SAMPLE LESSONS > O LEVEL WORK, ENERGY, POWER

# POWER

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### POWER

### EFFECIENCY

# 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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