Page 222 - Physics

Physics for Secondary Schools

Task 9.4 Thus,
Compare the rate of doing work by P = 1 hp × 36 watts
two cranes; A and B, when each crane 746 watts
lifts a body of mass 200 kg through a = 0.048 hp.
distance of 20 m. Crane A lifts the body Therefore, Power is 0.048 hp.
in 15 s while crane B requires 20 s to
lift it. Running upstairs is another area where

Assuming the cranes lift the body at a power is required. Running upstairs
constant velocity. increases one’s gravitational potential
(a) Do they do the same work? energy by the same amount as walking
(b) Which crane has more up, Figure 9.23. However, the energy
transferred during running upstairs is at
power?
a higher rate. When you run upstairs, you
dissipate more than when you decide to
Example 9.10 walk up.
A model of a car of mass 20 kg moves
at a speed of 6 m/s for 10 s. Find the
power developed by its engine.

(a) in watts.
(b) in horsepower.

Solution
(a) Data given
Mass, m of the model car = 20 kg
Speed, v of the car = 6 m/s Figure 9.23: Walking upstairs
Time, t taken = 10 s Crossing water bodies is energy-draining
Power, P is given by: because more power is required for an
mv 2 individual or animal to cross from one bank
P = of a river to the other. Figure 9.24 shows
2t an elephant crossing a river.
20 kg × 6 m/s) 2
(
P =
2 ×10s

= 36 W
Therefore, Power is 36 Watts

(b) Converting watts into horsepower
1 hp = 746 watts
? = 36 watts
Figure 9.24: An elephant crossing a river

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Student’s Book Form One

Physics Form 1 Final.indd 216 16/10/2024 20:58