Work done and power

WORK DONE AND POWER

At the end of this lesson:You should be able to:1. understand the concept of work in terms

of the product of force and displacement in the direction of the force.

2. define the joule.3. recall and use the equation W = Fs, where

F is a constant force along the direction of motion.

4. relate power to work done and time taken.5. define the watt.6. recall and use the equation W = Pt.

Work done – Intro (5 min)

‘ I am working hard to get a good result in my exam’ , The word work in the text does not have the same meaning of work in science.

Work in science refer to moving an object with force in a specific distance. For example ; pushing a 100N box in a distance of 2 metre.

Grouping – Conditions for work done Please divide yourself into the

following groups:

Group NamesRED Zahrina, Aziz, Erina, Irene &

HazrinYELLOW Liyana, Ris, Mas,

Dedie,Syahidah & AzizahBLUE Liza, Yuri, Thuraiya, Hirfian &

DayatGREEN Alice, Dayana, Amal, Eza,

Zatul & Jasmine

Video – Work or No Work?

ACTIVITY 1: 10 minutes

Look at the pictures given to your group, discuss with your group and state whether: WORK is DONE or WORK is NOT DONE

Tell the class the REASON(s) for your answer.

PICTURE 1

http://www.bt.com.bn/files/images/photos/2011-08-08/0708ps8.jpg

PICTURE 2

http://disney-stationary.com/coloring-book/birthday/Mickey-Mouse-presents-color.jpg

PICTURE 3

http://img10.glitterfy.com/graphics/105/elmo_reading.gif

PICTURE 4

http://www.rehovotisrael.com/img/park.jpg

PICTURE 5

http://farm4.static.flickr.com/3028/2645197100_e4a6812024.jpg

PICTURE 6

http://www.himho.com/wp-content/uploads/2010/12/Facebook-Cartoon-Profile.gif

PICTURE 7

http://files.whereiscat.com/uploaded_images/garfield-householdchores-742289.jpg

PICTURE 8

http://thumbs.dreamstime.com/thumblarge_293/12169882661ANq6N.jpg

PICTURE 9

http://smartpoodlepublishing.com/blog/wp-content/uploads/2009/06/up-disney-movie.jpg

PICTURE 10

http://1.bp.blogspot.com/-cf1tOnVfwa0/TdlVnNeFeOI/AAAAAAAABLc/MzUQWtyNlAg/s1600/the-big-bang-theory-102-1.jpg

PICTURE 11

http://images2.wikia.nocookie.net/__cb20110911222919/smurfs/images/3/30/Hefty_Comics.jpg

PICTURE 12

http://blog.thetravel.us/wp-content/plugins/wp-o-matic/cache/fcfdb_parasailing-or-jetskiing-at-Empire-Hotel-Country-Club-in-Jerudong-500x333.jpg

PICTURE 13 (Bonus)

http://www.cinema-suicide.com/wp-content/uploads/2011/01/calvin-hobbes.gif

Conditions for work done

There must be a force acting on the object.

The object must move. The effective part of the force is in

the direction of the movement.

Examples

Work Work is defined as: The product of force and the distance moved in the direction of the force. Work = Force × distance moved in the direction of the force.

• Units are newton metres (Nm) or joules

(J).

• Work is actually a scalar quantity despite

being the product of a vector quantity.

• When work is done, there must be

movement.

FsW W = Fs

Video - Work Done (1min 54s)

Example 1  These kids are trying to bump-start this car, but they cannot get it to move. Sweat is pouring off them. Explain why they have done no work.

The car has not moved.  Therefore the distance moved in the direction of the force is zero. Hence, they have done no work. 

Unless you enjoy trouble, don't mention that to them.          

Example 2  Ryan helps Maylenie to lift a parcel weighing 7N onto a shelf 2m high. How much work has she done?W = FsW = 7N x 2mW= 14Nm or 14 J

Example 3  Steven climbs 6 stairs vertically, walking up to his flat. If each step is 15cm high, how much work does he do? Assuming that he weighs 800N. W = FsW = 800N x (6 x 0.15m)W = 800N x 0.90mW= 720Nm or 720 J

EnergyEnergy and work are very closely related. Energy is the ability to do work. When work is done, energy is transferred.

Energy comes in many forms. Some kinds of energy can be stored, while others cannot. Energy is always conserved.

Example: A box is pushed 5 m across a room with a force of 30 N. What is the work done and how much energy is used? 

Work done, W = Fs W = 30N x 5m W = 150 J

Energy used = 150 J

Exercise

Power

In science, power means how fast a machine or thing or person can do work.

The actual definition for power is

“The rate of doing work or the rate of using up energy’.

What is power? Different definition?

What is power? Different definition?

What is power? Different definition?

Power equation

Power = work done

time taken

OR

Power = Energy used up

time taken

The S.I unit for power is watt (W)

• 1 watt = 1 joule per second. • Kilowatt (kW). 1kW = 1000 W. • Megawatt (MW). 1 MW = 1 × 106 W.

Examples

Hazrin and Zahrina is having a competition, they want to know who is more powerful. So they pushed a wheel barrow with a force of 600N over a distance of 100m. Hazrin took 50s to reach the finish line while Zahrina took 10s.Calculate the power produce by each person, which student is more powerful?P = W = 7N x 2mW= 14Nm or 14 J

Exercise

Summary of the whole lesson:Conditions for Work Done: There must be a force acting on the object. The object must move. The effective part of the force is in the

direction of the movement.

Summary of the whole lesson:1. Work = Force × distance moved in the

direction of the force.

Units are newton metres (Nm) or joules (J).

2. Energy is the ability to do work.

3. Power = work done

time taken

OR

Power = Energy used up

time taken

Summary of the whole lesson:

W = Fs

P = W t

P = Fs t