Answer:
(a)
As shown in figure above, simple pendulum consists of a
spherical metal bob suspended by a thread from a fixed support.
When the bob is displaced to end A, it gains potential
energy (= mgh) here KE is zero. As it is released from rest, its potential
energy begins to decrease and kinetic energy begins to increase. At an
intermediate position, say point Q, the energy is partly kinetic and partly
potential. At mean position, say O, the energy is totally kinetic, here PE is
zero. At the other end B, again the energy becomes totally potential,
here KE is zero.
Therefore, at all points, 'total mechanical energy remains
constant which is equal to mgh.
This is the law of conservation of energy. After sometime,
the energy of the bob gradually decreases as it is used in doing work against
the force of friction at the point of suspension and also against fricition of
air. This spent energy is converted into heat. Due to decrease in the energy of
the bob, its amplitude of oscillation decrease with time and eventually the bob
comes to rest.
(b) When saw is used to cut a log of wood, work is done against
the force of friction between the saw and log, therefore, the work done
(mechanical energy) is converted into heat energy. So, it becomes warm.
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