Chapter 3 Work
Table of Contents
L 1 Work & Power
Chapter 3 Forces in Fluids
L 1 Work and Power
How Is Work Defined?
The Meaning of Work
Work is done on an object when the object moves in the SAME direction in
which the force is exerted.
Example: Pushing a child on a swing, pulling books out of your backpack
Force
Motion
Force Motion
What are the conditions for work to happen?
1- No work without motion (NO MOTION= NO WORK)
To do work on an object, the object must move some
distance as a result of your force. If the object does not
move, there is no work is done even if you exert a force!
Example: trying to push a car that doesn’t move out of
deep snow.
2- Force in the same direction
The other really important thing to mention about this is
the force that you exert must be in the same direction as
the object’s motion.
Example: carrying the books to class.
Calculating Work
The amount of work you do depends on the amount of force you exert and the
distance the object moves.
Calculating work
The amount of work done can be calculated by multiplying force times distance.
The SI unit of work is the newton x meter ( ).
This unit is also called a joule.
One joule (J) is the amount of work you do when you
exert one newton of force to move an object one meter.
Calculating Work Examples
1. How much work do you do when you push a shopping cart with a
force of 50 N for a distance of 5 m?
Calculating Work Examples
A force of 6 N is used to push a couch along the floor a distance of 8 m.
How much work was done?
Calculating Work Examples
WORK = Force x Distance
1. How much work do you do when you push a shopping cart with a
force of 50 N for a distance of 5 m?
W = 50 x 5
W = 250 Joules
2. A force of 6 N is used to push a couch along the floor a distance of 8
m. How much work was done?
W=6x8
W = 48 Joules
What Is Power?
What Is Power?
Power is the rate at which work is done
Power equals the amount of work done on an object in a unit of
time. (how fast the work was done)
An object that has more power than another object can do more
work in the same time. It can also do the same amount of work in
less time.
What Is Power?
Which one has more power & why, The broom or the vacuum
cleaner?
The vacuum cleaner has more
power than the broom because it
can do more work in the same time.
It can also do the same amount of
work in less time.
What Is Power?
Calculating Power
Power is calculated by dividing the amount of work done by the amount of time it takes to
do the work.
Since work is equal force times distance, you can rewrite the equation for power as follows.
Power units
The SI unit for power is Joules/second (J/s ) or the Watt (W)
Power is often measured in larger units like kilowatts or horsepower.
1 Kilowatt = 1000 watts
1 horsepower = 746 watts
What Is Power?
A machine produces 4000 Joules of work in 5 seconds. How much power
does the machine produce?
A box that weighs 1000 Newtons is lifted a distance of 20 meters in 10
seconds. How many kilowatts of power are produced?
What Is Power?
1)A machine produces 4000 Joules of work in 5 seconds. How much power does the
machine produce?
work
Power =
time
4000J
Power =
5s
= 800 watt
2)A box that weighs 1000 Newtons is lifted a distance of 20 meters in 10 seconds. How
many kilowatts of power are produced?
𝟏𝟎𝟎𝟎𝐍 𝟐𝟎 𝐦
Power =
𝟏𝟎 𝐬
Power = 2000 watt
A bricklayer lifts a stack of bricks onto his shoulder, carries it across a
room, and then lifts the bricks onto a ledge above his head. Explain if
work is being done in each of these three situations.
Work is being done when the bricklayer lifts the stack of bricks to his shoulder and
again when he raises the stack to the ledge. In both situations the bricklayer’s force on
the stack and the motion of the stack are in the same direction. When the bricklayer
carries the stack, no work is being done because his force on the stack (vertical) and
the motion of the stack (horizontal) are not in the same direction.
Choose the correct answer for each of the following questions:
1-The rate at which work is done is called:
a)power b)efficiency
c)force d)energy
2-The amount of work done on an object can be determined by:
a)distance times power b)force times distance
c)Force divided by distance e)Mass times distance
3-The SI unit of power is all of the following except :
a)J/sec b)Watt
c)J/m d)kilowatt
Choose the correct answer for each of the following questions:
1-The rate at which work is done is called:
a)power b)efficiency
c)force d)energy
2-The amount of work done on an object can be determined by:
a)distance times power b)force times distance
c)Force divided by distance e)Mass times distance
3-The SI unit of power is all of the following except :
a)J/sec b)Watt
c)J/m d)kilowatt
4. For work to be done on an object,
a. some force need only be exerted on the object.
b. the object must move some distance as a result of a force.
c. the object must move, whether or not a force is exerted on it.
d. the object must not move.
5. If you exert a force of 20 newtons to push a desk 10 meters, how
much work do you do on the desk?
a. 200 joules
b. 30 joules
c. 10 joules
d. 100 joules
4. For work to be done on an object,
a. some force need only be exerted on the object.
b. the object must move some distance as a result of a force.
c. the object must move, whether or not a force is exerted on it.
d. the object must not move.
5. If you exert a force of 20 newtons to push a desk 10 meters, how
much work do you do on the desk?
a. 200 joules
b. 30 joules
c. 10 joules
d. 100 joules
6. Which of these is an example of work being done on an object?
a. holding a heavy piece of wood at a construction site
b. trying to push a car that doesn’t move out of deep snow
c. pushing a child on a swing
d. holding a door shut on a windy day so it doesn’t blow open
7. In order to do work on an object, the force you exert must be
a. the maximum amount of force you are able to exert.
b. in the same direction as the object’s motion.
c. in a direction opposite to Earth’s gravitational force.
d. quick and deliberate.
6. Which of these is an example of work being done on an object?
a. holding a heavy piece of wood at a construction site
b. trying to push a car that doesn’t move out of deep snow
c. pushing a child on a swing
d. holding a door shut on a windy day so it doesn’t blow open
7. In order to do work on an object, the force you exert must be
a. the maximum amount of force you are able to exert.
b. in the same direction as the object’s motion.
c. in a direction opposite to Earth’s gravitational force.
d. quick and deliberate.
