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Demonstration…

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Energy,Work & Power

Energy

capacity to do WORK

F x D

D = distance moved in the direction of force applied

1 Joule = 1 Newton meter

1 J = 1 Nm

- Which of the following are forms of energy?
- Sound
- Nuclear
- Elastic Potential
- Chemical Potential
- Jolues

Quiz

- In which of the following cases is work done?
- A. A person pushing a wooden box forward.
- B. A person pushing the wall of a building.
- C. An athlete trying to compress a shot put.
- D. Two opposing teams of people
- pulling a tug-of-war rope which
- is stationary.

Quiz

F x D

WORK =

5 m

20kg

- An object of mass 20 kg is pulled up a slope of 15m long as shown in Figure. The frictional force between the object and the slope is 30 N.
- What is the work done by the frictional force?
- What is the total work done in pulling the object up the slope?

Quiz

15 m

Gravitational Potential Energy

Energy a body has due to its position.

Ep = mgh

Ep = GPE (J)

m = mass (kg)

g = gravitational field strength (N/m)

h = height (m)

Kinetic Energy

Energy a body has

Ek = mv2

Ek = KE (J)

m = mass (kg)

v = speed of the body (ms-1)

COE

Conservation of Energy

Energy can neither be created nor 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.

Conservation of Energy

Assume negligible

air resistance

KE = 0

PE = max

KE = 0

PE = max

KE = max

PE = 0

Conservation of Energy

If air resistance

is NOT negligible

http://www.youtube.com/watch?v=CgqBg44azYk

Youtube video

45 45 - 49 12

Conversion of energy

Which one of the following correctly describes the energy conversion that occurs after a bungee jumper jumps from the bridge to the instant when the chord is extended to the maximum?

A. EPE KE GPE

B.GPE KE EPE

C.GPE EPE KE

D.KE GPE EPE

Quiz

EPE = elastic potential energy

KE = kinetic energy

GPE = gravitational potential energy

Homework

Workbook Page 49-52, Worksheet 6A (Skip Question 3)

Due: next lesson

Challenge

Where did all these energy come from?

Give examples of the various form of energy

state the principle of the conservation of energy

Give the formula for calculating KE and GPE?

apply the relationships for KE and GPE to new situations or to solve related problems

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

Recap

3 m

QuizConservation of Energy

An acrobat of mass 70 kg jumps down on to the seesaw and lift his partner upward. (Assume negligible air resistance and frictions Take g=10 N kg-1)(i) Calculate the loss of gravitational potential energy when the acrobat touches the see saw.

(ii) What is the speed of the acrobat just before touching the see saw?

(iii) Given that his partner has a mass of 60 kg, how high would he reach?

QuizWork done

A bullet of mass 50g was travelling at a speed of 200ms-1 before striking a sandbag. It travelled through 20cm of the sandbag before stopping.

What was the total resistive force

produced by the sandbag?

Rate of work done /

Rate of energy conversion

Power

Power

W = watt

s

J

1 W = 1 Js-1

A windmill is used to raise water from a well. The depth of the well is 5 m. The windmill raises 200 kg of water every day.

What is the useful power extracted from the wind?

Quiz

Efficiency

A

Quiz

Homework

Workbook Page 55-58, Worksheet 6B

Due: Next Lesson

Class Test

Chapter 5: Turning Effect of Forces

Chapter 6: Energy, Work & Power