PN HAZIAH BT AWANG LAH SAS 2007

2.10 UNDERSTANDING WORK, ENERGY, POWER AND EFFICIENCY

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Example : 1. Abu uses a horizontal force of 40 N to push a box. Calculate the work done to push the box a distance

of 180 m on a horizontal surface.

2. Izzul uses a force of 120 N to pull a crate along a straight corridor. The applied force is at an angle of 30° with the horizontal .Calculate the work done by him after pulling the crate a distance of 90 m.

TEACHER’S SIGNATURE : ………………………………….

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WORK

W = Fs

W = Work ( Joule)F = Force (Newton)s = Displacement (metre)

When work done, energy is transferred

ENERGY

Kinetic Energy (Ek)

Gravitational Potential Energy (Ep)

Energy carried by a moving object

Energy of an object due to its higher position in the gravitational field

Ek = ½ mv2

E p = mgh

Common Misconception :Momentum and Kinetic Energy are the same concept.

The Fact :Momentum is mv but Kinetic Energy is ½ mv2

PN HAZIAH BT AWANG LAH SAS 2007

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Fakhrul and rahmat are lifting the barbell from the same height, but Fakhrul is faster than Rahmat to lift up the barbell. Who is more powerful? Explain.

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_______________________________________________________________________________ SOLVING PROBLEM

1. If a mouse and an elephant both run with the same kinetic energy, can you say which is running faster? Explain in terms of the equation for kinetic energy.

2. An angkasawan in full space gear climbs a vertical ladder on the earth. Later the angkasawan makes the same climb on the moon. In which location does the gravitational potential energy of the angkasawan change more? Explain.

TEACHER’S SIGNATURE : ………………………………….

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The Principle of Conservation of Energy

Common MisconceptionEnergy is conserved only under certain condition.

The FactWhen energy changes from one form to another, it always transforms without net loss or gain

Power, P

Formulae

Power = work done Time taken = Energy Time taken

P = W = E (J) t t (s) Unit for power,Js-1 or Watt (W)

Quiz

Do you know?????The three main engines of the space shuttle can develop 33000 MW of power when fuel is burned at the enormous rate of 3400 kg/s. This is like emptying an average –size swimming pool in 20 seconds.

PN HAZIAH BT AWANG LAH SAS 2007

Efficiency of Energy Transformation

List down the importance of maximizing the efficiency of devices.

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SOLVING PROBLEM

1. A Kancil of mass 650 kg is approaching a hill. The hill has an angle of slope of 2.9°. The car moves up the hill to a distance of 200 m with uniform speed. How much work is done in moving the car up the hill? (Ignore friction and air resistance)

2. A trolley of mass 2 kg moving at 4 ms-1 collides with another trolley of the same mass at rest on a friction compensated runway. Both the trolley stick together and move at common velocity after collision. What is total change in kinetic energy resulting from the collision?

3. The turbine of electric power station is situated below the level of a lake as shown in the diagram. How much kinetic energy does 400 kg of water gain as it flow down pipe to a turbine?

4. An electric motor is used to lift a 2.0 kg mass through 3.0 m in 4.0 m as shown in figure below. The total amount of electrical energy used by the motor is 80.0 J. What is the efficiency of the motor?

TEACHER’S SIGNATURE : ………………………………….

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Effiency = Power output x 100% Power input

Turbine

2000 m

300 mPipe

lake

3 m

motor

2.0 kg