NOTES 11 – Energy, Work, & Power
What is energy & why do we need it?
• Energy – the ability to do work
• Work – moving an object by exerting a
force
• Force – a push or a pull
• Whenever force is used to move an
object, energy is required
– EX. When you walk, your muscles pull on
your bones in order to move your body.
This requires your muscles to use energy.
What is energy & why do we need it?
• Without energy, there could be no
motion
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Atoms and molecules could not move
Stars could not shine
Planets could not orbit around stars
Animals could not walk, run, swim, or fly
The wind could not blow
Messages could not be sent from your
brain to your body
Is work being done when you hold a
book over your head?
• No
• But it did take work to get
it there
• Whenever work is being
done, energy is being used
• The energy was
transferred from your
body to the book
How much work is being done?
• We can measure the amount of work being
done to move an object
• We need to know 2 things in order to measure
work:
1. The amount of force being used
2. The distance of the movement
• We can calculate work using the following
formula:
Work = Force x Distance (W = F x d)
Calculating Work – Units of Measurement
• When calculating work, use the following
units:
– Force is measured in newtons (N)
– Distance is measure in meters (m)
– Work is measured in joules (J)
• 1 joule equals 1 newton multiplied by 1 meter
• You do about 1 joule of work when you pick
up an apple and put it on a desk
Calculating Work – Sample Problem
• If you lift a 3N book 2 meters off the floor,
how much work did you do?
• W=Fxd
• W = 3N x 2m
• W = 6J
• You did 6 joules of work
Question
• When you carry a heavy bag of groceries from
your car to your kitchen, what does most of the
work, your arms or your legs? Explain why.
• Your legs, because they move you and the
groceries from the car to the kitchen. Your
arms only lift and hold the groceries.
• The legs apply more force over a greater
distance so they do more work
Energy Is Also Measured In Joules
• Since energy is required to do work, it is
measured using the same unit (joules)
• The amount of energy required to do work is
ALWAYS greater than or equal to the amount
of work being done
• EX. If you do 6J worth of work to lift a book,
you need at least 6J of energy to do it
Doing Work Gives Energy to Objects
• When work is done on an object:
– Energy is transferred from the object doing the
work to the object having work done on it
– The object doing the work loses energy
– The object having work done on it gains energy
• EX. A student pushing a desk across the floor
is doing work on the desk – energy is
transferred from the student to the desk and
the student loses energy
Why do we get tired after doing work?
• We give up our energy to all the objects we touch
and move around
• In any energy transfer in our body, some energy is
changed to heat and transferred to our
environment – the energy is LOST from our body
• Staying alive requires a lot of energy
– Heart beating, brain sending messages, cells moving
substances in and out, muscle contractions, etc.
• As our body’s energy gets low, we get tired
What is power?
• Power – the rate at which work is done
• More power means…
– More work is done in the same amount of time
– The same amount of work is done in less time
• EX. A person that is a more powerful runner is
faster and can run farther in the same amount of
time as a less powerful runner (more work in
equal time)
• EX. A car with a more powerful engine can
accelerate to 60mph faster than a car with a less
powerful engine (equal work in less time)
Calculating Power
• Power is measure in watts (W)
• We need to know 2 things in order to measure
power:
1. The amount of work being done
2. The amount of time it takes to do the work
• We can calculate power using the following
formula:
Power = Work / Time (P = W / t)
• 1 watt equals 1 joule divided by 1 second
Calculating Power – Sample Problem 1
• If an engine does 100,000 joules of work in 10
seconds, how much power did it use?
• P=W/t
• P = 100,000J / 10s
• P = 10,000W
• The engine used 10,000 watts of power
Calculating Power – Sample Problem 2
• Because work equals force multiplied by distance,
another way to write the power formula is:
Power = (Force x Distance) / Time
P = (F x d) / t
• P = W / t is the same as P = (F x d) / t
Calculating Power – Sample Problem 2
• If an engine exerts 3500 newtons of force to
move a car 50 meters in 10 seconds, how
much power did it use?
• P = (F x d) / t
• P = (3500N x 50m) / 10s
• P = 175,000J / 10s
• P = 17,500W
• The engine used 17,500 watts of power
A Watt Measures Work Done and
Energy Used In an Amount of Time
• A Watt equals 1 Joule per second
• The more watts, the more work is done each
second
• Joules also measure energy, so a watt also
measures energy use per second
• EX. A 100W light bulb uses 100J of energy
each second that it is on