# Situations where Work is Done

```PART I:
Lesson Objectives
What is meant by “work done”?
How can you tell when work is done and
when work is not done?
How is work done measured and
calculated?
A Product of Force &amp; Distance
What is WORK???
However , in physics,
work is done ONLY IF :
In normal usage, work
is about doing school
homework or
assignments, riding a
bicycle, sweeping the
floor, etc.
- a force is applied on an object
&amp;
- the object moves in the direction of force.
The force and the motion must
be in the same direction!
Situations where Work is Done
1
Situations where Work is NOT
Done
No Work Done:
Pushing against a
wall
Calculating Work Done
It is true that a force is exerted simply to hold a book,
but the book does not move through a distance.
Therefore the distance moved is 0, and the work
accomplished is also 0.
Calculating Work Done
Work Done =
Force
x
Distance moved in the
direction of that force
W=F•d
Units for Work
The SI unit for work is the
newton-meter (N • m)
which is also called a
Joule (J)
1 newton-meter = 1 Joule
1 N•m = 1 J
2
Try this:
Bob the Builder moves a 40.0 Newton
window up 9.00 meters to the second
floor.
360 Newton-meters = 360 Joules
His assistant moves uses a truck to slide
a 120.0 Newton concrete slab 30.0
centimeters across the floor.
3600 Newton-centimeters = 36.0 Joules
Units for Work (Con’t)
newton-meters (N • m)
• Are used to measure large amounts of work
newton-centimeters (N • cm)
• Can be used to measure small amounts of work
How much work did each person do?
We calculate work done by a force using:
W=F•D
work done = force x distance
• the units of work:
the Newton-meter ( N•m )
or the Newton-centimeter ( N•cm )
The amount of work done depends on two
factors, namely
1. the force applied
More work is done when a bigger force
is applied.
2. the distance moved in the direction of
the applied force
More work is done when the object
moves through a greater distance.
1 N•m = 1 Joule
Work (Practice Problems)
If I am holding a box that weighs 35 N, and
walk from where I am standing to a point
2.0 m away, how much work have I done
on the box?
W=F•D
work done = force x distance
3
If I lift a box that weighs 35 N up 2.0 m, how
much work have I done on the box?
If I lift a box that weighs 35 N up 2.0 m and
then carry it forward to a point 20.0 m
away, how much work have I done on the
box?
If I slide a box across a counter by pushing
on its side with 12 N of force, and the box
slides 3.0 m across the counter, how much
work have I done on the box?
If I slide a box that weighs 60 N across the
floor by pushing on its side with 12 N of
force, and the box slides 3.0 m across the
floor, and then I pick it up 2.0 m above the
floor, how much work have I done on the
box?
What is the work done in carrying a 25 N
box up a flight of stairs of to a floor 4.5 m
higher than the floor you started at?
What is the work done in climbing a flight of
stairs of height h?
W=F•d
W=F•d
d
d
4
What is the work done in climbing a flight of
stairs of height h?
POWER
W=F•d
Do you get more tired
when you run or when
you walk up the stairs?
What is the difference?
h
TIME
POWER
Power = Work / Time
POWER
POWER IS:
Electric Power = Current &times; Voltage
• the rate at which work is done
• how fast work is done
• how much work you can do per second
POWER
What are the units for Power?
Other units for Power:
Metric system units for Power are:
Watts (W)
1 Watt = 1 Joule per second
1 W = 1 J/s
Electrical Power is sold in kilowatt-hours (kWh):
1 kW = 3,600,000 J/hr
Cars and Machines are rated in Horsepower (hp)
1 hp = 746 W (a very strong horse! )
5
POWER
POWER
W
P
MEASURED IN
JOULES (J)
MEASURED IN
WATTS (W)
Watt Demonstration:
–Lift a 1-N mass a distance of 1
meter, and you have done 1 Joule
of Work.
–Do this once each second, and
your power output is 1 Watt.
t
MEASURED IN
SECONDS (s)
1 W = 1 J/s
Electric Power = Current &times; Voltage
What is the work done in climbing a flight of
stairs of height h?
What is the work done in climbing a flight of
stairs of height h?
W=F•d
W=F•d
Do you get more tired
when you run or when
you walk up the stairs?
What is the difference?
d
h
TIME
POWER
Power = Work / Time
• next…
POWER IS:
• the rate at which work is done
• how fast work is being done
• how much work you can do per second
6
Example:
What is the power output of a 100 N
person who runs up a 10 m high flight of
stairs in 2.5 s?
Power, Energy, Work …
all together now.
THE RATE OF DOING WORK OR
CONVERTING ENERGY.
W = Fd
= (100 N)x(10 m)
= 10,000 J
Power = Work / Time
P=W/t
= (10,000 J) / (2.5 s)
= 4000 W
h
Power = Energy / Time
UNIT- WATT = JOULE PER SECOND
7
```