Work

Work,WWork,W is defined as the product of the applied force,Fapplied force,F and displacement of an object in the displacement of an object in the direction of the applied force,s.direction of the applied force,s.

w = Fs• S.I unit - Joule

Can you define 1 Joule?

• 1 Joule1 Joule is the work done when a force of 1N1N moves an object through a distance of 1 m1 m in the direction of force.

• Sometime the object does not move in the direction of the applied force.

• Since the object is displaced horizontally, the horizontal component of the force is used to calculate the work done by the force.

Work done, = ( F cos Work done, = ( F cos ) x s

F

F

F Kos

s

Question 1

1. A gardener push a lawn-mover along a horizontal surface with a force of 40 N through a distance 10 m. What is the work done in pushing the lawn mover?

Questions

• How much work is done if a force of 12N moves an object a distance of 5m?

• If you use a 40N force to lift a bag and do 20J of work, how far do you lift it?

• A 60 kg of boy pushed a wall with 200N force. How much work has he done?

Work is not always done by a applied force!

• A student carrying his bag while waiting at the bus stop.

Since there is no motion in the direction of the force applied by the student, there will be no displacement in the direction of a force. No work is done by the student on his bag.

Work is not always done by a applied

• A waiter is carrying a tray of food and walking

The applied force act vertically upwards but the displacement of the tray is in a horizontal direction. Since there is no displacement in the direction of the applied force, no work is done on the tray of food.

Work is not always done by a applied

• A spaceship is moving through space with its engine switched off.

There is no work is done if an object moves with uniform velocity without the action of any force. There is no force acting on the spaceship in the forward or backward direction. Therefore, no work is done on the spaceship.

Energy transfer when work is done.

• Energy – capacity to do work.• An object that do work has energy.• Energy exist in different forms.• Can you name the different forms of energy?

Kinetic energy

Gravitational potential energy

Elastic potential energy

Sound energy

Heat energy

Chemical energy

Light energy

Electrical energy

Forms of energy

The work done and the Change in Kinetic Energy

• Kinetic energy is the energy of an object due to its motion.

Ek = ½ mv2

m = mass of an object

v = velocity

Work done and Gravitational Potential Energy

• Gravitational potential Energy is the energy of an object due to its higher position in the gravitational field.

Ep = mghm = mass

g = gravitational field strength

h = height

Principle of Conservation of energy.

• Energy can be changedcan be changed from one from to another, but it cannotcannot be created or destroyed.

• The total energy in the universe is constantconstant.

• When the baby falls, the gravitational potential energy changes to kinetic energy.

mgh = ½ mvmgh = ½ mv22

Power• When a force is applied on an object and there is motion in

the direction of the force, work is said to be done by the force.

• Power is defined as the rate at which work is done, or the amount of work done per secondamount of work done per second.

Power = Power = Work doneWork done Time takenTime taken

P = P = WW tt

• SI unit – Watt (w)SI unit – Watt (w)

Can you define 1 Watt?

1 Watt1 Watt is a power generated when 1 Joule1 Joule of work is done in 1 second1 second.

Question 1

• In the snatch event of a weightlifting competition, a weightlifter lifts 140 kg from the floor to a height of 1.2 m above the floor in one complete movement in a time of 0.8 s. What is the power generated by the weightlifter during this time?

• 2100 W

Question 2.• A 50 kg farmer climbed a 8 m tall coconut tree in 5 minute.

How much of power has he generated?Work = F x s

= 50kg ( 10 ms-1) x 8m = 4000 J

Power = Work done Time taken= 4000 J (5 x 60)s= 13.33 Watt

Question 2• The figure shows an electric motor lifting a box of mass 5 kg. The motor takes 4 s to

lift the box to a height of 0.8 m. What is the power of motor? [Assume g=10 ms-2].

• Ans 10W

0.8 m

load

motor

Efficiency

• The engine of a vehicle is unable to change all the chemical energy in the petrol to become the kinetic energy of the vehicle.

• Other forms of energy such as thermal energythermal energy and sound energysound energy are also obtained from the operation of the engine.

Kinetic energy

of the car

Thermal and sound energy lost to the surrounding

Chemical energy in petrol

• Energy input, Einput= chemical energy in petrol• Unwanted energy = thermal and sound energy• Useful energy, Eoutput = kinetic energy

Efficiency = Efficiency = Useful energy outputUseful energy output x 100% x 100% Energy inputEnergy input

Kinetic energy

of the car

Thermal and sound energy lost to the surrounding

Chemical energy in petrol

• Calculate the efficiency of the car= Useful energy outputUseful energy output x 100% x 100%

Energy inputEnergy input= 350 x 100% 1050

= 33.33%

350 J of kinetic energy

700 J of thermal and sound energy

1500 J of chemical energy in petrol

Question 1

• An electric motor of a crane can lift a 120 kg weight to a height of 4m in 8s. During this time the motor is supplied with 12kJ of electrical energy. Calculate a) the useful power output of the motorb) the efficiency of the motor.

• a) 588 Wb) 39.2 %

The importance of Maximizing the Efficiency of Devices.

• Consider two light bulbs: a bulb with a filament and a bulb with a fluorescent tube:

Heat energy =42 J

Light energy =18 J

Input energy = 60 J

Heat energy =24 J

Light energy =36 J

Input energy = 60 J

• The fluorescent bulb converts a higher percentage of the input energy to the useful form of energy. It is said to have a higher efficiency than the filament bulb.

Heat energy =42 J

Light energy =18 J

Input energy = 60 J

Heat energy =24 J

Light energy =36 J

Input energy = 60 J

When a device is operating at the maximum possible efficiency:

• Less input is required to produce the same useful output energy.

• The cost of operating the device is reduced• The unwanted output energy is reduced.• The energy resources in this world can be

used over a longer period of time.

Question• Figure shows an incomplete Sankey diagram for an electric

motor that lifts a load.

a) Complete figure aboveb) Calculate the efficiency of the motor.

• Explain why the efficiency of devices is always lower than 100%

• Give two reason why the efficiency of machines should be maximised.

Input energy: 16 kJ electrical energy

Useful output energy: 12 kJ of work done to lift a load.

Energy lost to:

(i) ___________

Energy lost as:

(ii) __________