Matt jumps backward off a moving skateboard, what's his landing speed?

[guru]

Matt is gliding on his skateboard at 4.00 m/s. He suddenly jumps backward off the skateboard, kicking the skateboard forward at 8.00 m/s. Matt's mass is 50 kg and the skateboard's mass is 7 kg. How fast is Matt going as his feet hit the ground?

This what I did:
P_f = P_i

v_f(50kg + 7kg) = 50kg*4m/s + 7kg*4m/s
v_f = 4.49m/s ?

 

[Moose352]

Hmmm, not sure what you did.
The initial momentum you calculated looks correct: (50+7)*4. But the total final momentum is Vf*50+7*8, where vf is the speed of Matt. Solving for Vf, I get 3.44 m/s. I'm probably wrong though.

 

[Doc Al]

Do not double post!

https://www.physicsforums.com/showthread.php?t=50625