Unit 4: Energy, heat and motion
Work and Power
What is work?
• Definition: work occurs when a force causes
an object to move in the direction of the
force.
• characteristic – the direction the object moves
MUST be in the same direction of FORCE to be
considered WORK
• Ex.
• Force
• Distance it moves
Work or not work?
Example
Josh pushing a box
across the floor.
Symone carrying
her books to her
next class
Ms. Tran lifting a
box off the ground
De’Andre placing a
box on the ground
Direction of force
Direction of
movement
Doing work?
(yes/no)
How do we calculate work?
• Formula: work = force x distance
• Ex. Find the work done on the box if a force of
55N was applied to the right and the box
moved a distance of 10m.
• 55N
• 10M
• Is work being done on this object?
Work out the problem
• Work = force x distance
• Work = 55N x 10m
• Work = 550 Newton-meters or JOULES (J)
Practice Problem #1
• Marquise applies a force of 500N to push a
truck 100m down the street. How much work
does he do?
Practice Problem #2
• In which situation do you do more work?
(Calculate the work done in each scenario)
– A. you lift a 75N bowling ball 2m off the floor.
– B. You lift two 50N bowling balls 2m off the floor.
Energy
• DEFINITION: Energy is the ability to do work.
• Energy is all around you! You can hear energy
as sound. You can see energy as light. You
can feel energy as wind. You use energy to
bounce a basketball or lift a backpack.
Energy & Work
• What do energy and work have to do with
each other?
• Energy and Work are directly related…in fact,
they are both measured in JOULES!
• Objects can gain energy because work is being
done on them. Can you think of any
examples?
Energy & Work: Examples
• When you kick a football, you give some of
your energy to the football to make it move.
• When you throw a bowling ball, you give it
energy to make it roll.
• When the bowling ball hits the pits, it loses
some of its energy to the pins, causing them
to fall!
Power
• What is power?
Power
• DEFINITION: Power is the rate at which work is
done.
• FORMULA: Power = Work ÷ Time
• Or: Power = Energy ÷ Time
• Or: (Force x Distance) ÷ Time
• Unit of measure: Watt (W)
Power: Let’s Practice…
• An Antarctic explorer uses 6000 Joules of work
to pull his sled for 60 seconds. What power
does he need?
• Power = Work ÷ Time
• Work = 6000J, Time = 60sec
• Power = 6000J ÷ 60sec = 100 Watts
Power: Let’s Practice…
• If a conveyor belt at a candy factory uses 10 J
to move a piece of candy a distance of 3
meters in 20 seconds, what is the conveyor
belt’s power?
• Power = Work ÷ Time
• Work = 10 J, Time = 20 sec
• Power = 10 J ÷ 20 sec = 0.5 Watts
Light Bulbs
• But wait! Aren’t watts used to describe light
bulbs???
• Yes! For example, a 60-watt light bulb uses 60
joules of energy every second to shine!
Power: Let’s Practice…
• A light bulb uses 600 J of energy in 6 seconds.
What is the power of the light bulb?
• Power = Energy ÷ Time
• Energy = 600 J, Time = 6 sec
• Power = 600 J ÷ 6 sec = 100 watts
More Practice…
• What two factors do you need to know in
order to calculate how much work was done
in any situation?
• Answer: You need to know FORCE and
DISTANCE.
More Practice…
• If you push very hard on an object but it does
not move, have you done work? Why or why
not?
• Answer: NO! The object must move for work
to be done.
More Practice…
• Was work done on a book that fell from a desk
to the floor? If so, which force was involved?
• Answer: YES! The force of GRAVITY was
involved.
More Practice…
• Work is done on a ball when a soccer player
kicks it. Is the player still doing work on the
ball as it rolls across the ground? Why or why
not?
• NO! The player is no longer doing work on the
ball while it is rolling. The ball keeps rolling
because of inertia.
More Practice…
• How is Power related to Work?
• Answer: Power is the rate at which work is
done. The faster you do work, the greater
your power.
More Practice…
• By increasing the SPEED at which you work,
you can increase your:
a) Force
b) Work
c) Energy
d) Power
Answer: d) Power
More Practice…
• Which takes more power: using 15 Newtons
to lift a ball 2 meters for 5 seconds, or using
100 Newtons to push a box 2 meters in 50
seconds?
• Answer: The first situation.
• Power = (15N x 2m) ÷ 5 sec = 6 W
• Power = (100N x 2m) ÷ 50 sec = 4W
– 6 Watts is greater than 4 Watts
More Practice…
• How can you tell if a force you apply to an
object is doing work?
• Answer: You can tell the force is doing work
because the object will MOVE.
More Practice…
• How much work is done if you drag a box
weighing 150 Newtons a distance of 30
meters?
• Work = Force x Distance
• Force needed to drag suitcase = 150N
• Distance = 30m
• Work = 150N x 30m = 4,500 Joules (J)
More Practice…
• A light bulb uses 400 J of energy in 8 seconds.
What is the power of the light bulb?
• Power = Energy ÷ Time
• Energy = 400 J, Time = 8 sec
• Power = 400 J ÷ 8 sec = 50 watts
Exit Ticket: So What Did We Learn?
•
•
•
•
What is WORK?
What is ENERGY?
What is POWER?
How do they all relate to what we’ve already
learned about FORCE?