# Homework Help: Dynamics 2nd law of newton

1. Aug 23, 2016

### Nanu Nana

1. The problem statement, all variables and given/known data
A skier glides down a slope, and has at the bottom of the slope, at the beginning of a horizontal piece, a velocity of 30 m / s. He begins to brake and after 4 seconds later has a speed of 10 m / s. The friction between the skis and the snow is 400 N during braking.
a. What is the acceleration of the skier during braking?
b) What is the mass of the skier??

2. Relevant equations
F= m x a
vf=af x t + v(i)

3. The attempt at a solution
10m/s= af x 4s + 30m/s
af = -5m/s^2
Thats what i thought answer would be but answer should be +5m/s^2
Why is that ???
And for mass F= m x a
m = F/a
m = 400N / (-5)
m=-80 kg
Maybe I have to exchange value for v (initial ) and v (final )??

2. Aug 23, 2016

### Staff: Mentor

Your answer is perfectly fine. The minus sign means that the skier is slowing down. The magnitude of the acceleration is just 5 m/s^2; that's probably what they wanted for the answer.

Realize that F and a point in the same direction and thus must have the same sign.

If you use a = -5 m/s^2, then you must use F = -400 N. (But you can just make them both positive, since they are in the same direction.)

The main thing is to be consistent with signs.

3. Aug 23, 2016

### Nanu Nana

Now i understand . Thank you very much