A 18 kg child slides down a 4.0 m-high playground slide. She starts from rest, and her speed at the bottom is 2.8 m/s. What is the change in the thermal energy of the slide and the seat of her pants?

Answer 1

loss of potential energy = m g h = 18*9.8*4
= 705.6 Joules

Kinetic energy at bottom if there were no friction losses also is 705.6 Joules

However the actual kinetic energy = (1/2) m v^2 = 9*2.8^2 = 70.6 Joules
so
705.6-70.6 = 635 Joules goes into heat