Sc(phys) chapter 7 work, energy and power

Before we move on,

We have…

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Pupils should be able to

define work done as work done = force distance moved in the direction of the force.

apply the relationship between work done, force and distance moved in the direction of the force to new situations or to solve related problems.

Lesson objectives

Work, Energy & Power

Lesson Trigger

Class, have you done your

work?

I have done my work, Teacher.

Work is a force related quantity.

Applying a force to strike the tennis ball – you are doing work on the ball.

Applying a force to lift your body weight up the stairs – you are doing work against gravity.

Pulling you down during a dive – the Earth is doing work on you.

Applying a force to drag the bag – you are doing work against friction.

A force MUST be applied on an object in order for work to be done.

(a) you push against the wall

(b) a man carries a bag of gold on his hand and SLIDES across the room.

100 N

No work is done when…

A force is applied but the object does not move.

A force is applied, but the object does not move in the direction of the applied force.

The SI unit for work is the joule (J).

To a scientist, work WW is done whenever a force FF makes an object move a certain distance DD in the direction of the force.

WWork = FForce DDistance in the direction of applied force

1 J = 1 N mN m

The greater the force, and the further it moves, the more work is done.

Defining Work

1 joule is defined as the amount of work done when a force of 1 N moves an object 1 m in the direction applied force.

(a) Work done against friction

Work is done against friction when a force is applied to move an object in contact with a surface over a certain distance.

Need to apply a force = 28 N to overcome friction.

(b) Work done against gravity

Work is done against gravity when a force is applied to lift an object to greater height in a gravitational field.

Nature of Work Done

1.5 m200 N

Need to apply a force = weight of the load to overcome gravitational pull.

A piece of log is dragged 1.5 m along a slope with a pulling force of 1 600 N. Friction between the log and the slope surface is 1 200 N.

(a) Calculate the work done against friction.

(b) State and explain whether the log will move up the slope with a uniform speed.

W friction = Friction distance= 1 200 1.5= 1 800 J

The log should accelerate up the slope, as there is a net force acting on it.

Fnet = 1 600 – 1 200 = 400 N

Sample Calculation 1

Mr Tan of mass 95 kg is running up to the 5th floor of his HDB flat, 12.0 m away from the ground floor. How much work does he do against gravity?

Mr Tan is moving up against gravity. He has to apply a force = his body weight to overcome the pull of gravity.

WD gravity = Force distance

= weight distance

= 950 12.0

= 11 400 J

Sample Calculation 2

Pupils should be able to

understand that kinetic energy, elastic potential energy, gravitational potential energy; chemical potential energy and thermal energy are different forms of energy.

state and apply the principle of the conservation of energy.

state that kinetic energy is Ek = ½ mv2 and gravitational potential energy Ep = mgh (for potential energy changes near the Earth’s surface).

apply the relationship for kinetic and potential energy to new situations or to solve related problems.

Lesson objectives

Work, Energy & Power

Think about and write down what you know about any kind of energy. Write down whatever that comes to your mind when you think about the term energy.

Energy

Kinetic energy – energy due to motion, – all moving objects have it.

Potential energy

Electrical energy

Nuclear energy

Sound energy

Heat energy

Mechanical energy

internal energy

Energy is defined as the ability to do work. You make use of different types of energy to help you do work in your everyday life.

Defining Energy

The SI unit of energy is joule, symbol J.

Potential energy is defined as stored up energy, waiting to be used. When released, it is capable of doing work.

Potential energy

1 Elastic potential energy

2 Chemical potential energy

3 Gravitational potential energy

gravitational potential energy

elastic potential energy

chemical potential energy

When an object stores energy as the result of its position in a gravitational force field, the object is said to possess Gravitational Potential energy, PE gravity

The ram of a pile driver possesses gravitational potential energy.

It is capable of doing work on the wooden pole.

When released, it applies a force F to move the wooden pole a distance h into the ground.

Example

ram

wooden pole

hWork done = F D = mgh

= mgF

ram

wooden pole

F = mg

h

PE gravity = mgh

Hence gravitational potential energy PE gravity is given as:

mass in kg

gravitational field strength g = 10 N/kg

height in m

The SI unit of PE gravity is joule, symbol J.

In a rescue operation, a 75 kg rescuer is raised to a height 12 m above ground. Calculate his gain in Gravitational PE.

PE = mghQuick Practice (3 min)

TB pg 121 Q3= 75 10

12 = 9 000 J

A body in motion is capable of doing work. It is said to possess Kinetic Energy, KE.

A swinging mallet has kinetic energy, When it strikes the ball with a force F, it causes the ball to move a certain distance D in the direction of the force, producing work.

W = F D

The kinetic energy KE of a body is given as:

KE = ½ mv2

The greater the speed v of a moving body, the greater is its kinetic energy. The greater the mass m of the moving body, the greater is its

kinetic energy.

J kg

m/s

We are equally fast, but I have greater KE than you

I am faster. I have greater KE than you

(i) Ali jogs at a uniform speed of 7.5 m/s. Calculate his kineticenergy if his mass is 65 kg . (ii) How will his KE change if he slows down his motion?

As KE is depending on speed of motion, if he slows down, his KE will decrease.

KE = ½ mv2 = ½ 65

7.52V = 1 828.125 J 1.8 103 J

Quick Practice (3 min)

TB pg 121 Q2

The energy of a body is always converted from one form to another during work done.

Work is done against gravity when a car is driven up a slope.

Example

Chemical Ep (petrol) ➔ Gravitational Ep (car gains

height)

If the car accelerates up the hill, then:

Chemical Ep (petrol) ➔ Gravitational Ep (car gains height) + Ek (car

speeds up)

More petrol will be burnt to release more Chemical Ep to accelerate the car.

(a) A car is brought to rest when the driver applies brake.

(i) What kind of work is done?(ii) State the energy change.

Work is done against friction when the car is braked.

Quick Check

Ek moving car Heat Energy + Sound Energy➔

Energy cannot be created nor destroyed. It can be converted from one form into another, during which time, work is always done.

In any closed system, the total amount of energy remains constant before and after work done, regardless of any process which takes place.

(a) Mr. Tan lifts a 200 kg weight to a point 1.8 m above ground. Calculate the gain in gravitational Ep at its greatest height.

(b) Mr Tan releases his grips and the weight drops vertically down. (i) What is the kinetic energy just before it strikes the ground ? (ii) Determine the maximum speed of the weight.

Ep = mgh=

200 10 1.8= 3 600 J

1.8 m

Ek = Ep

= 3 600 J

Ek = ½ mv2 3 600 = ½ 200

v2

v = 6 m/s

(c) (i) State the assumption you are making in the calculation in (b). (ii) what is the significant of this assuption?

Ep = mgh= 200

10 1.8 = 3 600 J

1.8 m

Ek = Ep = 3

600 JWe assume that air resistance is negligible, no work is done to overcome air resistance. When air resistance is negligible:

Ep lost = Ek gain .

Otherwise,

Ep lost = Ek gain + work done against air resistance.

Quick Practice (3 min)

TB pg 123 Q8

Mechanical energy is energy of motion (Ek) or of potential for motion (Ep) on a macroscopic scale (a system).

A catapult flying through air has Ek and Ep. Is there a special name for such a body possessing two such energies simultaneously?

http://www.youtube.com/watch?v=HNkqy-qsheY

Pupils should be able to

Recall the relationship power = work done time taken.

apply the relationship between power, work done and time taken to new situations or to solve related problems.

Lesson objectives

Work, Energy & Power

http://youtube.com/watch?v=xzKrSTx4Imo http://youtube.com/watch?v=X666_Y7C_tg

How do we measure power?

SI unit - joule per second (J/s)

Power is a force related quantity. It measures the rate of work done or energy conversion.

Power =work done

timeEnergy converted

time=

J

s

Another unit, watt (W) is also used.

1 W = 1 J/s

Mr Tan of mass 95 kg is running up to the 5th floor of his HDB flat, 12 m away from the ground floor. If he is able to reach the 5th floor within 35 s, What is the power developed by him?

Mr Tan is moving up against gravity. He has to apply a force of at least equal to his body weight to overcome the pull of gravity.

His work done = Force distance

= 950 12

= 11 400 J

His power = work done time

= 11 400 35

= 325 W

Sample Calculation 6

Summary

Understand the examples of different forms of energy.

By the end of this lesson pupils are able to:

State the principle of the conservation of energy.

Solve problems using the principle of the conservation of energy.

State that kinetic energy is Ek = ½ mv2 and gravitational potential energy Ep = mgh.

Solve problems using the relationships for kinetic energy and potential energy.

Solve problems using the relationship work done = force distance moved in the direction of the force.

Solve problems using the relationship power = work done time taken.