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TITLE:WorTITLE:Work, Energyk, Energyand Powerand PowerISN PAGE ISN PAGE 104104

TITLE:WorTITLE:Work, Energyk, Energyand Powerand PowerISN PAGE ISN PAGE 104104

*FORCE IS A PUSH OR PULL * FORCE IS MEASURED IN NEWTON'S (N)

What is Force?

WorkWorkDefinition:Work done by a constant force on an object is given bythe product of the force and the distance moved by theobject in the direction of the force. (TB)

Work = Force x distance moved

W = F x d

F F

d

NO WORK DONE IF1. the body does not move.

2. the force is perpendicular to the direction of movement.

Work

Work done by force = 0

WorkWork is done by a force.

Example 1: Motion in different directions

Powerful cranes

Power• *Power is the rate of work done or

rate of energy conversion.

• Formula(Write both formulas)P = W t

• The SI unit = watt (W)

• 1 W = 1 J/s

P = E t P = Fv

REVIEW

What unit is Force measured in?

What is measured in watts?

What is the equation for Work?

Principle of Conservation of Energy

• It states that energy cannot be created or destroyed in any process.

• It can be converted from one form to another or transferred from one body to another, but the total amount remains constant.(TB)

Worked Example 4

A bricklayer lifts 50 bricks each weighing 15 N through a vertical height of 1.2 m in 1 min and places them at rest on a wall. Calculate(a) the work done

(b) the average power needed.

Energy• Energy is the capacity to do work.

• The SI unit for energy is joule (J).

• Different forms of Energy

KineticEnergy

HeatEnergy

SoundEnergy

ElectricalEnergy

PotentialEnergy

Gravitational ElasticNuclear

Energy

Chemical

Energy

Energy Transformation

1. Fossil fuel is used to generate electricity in a power station.

Chemical energy in fuel

Heatenergy

Kineticenergy of turbine

Electricalenergy

2. Stored water in a dam is used to generate electricity in a hydroelectric power station.

GravitationalPotential

energy of water

Kineticenergy of water

Kinetic energy of turbine

Electricalenergy

burn Turbine Generator

Gravitational Potential Energy (GPE)

• GPE is the energy possessed by a body due to its height above the ground.

• Formula:G.P.E. = mgh (from conversion of WD to GPE)

• where m= mass of body (in kg) g = gravitational field strength (in N/kg)

• h = vertical height above ground (in m)

Factors affecting GPE

Worked Example 1

A brick of mass 2 kg is at a height of 3 m

above the ground. What is the gravitational

potential energy of the brick?

[Take g as 10 N/kg ]

Kinetic Energy (KE)

• Kinetic energy is the energy possessed by a body due to its motion.

• Formula:

K.E. = ½ m v2

where m = mass (in kg)

v = speed (in m/s)

What is the rearranged version of this equation for calculating speed?

v = 2KE m

Rearranging the KE equation

Worked Example 2

A bullet of mass 200 g is travelling at a speed of 60 m/s. What is the kinetic energy of the bullet?

Use the KE = ½mv2 equation to fill in the kinetic energy values in the table below for two cars each travelling at two different speeds.

What factor – mass or speed – has the greatest effect on the kinetic energy of a moving object?

Dangerous speeding?

KE = 160 kJ KE = 320 kJ

1,000 kg 2,000 kg

20 mph

40 mph

KE = 40 kJ KE = 80 kJ

Doubling the mass of a moving object doubles its kinetic energy, but doubling the speed quadruples its kinetic energy.

If the speed of a car is slightly above the speed limit, its kinetic energy is much greater than it would be at the speed limit. This means that:

Too much kinetic energy

It is more difficult to stop the car and there is more chance of an accident.

GPE and KEWhen a body falls from a height, its GPE is changed into

KE during the fall. GPE

GPE+K.E.

K.E.

As the body gains speed, more and more of its GPE is changed into KE.

Just before the body strikes the ground, all its GPE is changed into KE.

The total amount of energy at the beginning and the end is the same.

Worked Example 3A ball of mass 2 kg is released from rest at the top of a building of height 12 m. (a) What is the speed of the ball when it reaches the

ground?

(b) What happens to its kinetic energy after it has struck the ground?

Worked Example 5Solution(a) By the principle of conservation of energy,

K.E. = GPE ½ m v2 = mgh

v = = 15.5 m/s

(b) The kinetic energy is changed into elastic PE and heat and sound.

2x10x122gh

Note

• Whenever there is friction or impact:

• heat (mostly) and sound are produced.