# Homework Help: Total power developed and kinetic energy

1. Dec 20, 2011

1. The problem statement, all variables and given/known data
A horizontal force of 2000N is applied to a vehicle of mass 400kg which is initially at rest on a horizontal surface. If the total force opposing motion is constant at 800N, calculate
(i)the acceleration of vehicle
(ii)the kinetic energy 5s after force is applied
(iii)the total power developed after 5s the force is applied

2. Relevant equations
F=ma
P=W/t
1/2(m)(v)^2
P=Fv
v=u+at

3. The attempt at a solution
(i)2000-800=400a
a=3ms^-2
(ii)v=(3)(5)
=15m/s
K.E=0.5(400)(15)^2
=45000W
(iii) This part Im not sure of...i tried adding 2 powers together:
P=Fv
=(1200)(15)=18000
18000+(45000/5)=27000 but its not the right awnser

2. Dec 20, 2011

### LawrenceC

Power is force times velocity.

3. Dec 20, 2011

### Serac

You have the acceleration, and you have a time. The object is initially at rest, so it's velocity is: 5a.

From this velocity, you must multiply it by the net force on the object.