A 2.5kg stone is moving 3.5m/s(W). What force would be needed to stop the stone in time of 3.5x10^-4s?

Question

The answer is 2.5 x 10^4N (E) but, I keep getting the answer wrong.I find the distance by multiplying velocity and time and that give mes 0.001225m.

Answer 1

F*t=0.5*m*v^2

F=4.375x 10^4N

Answer 2

Using F = momentum / time

Rearranging, F X t = m X v

F X (3.5x10^-4) = 2.5 X 3.5

F = 25 000 N

Since the force is used to stop the stone travelling West, it must be acting against the stone in the opposite direction, ie to the East.