Power – problems and solutions
1. A 50-kg person runs up the stairs 10 meters high in 2 minutes. Acceleration due to gravity (g) is 10 m/s2. Determine the power.
Known :
Mass (m) = 50 kg
Height (h) = 10 meters
Acceleration due to gravity (g) = 10 m/s2
Time interval (t) = 2 minute = 2 (60) = 120 seconds
Wanted : Power (P)
Solution :
Formula of power :
P = W / t
P = power, W = work, t = time
Formula of Work :
W = F s = w h = m g h
W = work, F = force, w = weight, d = displacement, h = height, m = mass, g = acceleration due to gravity
W = m g h = (50)(10)(10) = 5000 Joule.
P = W / t = 5000 / 120 = 41.7 Joule/second.
2. Calculate the power required of a 60-kg person climbs a tree 5 meters high in 10 seconds. Acceleration due to gravity is 10 m/s2.
Known :
Mass (m) = 60 kg
Height (h) = 5 meters
Acceleration due to gravity (g) = 10 m/s2
Time interval (t) = 10 seconds
Wanted : Power
Solution :
Work :
W = m g h = (60)(10)(5) = 3000 Joule
Power :
P = W / t = 3000 / 10 = 300 Joule/second.
3. A rotary comedy with power of 300 watts and period 5 minutes rotates 5 rounds. The energy it uses is ….
A. 15 kJ
B. 75 kJ
C. 90 kJ
D. 450 kJ
Known :
Power (P) = 300 Watt = 300 Joule/second
Period (T) = 5 minutes = 5 (60 seconds) = 300 seconds
Number of rotation = 5
Wanted: Energy used by the rotary comedy
Solution :
The correct answer is D.
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