Page 224 - Physics

Physics for Secondary Schools

Assume there are n stairs and the height Therefore, the power developed by the
of each stair is h cm. If your mass is m kg man is 0.4 kW.
and runs upstairs in t seconds, then, the
total work done is given by
Exercise 9.3
W total = mg ( )nh
Answer all questions
The power developed
1. A forklift is used to raise a mass
Total work done
by a man = of 400 kg to a height of 2 m in 4
time seconds. Determine the power
nmgh developed.
= ⋅
t 2. A truck for transporting sand is fi lled
to its capacity. The total mass of the
Example 9.12
sand was 5 000 kg. If a digger had to
A man whose mass is 80 kg walks up climb through a height of 2 m 100
a fl ight of 25 steps each 20 cm high in times to fi ll the truck, calculate:
10 seconds. Find the power developed
by the man in kilowatts. (a) the work done in loading the
truck; and
Solution (b) the power developed if the
Given data digger took 5 seconds per climb.
Mass of the man (m) = 80 kg
Number of steps (n) = 25 3. If a crane lifts a load weighing
Height of each step (h) = 20 cm = 0.2 m 3 000 N through a height of 10 m
in 500 seconds, what is the power
Time taken (t) = 10 s of the crane?
nmgh
P = 4. Convert the following
t
(a) 11 936 W into hp.
25 × 80 kg × 10 N/kg × 0.2 m
P = (b) 2.5 hp into W.
10 s
P = 400 watts. 5. A body of mass 50 kg is raised to a
height of 2 m above the ground.
Conversion of watts into kilowatts

1 kW = 1 000 W (a) What is its potential energy?
? = 400 W (b) If the body falls down, fi nd its
kinetic energy:
Thus, (i) when halfway down.
1 kW × 400 W
P = (ii) just before impact with the
1 000 W
ground.
P = 0.4 kW.

218
Student’s Book Form One

Physics Form 1 Final.indd 218 16/10/2024 20:58