Page 223 - Physics

Work, energy and power

Example 9.11 Acti vity 9.3

How much power is required for a car of mass Aim: To determine
1 000 kg to change speed from 10 m/s to 40 m/s the power
developed by
in 8 s?
your legs.
Solution Materials: A fl ight
The work done on the car increases its kinetic of stairs,
energy. weighing scale,
Work done = ∆KE stopwatch, and
1 1
= mv 2 − mv tape measure
2
2 f 2 i
2 1 Procedure
(
(
= 1 ×1 000 kg × 40 m / s) − ×1 000 kg × 10 m / s) 2 1. Measure your weight in
2 2 Newtons and note it down.
=750 000 J 2. Measure the vertical height
of the stairs as in Figure 9.25.
The energy used by the car is 750 000 J.
The power required is given by:

change in energy
P !
time
750 000 J
= = 93 750 W.
8 s

Car engines are often rated in horsepower (hp). Figure 9.25
Thus, the engine must produce a power;
3. Record the height in
1hp
P = 93 750W × metres.
746W 4. Using a stopwatch, record
= 125.67 hp. the time taken to run from
the bottom to the top of the
Since work causes a change in energy, power can stairs.
also be considered as the rate of change of energy. 5. Other individuals can
change in energy repeat step 4.
P =
t Questions
For example, a light bulb converts electrical (a) Calculate the amount of
energy into light and heat. A 60-Watt light bulb work done.
converts 60 J of electrical energy each second (b) Calculate the power
dissipated in running
from electrical to light and heat energy.
upstairs.

Most appliances have a rating plate on them showing (c) Find out who developed
exactly how many watts they use. the highest power.

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Physics Form 1 Final.indd 217 16/10/2024 20:58