Help w/ friction problems for physics?

Question

Hi, I am having a really hard time with these problems. The static, kinetic, and coefficient are giving me the hard time. Someone help me plz! Thanks.

1. At a wedding receoption, you notice a small boy who looks like his mass is about 25kg running part way across the dance floor, then slidding on his knees until he stops. IF the kinetic coefficient of friction between the boy's pants and the floor is 0.15, what is the frictional force acting on him as he slides?

2. If you use a horizontal force of 30 N to slide a 12kg wooden crate across the floor at a constant velocity, what is the coefficient of kinetic friction between the crate and the floor?

3. A 225 kg crate is pushed horizontally w/ a force of 710N. If the coefficient of fricition is 0.20, calculate the accleration of the crate.

4. A force of 40.0 N accelerates a 5.0kg block at 6.0 m/s squared along a horizontal surface. How large is the frictional force and what is the coefficient of friction.

Answer 1

If it's moving it is kinetic if not it's static.

1. f=uN
f=umg=0.15 x 25 x 9.81=36.8 N (kinetic friction )

2. again f=umg
so u=f/mg and since it was constant speed f=F we have

u=30/(12 x 9.81)=0.254

3. Ft=F1 - f = F1 - umg
F=ma
so a= Ft/m=(F1 - umg)/m

You can do the rest now.

Have fun