Page 8 - TRIAL 2

Solution
Example 1
According to the Principle of conservation of
energy: If 4 kW of power is dissipated for 30 min, how much
Kinetic energy gained = Potential energy lost energy was involved?
1 2
= ℎ
2 m is the
1 2 Solution
× = 9.81 × 6
2 mass of the 3
= √58.86 coconut 4kW  4 10 W
= 7.6720/ 30min  1800 s
P  change of energy
Power, time taken, t
Example 5 3 E
4 10  1800
Figure shows a man bouncing on a Energy  7.2 6
trampoline. He moves upward at 10 J
-1
A velocity of 10 ms . How high will he
reach?
Example 2
Solution How many kilocalories of food energy do a person
with a metabolic rate of 97.0 W burns per day? (1
Potential energy gained = kinetic energy lost
kilocalories=4184 J).
1 2
ℎ =
2
1
2
9.81 × ℎ = × 10 Solution
2
h= 5.0968 m change of energy
Power, P 
time taken, t

97  E
4.5 Power 3600
Energy  349200 J
1 kilocalori es  4184 J
1. Power is a rate of work done measure in a unit
of time. x  349200 J
4184x 349200
2. Usually work done by two different people x 
doing the same work is equal, despite of body 83.46 kilocalori es
size and ability.
3. For example Ali is young healthy man and Mrs. Example 3
Wong is an old age lady, both carrying a pail of At a velocity of 70 km/h, the force act on a light car
water from first floor to the second floor. Both is 1050 N. What power must the motor generate to
of them are doing the same amount of work maintain this velocity?
done, but to determine who is more powerful,
time have to be taken.
4. Therefore, the least time taken to complete a
work is more powerful.
Work done, W
Power, P =
Time taken, t

Fd
Power, P =
t
Power, P = Fv or

Change of energy
Power, P =
Time taken

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