2.10 work, energy, power and efficiency

Chapter 2 Forces and Motion

2.10 Work, energy, power and efficiency

2.10 Work, energy, power and efficiency

WORK

2.10 Work, energy, power and efficiencyWork

Work is defined as the product of the force, F and the distance travelled, s in the direction of force.

2.10 Work, energy, power and efficiencyWork

Work = F(N) x s(m)

*Unit = Nm or J

2.10 Work, energy, power and efficiencyWork

When the force and the direction of motion of an object are perpendicular to each other, the work done is equal to zero.

2.10 Work, energy, power and efficiencyWork

If the direction of the force acts on an object is different from the direction of the motion, therefore we need to get the component of the force in the direction of motion.

sFW cos Work,

2.10 Work, energy, power and efficiencyWork

2.10 Work, energy, power and efficiencyWork

2.10 Work, energy, power and efficiencyWork

Work done = area under the F vs s graph

2.10 Work, energy, power and efficiency

ENERGY

2.10 Work, energy, power and efficiencyEnergy

• When work is done, energy is consumed.• Energy is the ability to do work.• Mechanical energy is divided into–Kinetic energy –Potential energy

–The unit for energy is J (Joule)

2.10 Work, energy, power and efficiencyKinetic Energy

Kinetic energy is the energy possessed by an object due to its motion.

2.10 Work, energy, power and efficiencyKinetic Energy

* What are the factors that affect kinetic energy?

2.10 Work, energy, power and efficiencyKinetic Energy

Example 1A 0.6 kg trolley moves across the floor at a velocity of 0.5 m/s. What is the kinetic energy of the trolley?

2.10 Work, energy, power and efficiencyPotential Energy

Potential energy of an object is the energy stored in the object because of its position or state.

2.10 Work, energy, power and efficiencyPotential Energy

2.10 Work, energy, power and efficiencyPotential Energy

* What are the factors that affect potential energy?

2.10 Work, energy, power and efficiencyPotential Energy

2.10 Work, energy, power and efficiency

PRINCIPLE OF CONSERVATION OF ENERGY

2.10 Work, energy, power and efficiency

PRINCIPLE OF CONSERVATION OF ENERGY

Principle of conservation of energy states that ENERGY CANNOT BE CREATED OR DESTROYED. It can be transformed from one form to another, but the total energy in a system is constant.

2.10 Work, energy, power and efficiency

PRINCIPLE OF CONSERVATION OF ENERGY

2.10 Work, energy, power and efficiency

Example A durian falls from a height of

6m. What is the velocity of the durian just before it strikes the ground?

2.10 Work, energy, power and efficiency

POWER

2.10 Work, energy, power and efficiency

Power, P is the rate at which work is done or energy is changed or transferred.

*Unit is J/s or Watt (W)tW

P

takentime

DoneWork Power

2.10 Work, energy, power and efficiency

EFFICIENCY

2.10 Work, energy, power and efficiency

Work can be done by machine when input energy is supplied.

If a machine does least work from the supplied energy, it is said to be non-efficient.

EFFICIENCY

2.10 Work, energy, power and efficiency

The efficiency of a machine is defined as

EFFICIENCY

%100inputEnergy

outputenergy UsefulEfficiency

2.10 Work, energy, power and efficiency

If efficiency = 100%, then useful energy output = energy input perfect device

In general, the efficiency for all machines is less than 100% because the work done against friction when operating a machine.

EFFICIENCY

2.10 Work, energy, power and efficiency

EFFICIENCY

ExampleA crane lifts a load of 500 kg to a

height of 120 m in 16 s.If the power input is 45000 W, what is

the efficiency of the motor used in the crane?

[Take g = 10 N/kg]

2.10 Work, energy, power and efficiency

EFFICIENCY

ExampleA crane lifts a load of 500 kg to a

height of 120 m in 16 s.If the power input is 45000 W, what is

the efficiency of the motor used in the crane?

[Take g = 10 N/kg]

2.10 Work, energy, power and efficiency

EFFICIENCY

J 6000002010500

outputenergy Useful

mgh

J 7200001645000TimePowerinputEnergy

%3.83

%100720000600000

%100)useful(

Efficiency

iE

E