SAMPLE LESSONS > O LEVEL WORK, ENERGY, POWER

POWER

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POWER

EFFECIENCY

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WORKED EXAMPLES

EASY

A student of mass 60 kg takes 10 s to climb a flight of stairs that has 20 steps. The height of each step is 15 cm, calculate the power of the student.

A formula can be derived from the following three formulae:
P = Work / t … (1)
Work = Fd … (2)
v = d/t … (3)
Substitute (2) into (1),
P = (Fd) / t
P = F (d/t)
Substitute (3), P = Fv

Answer

MEDIUM

A car has a mass of 2000 kg. It accelerates from rest to 3 m/s in 12 s. The frictional force on the road is 300 N.
a) Calculate the force exerted by the car engine.
b) What is the total work done by the engine during the acceleration of the car?
c) Calculate the power of the car engine.
Answer

HARD

The diagram below shows a load of mass 800 kg being pulled by a truck through a rope
When the truck moves at a constant speed of 4m/s horizontally, the tension in the rope is 3200 N.
a) Use a free body diagram to show all the forces acting on the load. Label the forces.
b) When the load accelerates uniformly, the tension of the rope increases to 4000 N. What is the acceleration of the load?
c) Hence, using your result in (b), determine the speed of the load after 40 s.
d) What is the work done in accelerating the load for 40 s?
e) When the driver applies the emergency brakes of the truck, the load collides with the truck. Explain why it is so.
Answer

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