Middle School

A child weighing 25 kg is using a 1 kg skateboard to travel down the hill. If the child has 165 J of kinetic energy after traveling down the hill, what is the child’s speed in meters per second?

Answer :

KE= 0.5mv^2

165J= 0.5×(825kg+1kg)×v^2

165J = 413kg×v^2

v^2=165J÷413kg

v^2=0.39952 m^2/s^2 (5s.f.)

v= √0.39952= 0.632 m/s (3.s.f.)

That poor child weighs 1,818 pounds ! He can't stand up, can't hold himself upright on a skateboard, has probably been banned from every hill in town, and no 1-kg skateboard can hold all of him.

This must be a big part of the reason why his speed down the hill is only 0.632 meter/second . . . about 1.4 miles per hour.

You can WALK at east twice that fast.

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