A 45 kg student runs down the sidewalk at a speed of 4.20m/s and jumps onto the stationary skateboard. The student and skateboard then move down the sidewalk at 3.95 m/s. What type of collision is this? What is the mass of the skateboard?

Answer 1

conservation of momentum
45*4.2+0=(45+M)3.95
solve for Mass M.
Now to see if it was an elastic collision (it wont be when the masses are joined in a collision), do this.
does final energy=initial energy ?

1/2 45 4.2^2=? 1/2 (45+M)3.95^2
check that.

Answer 2

2s