Transcript 3-Trans of energy - Power

Transformation of Energy and
Power
Work is done whenever energy is
transformed from one form to another
Eg. Drop a ball and potential energy
is transformed into kinetic energy.
Work is done by gravity.
Conservation of Energy
 Energy is never lost, just changed into new
forms. The TOTAL energy of an system is
constant.
 However, it may appear otherwise due to
loss of energy to heat, sound, light etc.
 Eg. In a car crash, kinetic energy is
transformed into the work in deformation of
the car, heat and sound. No energy is lost.
Conservation of Energy
(efficiency)
 The amount of energy is the same in a
transformation
 NOT all the energy converted is useful
 Efficiency (%) =
Conservation of Energy
 Kinetic Energy + Potential Energy =
Total
Mechanical
Energy
Ek(initial) + Ug (initial) = Ek(final) + Ug (final)
1/2 mu2 + mghinitial = 1/2 mv2 + mghfinal
Example 1
(a) A roller coaster car of mass 120 kg is travelling at an
initial speed of 14 ms-1 at a height of 15 m. It falls to a
height of 3 m. What is its new speed?
(b) It goes back up again and has a speed of 10 ms-1. What
is its new height?
Example 2
(a) How much work is done in compressing a spring in a
pinball machine by 20 cm if the spring constant is 800
Nm-1?
(b) What will be the final speed of a pinball of mass 50 g if
the spring is released?
Power
 A measure of how much work is done every
second (the rate of work)
Power = work done
time taken
P = W/t = E/t
Units: J/s (Joules/second) or W (watts)
Example 3
 What is the power rating of a lift motor that
can raise a 300 kg lift a distance 30 m in 12 s?