# Homework Help: Dynamics, Newton's laws, Problem with free body diagram and work

1. Apr 2, 2013

### akmenio

1. The problem statement, all variables and given/known data
A boy skateboards down a curved playground ramp. If we treat the boy and his skateboard as a
particle of total mass m = 25:0 kg, he moves through a quarter-circle with radius R = 3:0 m. He
starts from rest at point A and there is no friction.
(a) Find his speed at the bottom of the ramp
(b) Draw free-body diagrams for points A, B, C and D.
(c) Find the normal force that acts on him at the bottom of the curve
(d) Find work done by the normal force
2. Relevant equations
E=mgh
E=(mv^2)/2
3. The attempt at a solution
I find answer for a, I use that change of potential energy is equal to kinetic. Also I done c because the normal force is equal change of potential energy. But i haven't got idea how to draw free-body diagrams and whywork done by normal force is equal 0 (Our teacher give us this answer)

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2. Apr 2, 2013

### Staff: Mentor

Good.

Not sure what you mean.

Just label the forces acting on the particle, indicating their direction.

At any point along the path, how does the direction of the normal force compare to the direction the particle is moving?