Energy, Work & Power

Unit 6

Learning ObjectivesLearning Objectives

You will learn• the relationship between work and energy

change• kinetic energy = ½mv2 • gravitational potential energy = mgh• conservation of energy

You will learn• the relationship between work and energy

change• kinetic energy = ½mv2 • gravitational potential energy = mgh• conservation of energy

Energy and WorkEnergy and Work

When we do work, we use energy.

If we do more work, we use more energy.

Hence,

When we do work, we use energy.

If we do more work, we use more energy.

Hence,

Work Done (J) = Energy Used (J)Work Done (J) = Energy Used (J)

When one joule of work is done, one joule of energy is used up.

Def: Energy is the capacity to do work.

Qn: Where do we get this energy?

Some examples of energy Some examples of energy

Different forms of energyDifferent forms of energy ExamplesExamples

Potential Energy

Elastic P.E.Strained body: wound up spring of a clock or a stretched elastic band

Gravitational P.E. Object placed at a height

Chemical P.E.Substances that can be burnt such as food we eat, petrol, wood and plants

Kinetic Energy

K.E.Object that is moving: a flying arrow or a moving bus

Energy- the ability to do workEnergy- the ability to do work

Kinetic energy (K.E.) Kinetic energy (K.E.) energyenergy

Kinetic energy (K.E.) is the energy that an object has due to its motion.Kinetic energy (K.E.) is the energy that an object has due to its motion.

FormulaFormula

How does speed affect kinetic energy for an object?The faster the object moves, the greater is its kinetic energy.

How does speed affect kinetic energy for an object?The faster the object moves, the greater is its kinetic energy.

Where K.E. is measured in J m is measured in kg v is measured in m/s

Where K.E. is measured in J m is measured in kg v is measured in m/s

K.E. = ½ mv2K.E. = ½ mv2

Kinetic energy (K.E.) Kinetic energy (K.E.)

energyenergy

A car of mass 1000 kg is moving at a speed of 10m/s. What is its kinetic energy?

If the speed is increased to 20m/s, how many times will the kinetic energy be increased by?

A car of mass 1000 kg is moving at a speed of 10m/s. What is its kinetic energy?

If the speed is increased to 20m/s, how many times will the kinetic energy be increased by?

Kinetic energy = ½ x m x v2 = ½ x 1000 x 10 x 10 = 50 000J

Kinetic energy = ½ x m x v2

= ½ x 1000 x 20 x 20 = 200 000J = 4 (50 000) J = 4 times

Kinetic energy = ½ x m x v2 = ½ x 1000 x 10 x 10 = 50 000J

Kinetic energy = ½ x m x v2

= ½ x 1000 x 20 x 20 = 200 000J = 4 (50 000) J = 4 times

GPE = mghGPE = mgh

h

Gravitational potential energy (G.P.E.)Gravitational potential energy (G.P.E.)

Gravitational potential energy is the energy a body possesses because of its position relative to the ground.Gravitational potential energy is the energy a body possesses because of its position relative to the ground.

How does height affect GPE?The further a body is from the ground, the greater is its gravitational potential energy.

How does height affect GPE?The further a body is from the ground, the greater is its gravitational potential energy.

gravitational potential energy at reference level is defined as zero

energyenergy

Where GPE is measured in J m is measured in kg g is a constant at 10 m/s2

h is measured in m

Where GPE is measured in J m is measured in kg g is a constant at 10 m/s2

h is measured in m

mg

GPE = 0 JReference level

• Note that gravitational potential energy is never measured absolutely.

• It is ALWAYS measured with respect to a certain level

• This level is called reference level

• we only consider the change in vertical height.

Gravitational potential energy (G.P.E.) Gravitational potential energy (G.P.E.)

energyenergy

A car of mass 1000 kg is raised up to a height of 5m by an electromagnet. What is the gain in gravitational potential energy of the car?

Another car of weight 800 kg is raised to a height of 7m. What is the gain in gravitational potential energy of the car?

A car of mass 1000 kg is raised up to a height of 5m by an electromagnet. What is the gain in gravitational potential energy of the car?

Another car of weight 800 kg is raised to a height of 7m. What is the gain in gravitational potential energy of the car?

Gravitational potential energy = m x g x h = 1000 x 10 x 5 = 50 000J

Gravitational potential energy = m x g x h = 800 x 10 x 7 = 56 000J

Gravitational potential energy = m x g x h = 1000 x 10 x 5 = 50 000J

Gravitational potential energy = m x g x h = 800 x 10 x 7 = 56 000J

EnergyEnergy

principle of conservation of energy principle of conservation of energy

The principle of conservation of energy states that energy cannot be created or destroyed, but only changes from one form to another.

Energy Conversion and Conservation Energy Conversion and Conservation

Which means to say energy can convert from one type to another type of energy. E.g. electrical energy from a battery into light energy from a light bulb.

conversion of energy: some examplesconversion of energy: some examples

energy conversion and conservation energy conversion and conservation

conversion of energy: some examples conversion of energy: some examples

energy conversion and conservation energy conversion and conservation

conversion of energy conversion of energy

energy conversion and conservation

energy conversion and conservation

Energy can change from one form into another.

1. The pile hammer is raised to a height. It has gravitational potential energy.

2. Gravitational potential energy of the pile hammer changes into kinetic energy during the fall

3. This kinetic energy is used to drive the pile into the ground

energy conversion and conservation energy conversion and conservation

Ignoring effects of friction and air resistanceIgnoring effects of friction and air resistance

all potential energy (stops for

an instant) all kinetic energy (greatest speed)

all potential energy (stops for an

instant)

principle of conservation of energy principle of conservation of energy

An object of mass 4kg slides down a smooth curved ramp. What is the speed of the parcel when it reaches the bottom?

4 m

By the principle of conservation of energy,

Gain in K.E. = loss in G.P.E

½ mv2 = mgh v2 = 2gh (remove common terms i.e. m and bring ½ over)

= 2 x 10 x 4

v = √160

= 12.6 m/s (3 sig fig)

Is there any assumption that you have made?

We assume that no energy is lost to the surrounding, ie negligible friction and air resistance.

By now you have learnt…By now you have learnt…

• Forms of energy• kinetic energy = ½mv2 • gravitational potential energy = mgh• conservation of energy

• Forms of energy• kinetic energy = ½mv2 • gravitational potential energy = mgh• conservation of energy