Efficiency

The efficiency of a device is defined as the percentage of the energy input that is transformed into useful energy.

Example
In the example above, the input power is 100J/s, the desire output power (useful energy) is only 75J/s, the remaining power is lost as undisire output. Therefore, the efficiency of this machine is

75/100 x 100% = 75%

Air Conditioner

1. Switch off the air conditioner when not in use.
2. Buy the air conditioner with suitable capacity according to the room size.
3. Close all the doors and windows of the room to avoid the cool air in the room from flowing out.

Refrigerator

1. Always remember to close the door of refrigerator.
2. Open the refrigerator only when necessarily.
3. Always keep the cooling coil clean.
4. Defrost the refrigerator regularly.
5. Choose the refrigerator with capacity suitable for the family size.
6. Refrigerator of large capacity is more efficient compare with refirgerator of small capacity.

Lamp or Light Bulb

1. Use fluorecent bulb rather than incandescent bulb. Fluorescent bulbs are much more efficient than incandescent bulbs.
2. Use a lamp with reflector so that more light is directed towards thr desirable place.

Washing Machine

1. Use front-loading washing machine rather than top-loading wahing machine because it uses less water and electricity.
2. Use washing machine only when you have sufficient clothes to be washed. Try to avoid washing small amount of clothes.

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Power

Power is the rate at which work is done, which means how fast a work is done.

Formula:
Example 1
An electric motor takes 20 s to lift a box of mass 20kg to a height of 1.5 m. Find the amount of work done by the machine and hence find the power of the electric motor.

Answer:
Work done,
W = mgh = (20)(10)(1.5) = 300J

Power,
\[ P = \frac{W}{t} = \frac{{300}}{{20}} = 15J\]

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Work

1. Work done by a constant force is given by the product of the force and the distance moved in the direction of the force.
2. The unit of Nm(Newton metre) or J(Joule).
   
3. Work is a scalar quantity.

Equation of Work

When the direction of force and motion are same, θ = 0^o, therefore cosθ = 1

Work done,

W = F × s

Example 1

A force of 50 N acts on the block at the angle shown in the diagram. The block moves a horizontal distance of 3.0 m. Calculate the work being done by the force.

Answer:

Work done,
W = F × s × cos θ
W = 50 × 3.0 × cos30^o = 129.9J

Example 2

Diagram above shows a 10N force is pulling a metal. The friction between the block and the floor is 5N. If the distance travelled by the metal block is 2m, find

1. the work done by the pulling force
2. the work done by the frictional force

Asnwer:

(a) The force is in the same direction of the motion. Work done by the pulling force,

W = F × s = (10)(2) = 20J

(b) The force is not in the same direction of motion, work done by the frictional force

W = F × s × cos180o= (5)(2)(-1) = -10J

Work Done Against the Force of Gravity

Example 3
Ranjit runs up a staircase of 35 steps. Each steps is 15cm in height. Given that Ranjit's mass is 45kg, find the work done by Ranjit to reach the top of the staircase.

Answer:

In this case, Ranjit does work to overcome the gravity.
Ranjit's mass = 45kg
Vertical height of the motion, h = 35 × 0.15
Gravitational field strength, g = 10 ms-2
Work done, W = ?

W = mgh = (45)(10)(35 × 0.15) = 2362.5J

Finding Work from Force-Displacement Graph
In a Force-Displacement graph, work done is equal to the area in between the graph and the horizontal axis.

Example 4

The graph above shows the force acting on a trolley of 5 kg mass over a distance of 10 m. Find the work done by the force to move the trolley.

Answer:

In a Force-Displacement graph, work done is equal to the area below the graph. Therefore, work done

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