Page 221 - Physics

Work, energy and power

Task 9.3 This means that whenever work is done, energy
changes into a different form.
Identify and discuss the energy
changes in: Power is measured in Joules per second (J/s)
(a) Rotating fan. or Watts (W). 1W=1J/s.
(b) Wood splitting.
(c) Throwing up a stone. Mathematically, power may be expressed as :
(d) Burning of charcoal. work done W ( )
(e) Striking a tuning fork. Power P ( ) = time t ( ) ⋅

(f) Eating potatoes. W
(g) Lighting a torch. P = .
(h) Swimming. t
Is the law of conservation of energy Therefore, a watt is defi ned as the power
obeyed? dissipated when one joule of work is done in
one second.
Concept of power Power may also be calculated using alternative

Imagine you are tasked to design formulae as presented in Table 9.3.
an elevator system for a high-rise Table 9.3: Alternative formulae for
building. The elevator needs to lift calculating power.
a certain load, say 1,000 kg to a Power, P In terms of potential energy;
height of 50 m. If the elevator must mgh
complete the lift in 20 s, then you P =
need to calculate the power required t
by the motor to perform this task In terms of kinetic energy;
effi ciently. Power, in this context, mv 2
is the rate at which work is done or P = 2t
energy is transferred. The faster the
elevator needs to lift the load; the In terms of force and velocity;
more power is required. This concept P = Fv
is crucial in engineering because In terms of momentum;
it helps determine the appropriate P = mvg, where mv- momentum.
specifi cations for motors, engines,
and other machinery, ensuring they Note: These alternative formulae may be
can perform tasks within the desired used depending on the given situation.
time frame without overloading or Other units of power are: kilowatts (kW),
wasting energy. horsepower (hp) and Megawatts (MW). Their
equivalence to the Watt is:
Power is the rate at which work is 1 kW = 1 000 W
done. It is a measure of the rate at 1 MW = 1 000 000 W
which energy changes. 1 hp = 746 W

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