variable forces question.

[z1m2v3]

From MathsQuest Exercise 11F 5a) -

A 2 kg mass initially travelling at 2 m/s is brought to rest by a force F = 40/(t+5)^2 N where t is equal to or greater than 0
Calculate the time at which the body comes to rest.

I know you have to use F = ma and transpose to find acceleration, then integrate that to find velocity, then sub in v(0) = 2 to find c, then make v(t) = 0
However the answer i got was t = -5/3 seconds

What have i done wrong? Thanks.

[Martoman]

That force is a RETARDING force, it is negative. If you do that, you get t = 5.