Energy, Work & Power

Work

Lesson Objectives

O Recall and apply the relationship work

done = force x distance moved in the

direction of the force to new situations or

to solve related problems.

Work Done

O Work done by a constant force on an object is the product of the force and the distance moved by the object in the direction of the force.

O WD = F x sF = constant force (in N) s = distance moved by the object in the direction of the force (in m)i.e displacement

O SI Unit: Joule (J)

WD

F s

Scenario 1A file is pushed from left to right with a force of 10 N for 0.5 m.

Direction of

force applied

Is the object

moving?

If yes, what is

the direction?

Is the direction

of force and

movement the

same?

Calculate the

work done

Left to rightYes.

Left to rightYes

𝑊𝐷 = 𝐹 × 𝑠= 10 × 0.5= 5 𝐽

This is work done against friction as

friction is overcome in order for the file to

move.

motion

Ff

Scenario 2A wall is being pushed from the left side with a force of 10 N.

Direction of

force applied

Is the object

moving?

If yes, what is

the direction?

Is the direction

of force and

movement the

same?

Calculate the

work done

Left to right No. No movement

𝑊𝐷 = 𝐹 × 𝑠= 10 × 0= 0 𝐽

There is no movement and therefore no

work done.

F

Scenario 3A file is lifted off the table by a force of 10 N for a distance of 1.2 m.

Direction of

force applied

Is the object

moving?

If yes, what is

the direction?

Is the direction

of force and

movement the

same?

Calculate the

work done

UpwardsYes.

UpwardsYes

𝑊𝐷 = 𝐹 × 𝑠= 10 × 1.2= 12 𝐽

This is work done against gravity as gravitational

force (weight) is overcome in order for the file to

move.m

oti

o

n

F

W

Scenario 4A file is held at a fixed height of 1.2 m by a force

of

10 N.

Direction of

force applied

Is the object

moving?

If yes, what is

the direction?

Is the direction

of force and

movement the

same?

Calculate the

work done

Upwards No. No movement

𝑊𝐷 = 𝐹 × 𝑠= 10 × 0= 0 𝐽

There is no movement and therefore no work done.

F

W

Work Done

O No work is done if:

No applied force

No motion

No motion in the direction of the force

Practice

O Complete the practice questions for work done.

Lesson Objectives

O Show understanding that kinetic energy,

potential energy (chemical, gravitational,

elastic), light energy, thermal energy,

electrical energy and nuclear energy are

examples of different forms of energy.

Lesson Objectives

O State that kinetic energy 𝐸𝑘 =1

2𝑚𝑣2 and

potential energy 𝐸𝑝 = 𝑚𝑔ℎ (for potential

energy changes near the Earth’s surface)

O Apply the relationships for kinetic energy

and potential energy to new situations or

to solve related problems

Energy

O Energy is defined as the capacity to do

work.

O SI Unit: joule (J)

O Energy is a scalar quantity.

Electrical energy

(covered in Ch 16 –

22)

Thermal energy

(heat)

(covered in Ch 9 – 11)

Light energy

(covered in Ch 13 –

14)

Sound energy

(covered in Ch 15)

Nuclear Energy

Types of Energy

Types of Energy

2 main types of energy to be calculated in

this chapter:

O Potential energy (elastic, gravitational and

chemical)

O Kinetic energy

Potential Energy

O Potential Energy is the energy stored in

an object due to its position, state or

shape.

O Elastic potential energy is the energy

stored in the wound up spring or a

stretched rubber band.

Potential Energy

O Chemical potential energy is energy

stored in fuels and food.

Gravitational Potential Energy

O An object placed at a height h from the

ground possesses gravitational potential

energy.

h

ground

Gravitational Potential Energy

O Gravitational potential energy is the

energy which a body possesses because

of its position relative to the ground.

O Formula: 𝑮𝑷𝑬 = 𝒎𝒈𝒉

O m = mass of the object (kg)

g = acceleration due to gravity (10 N/kg)

h = height above the ground (m)

Gravitational Potential EnergyPractice Qn 1

A girl lifts her school bag of mass 3 kg from the floor

onto her lap through a height of 0.5 m. What is the

gravitational potential energy gained by the bag?

𝐆𝐏𝐄 = 𝒎𝒈𝒉= 𝟑 × 𝟏𝟎 × 𝟎. 𝟓= 𝟏𝟓 𝐉

Gravitational Potential EnergyPractice Qn 2

How much gravitational potential energy does a 20 kg

box gain when it is moved from a shelf of 0.3 m high to

a shelf of 1.8 m high?

𝐆𝐏𝐄 = 𝒎𝒈𝒉= 𝟐𝟎 × 𝟏𝟎 × 𝟏. 𝟖 − 𝟎. 𝟑= 𝟑𝟎𝟎 𝐉

Kinetic Energy

O Kinetic Energy is the energy a body possesses due to its motion.

O Everything that moves has kinetic energy.A stationary object has NO kinetic energy.

O Formula: 𝑲𝑬 =𝟏

𝟐𝒎𝒗𝟐

O m = mass of the object (kg)v = velocity of the object (m/s)

Kinetic EnergyPractice Qn 1

When a trolley of mass 5 kg moves with a velocity of

10 m/s, what is its kinetic energy?

K𝐄 =𝟏

𝟐𝒎𝒗𝟐

=𝟏

𝟐× 𝟓 × 𝟏𝟎𝟐

= 𝟐𝟓𝟎 𝐉

Kinetic EnergyPractice Qn 2

The Brazuca ball used in FIFA World Cup 2014 weighs

about 500 g. If a ball that has been kicked has kinetic

energy of 100 J, calculate the speed of the ball.

Did you know?

The fastest football shot

ever recorded has the ball

travelling close to 60 m/s.

K𝐄 =𝟏

𝟐𝒎𝒗𝟐

𝟏𝟎𝟎 =𝟏

𝟐× 𝟎. 𝟓 × 𝒗𝟐

𝒗𝟐 = 𝟏𝟎𝟎 × 𝟐 × 𝟐= 𝟒𝟎𝟎

𝒗 = 𝟒𝟎𝟎= 𝟐𝟎𝐦/𝐬