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Question

A constant friction force of 10 N acts on a 60 kg skier for 20 s. What is the skier's change in velocity?

Answer should be m/s.

Answer 1

Using Newton’s second law,
Force = mass * acceleration,
F = ma
We know that a force causes a mass to accelerate.

Acceleration is a change in velocity with respect to time,
a = Δv / Δt

We can solve Newton’s second law for acceleration as,
a = F / m

Now we have two equations for the acceleration of the skier which we can set equal to each other,
F / m = Δv / Δt
And now solving for Δv,
Δv = F * t / m

We are told that the force acting on the skier is 10 Newtons,
F = 10 N
We are told that the time which the force acts upon the person is 20 seconds,
Δt = 20 seconds
And we are told that the skier has a mass of 60 kg,
m = 60 kg

Plugging in what we know into the formula we have derived for the change in the skier’s velocity we get,
Δv = (10 N) * (20 s) / (60 kg)
Δv = (200 N s) / (60 kg)
Δv = 3.33 m/s

So the skier experiences a change in velocity of about 3 m/s.
And, of course, this change in velocity would be in the direction of the frictional force (which is in the direction opposite that of the motion of the object).