A constant friction force of 25N acts on a 6kg skier for 20s. what is the skier's change in velocity?

Question

Plzzzz help me i dont know which formula do i have to use for answer this question.
plzzzz explain me in a simple way and with formula.

Answer 1

Well use Newtons second law. The sum of all Forces = mass times acceleration
F = m*a
In the x direction the only force is friction, and the mass is the skier. The force is negative because it is slowing down the skier.
-25 = 6*a
a = -25/6 = -4.2

Now The change in velocity is the constant acceleration times the time.
(v - v0) = at.
(v -v0) = -4.2 m/s^2 *20 s = -84 m/s

This is a very ridiculous problem. Most skiers are about 10 times as massive, and a speed of 84m/s is equal to 190 miles per hour.