

### fizzynoob

Ha yeah your right, got my U and V mixed up for some reason.