Acceleration and force

Question

A 5 kg mass object is moving with an acceleration of 2 m/s^2. After 4 s from the start of the observation, its measured velocity was 12 m/s. What is the force acting on the body? (Answer 10 N)

I don't really understand the problem, it seems obvious at first using F=ma=5*2=10 N, but using formula for acceleration a=v-v0/t=12-0/4 = 3 m/s^2, so F=ma=5*3=15 N. What am I doing wrong?

Answer 1

It's sort of a trick problem. You are told the acceleration but NOT the initial velocity. You are given the final velocity and the time that went by. If you wanted to, you can calculate the initial velocity using:

but that isn't the question.

Since the question is about the force, you just multiply mass and acceleration