22. A 20.0 kg piece of rock start from rest at the top of the first hill (point

Question

22. A 20.0 kg piece of rock start from rest at the top of the first hill (point A ). It slides down the hill and follows the track as illustrated in the drawing. We will assume that there is no friction for part (a) and (b). (a) Using the conservation of energy concepts, find the kinetic energy of the rock at the bottom of the first hill (point B). (b) What is the rock?s speed at point C? (c) A second, identical rock slides down the hill, but by now the track has become rough, resulting in a kinetic frictional force exerted on the rock. As a result, the kinetic energy of resulting in a kinetic frictional force exerted on the rock. As a result, the kinetic energy of the rock at point B is only half the value obtained in part(a). Find the work done by the kinetic frictional force when the rock reaches point B .

Explanation / Answer

a)

at A, energy=potential energy=20*9.8*9=1764 J

so as at B height=0

the total energy is completely kinetic eenrgy

hence kinetic energy=1764 J

b)if the speed is v m/s,
then 0.5*22*v^2=1764

v=9.1652 m/s

c)at C, potential energy=5*22*9.8=1078 J

at B kinetic eenrgy=0.5*1764=882 J

so work done against friction=1078-882=196 J

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