Question
How does gas make a car go?
Energy and Work
Which of these will take more “energy” from your body?
Which will make you more tired to move?
Energy and Work
Which of these will take more “energy” from your body?
Which will make you more tired to move?
Energy and Work
Which path below will take more energy as you push the
lawnmower?
Energy and Work
Which path below will take more energy as you push the
lawnmower?
Energy and Work
Definition of
energy
To “use up” more energy, we can either
push really hard or push really far.
So energy is how we combine force
times distance.
Vocabulary
Energy
How we use force to move something
over a distance.
Energy and Work
How this
helps us
If we can’t figure out how much energy
something will take to move, we can’t
move things well!
Energy and Work
What is
work?
In physics, we call using energy “work”.
This isn’t like when you use the everyday
word for “work”. In physics, it means
something very specific.
Vocabulary
Work
A force applied over a distance. How
hard we push something and how far it
goes.
Energy and Work
Units
So how do we measure work?
Energy and Work
Units
So how do we measure work?
We need a new unit. One that can
describe energy.
We call it a Joule.
(N*m)
(kg * m/s2 *m)
(kg * m2 /s2)
This is energy it takes to lift up 1 kg a
distance of 1 meter.
Your body releases 100J as heat every
second.
Energy and Work
A few things
to know
Look at some extreme cases
If you push super hard and something
moves only a little, you still do a lot of
work. (big force, small distance)
Energy and Work
A few things
to know
Look at some extreme cases
If you push super hard and something
moves only a little, you still do a lot of
work. (big force, small distance)
If you barely push at all but something
moves really far, you still do a lot of
work. (small force, big distance)
Energy and Work
A few things
to know
Look at some extreme cases
If you push super hard and something
moves only a little, you still do a lot of
work. (big force, small distance)
If you barely push at all but something
moves really far, you still do a lot of
work. (small force, big distance)
If you push really hard, but something
doesn’t move at all, you do zero work.
Question
This kid is pushing
against the door with a
force of 1,000 N, but
the door isn’t moving.
Is he doing any work
on the door?
Equation
Work
w=F•d
w= work. (measured in joules) J
F= Force. (measured in newtons) N
d= distance (measured in meters) m
Tells us the energy it takes if we use
force to push a mass over a distance.
Joule
(N*m)
(kg * m/s2 *m)
(kg * m2 /s2)
Efficiency
X 100%
Efficiency = Output Work / Input Work X 100%
1.
You do 20J of work while using a hammer. The
hammer does 18J of work on a nail. What is the
efficiency of the hammer?
 2. Suppose you left your lawn mower outdoors all
winter. Now it’s rusty. Of your 250,000J of energy only
100,000J go to cutting the lawn. What is the efficiency
of the lawn mower now?
3. What is the efficiency of a cyclist who applies 9 N of
force to his bicycle to climb a hill 100m long while the
bike needs 75 J to actually climb the hill? What could be
the reason for this level of efficiency?
4. A certain light bulb consumes 200J of electrical
energy per second, but only emits 25J of light energy
per second. Calculate the efficiency of this bulb
5. A certain large wind turbine is able to transform
1,500,000J of mechanical energy into 1,000,000J of
electrical energy every second. Calculate the efficiency
of this turbine.
6.
A certain solar panel is capable of absorbing 750J of
light energy every second, and converting 150J of that
energy into electrical energy.
a.
How much energy is ‘lost’ in the form of heat by the
solar panel every second?
b. What is the efficiency of this solar panel?
7.
An automobile gasoline engine is able to do 225 J of
useful work with each stroke of its pistons.
a. If the engine has an efficiency of 29.0 percent, what is
the amount of work that must be done to the pistons in
the engine?
b. How much energy is lost as heat with each cycle?