*22.  A 40.0-kg boy, riding a 2.50-kg skateboard at a velocity of +5.30 m/s across a level sidewalk, jumps forward to leap over a wall. Just after leaving contact with the board, the boy’s velocity relative to the sidewalk is 6.00 m/s, 9.50° above the horizontal. Ignore any friction between the skateboard and the sidewalk. What is the skateboard’s velocity relative to the sidewalk at this instant? Be sure to include the correct algebraic sign with your answer.

 

Consider the boy and the skateboard to be a single system.  If we ignore the friction between the skateboard and the sidewalk, then there is no external horizontal forces acting and the momentum in the x direction must be conserved.  Note, since the boy is airborne and the skateboard cannot go through the sidewalk,  there must be some force in the y direction and so the y momentum is not conserved in this case.

 

 

 

 

 

 

 

Minus sign indicates that skateboard moving opposite direction as the boy!

 

 

 

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This page last updated on January 11, 2020