# Momentum and Impulse questions

1. Dec 3, 2014

### sugz

1. The problem statement, all variables and given/known data
The 45-kg boy has taken a running jump from the upper surface and lands on his 5-kg skateboard with a velocity of 5 m/s in the place of the figure as shown. If his impact with the skateboard has a time duration of 0.05 s, determine the total normal force N exerted by the ground on the skateboard wheels during the impact. The speed is 30 degrees below the horizontal.
a. 1.73 kN b. 2.22 kN c. 2.74 kN d. 2.44 kN e.None of these

2. Relevant equations

(Fnet)(delta t) = (delta p) = mvf-mvi, where vi=0
vfx=(-1)*(5sin30)
vfy=5cos30
3. The attempt at a solution
Fx = mvf/(delta t)
Fx = [(50)(5)*cos(30)]/0.05 = 4330.12 N
Fy = mvf/(delta t)
Fy = [(50)(5)*(-1)*sin(30)]/0.05 = -2500

The force equations for the boy on the skateboard:

(Fnet)y=-2500=Fn-mg
=-2500+(50)(9.81)
=-2009.5N

Last edited: Dec 3, 2014
2. Dec 3, 2014

### collinsmark

Does the skateboard move with a vertical component, along with the boy, when the boy is in the air? The problem statement sounds to me as if the boy lands on the skateboard, when the skateboard was already, firmly on the ground.

If only the boy was moving in the air with a vertical component, what is the mass of what was moving vertically? (50 kg or 45 kg?)

[Edit: In other words, make sure you are using the correct mass when calculating the initial momentum components before the impact.]

I think you might have subtracted in there somewhere when you meant to add. [Edit: remember, impulse is equal to the body's change in momentum. You must subtract off the initial momentum component from the final momentum component to find the change in the vertical momentum. I think you got your signs mixed up in there somewhere.]

Last edited: Dec 3, 2014