Transcript Energy - WebPhysics

Goal: To understand Energy
Objectives:
1) To learn about What energy is
2) To learn about Work
3) To learn about Power
4) To understand the relationships and differences
between Potential and Kinetic energy
5) To learn about the different Forms of energy
6) To understand the relationships between Work and
Kinetic energy
7) To understand theTransfer of energy
8)
To understand the uses for Energy and our world –
present and future
What is energy?
•
•
•
•
•
•
Energy is what is needed to do stuff.
Energy is required to heat.
Energy is required to power an AC to cool.
Energy is needed to move things
Energy is needed to build things
Energy lights our lights and powers our
TVs.
• Energy drives our cars.
Work
• One way to measure the use of energy is
by measuring work (work is an energy).
• Work = Force * distance
• Net Work = Net Force * Net distance
• Units of Work/Energy:
• Work = Force * distance = Newton * m
• Newton * m = Joule
Other Work/Energy Units
•
•
•
•
•
Watt-hour
Kilowatt-hour (more on these later)
BTU (British Thermal Unit)
Therm (= 100,000 BTUs)
Calories
• Mostly we will use Joules
Example:
• You walk up a set of stairs. Lets assume
your weight is 700 N. If the top stair is 3 m
higher than the bottom stair then how
much work have you done on yourself?
Batman
• Batman slides across the lake at a
constant velocity.
• If he travels 800 m horizontally and has
weight of 900 N then what work has been
done on Batman?
Dropping the ball
• You want to sit on the couch but for some
reason a bowling ball of mass 7 kg is sitting on
the couch right where you want to sit.
• Throwing the bowling ball would be a bad idea,
so you pick up the bowling ball, lift it 0.2 m
above the couch seat and then gently set it onto
the floor.
• If the floor is 0.3 m below the couch seat then
what is the total work done on the bowling ball
by you?
• What is the total work done by the earth?
Push start
• Ignoring friction, you push your 600 kg car
with a force of 2000 N for a distance of 10
m.
• How much work have you done on the
car?
• Where is this work going to go?
Power
• Power is quite simply the rate at which you
do work.
• Power = Work / time = Energy / time
• So, the faster you do work, the more
power you are providing.
• Units: Power = Energy / time = J/s
• J/s = Watts
Power example
• A forklift which generates a force of 2500
N lifts a 225 kg bundle up a distance of 1
m in a time of 2 seconds.
• What is the work done by the forklift?
• What is the power generated by the
forklift?
Lights out
• An incandescent light bulb has a power of
60 Watts.
• If you turn the light bulb on for 10 hours
how much energy does the light bulb use?
• NOTE 1 Watt * 1 hour = 1 watt-hour of
energy.
• If 1000 Watt-hours costs you $0.10 then
what is the daily cost of a single light bulb?
• What is the yearly cost of this single light
bulb?
Fluorescent bulbs
• For the same light, they use 4 times less
power!
• How much money could 1 Fluorescent
bulb save you in 1 year?
• If you have 10 bulbs to replace how much
do you save per year by going Fluorescent
and how does that compare to the $3 each
that the bulbs cost?
Potential and Kinetic Energy
• All energies are comprised of Potential
and Kinetic Energies.
• Kinetic Energy = energy of motion
• Potential Energy = stored energy
Potential Energies:
•
•
•
•
•
•
Spring potential (Mechanical Energy)
Chemical potential
Gravitational potential
Electromagnetic potential
Nuclear potential
Mass Energy
Kinetic Energy
•
•
•
•
•
Moving energy
Heat energy
Light Energy
Wave energy
Sound energy
Work vs. Kinetic energy
• Work = Force * distance
• Force = mass * acceleration
• Distance = ½ acceleration * time * time
• So,
• Work = mass * acc * ½ acc * time * time
• Acceleration * time = velocity
• Therefore,
• Work = ½ mass * velocity * velocity
• Notice that the above is the equation for Kinetic energy!
Lets prove it!
•
•
•
•
•
•
You push a box across a frictionless floor.
You apply a 300 N force to the 20 kg box.
You push the box for 5 m.
What is the work you have done to the box?
What is the acceleration on the box?
How long does it take to push the box 5 m (we
have distance and acceleration…)?
• Using v = at, what velocity is the box traveling
when you get to the 5 m mark.
• Now, find the Kinetic energy of the box
(KE = ½ mass * velocity * velocity)
Energy transfers
• Energy is not created or destroyed, but it
is always moving from one form to
another.
• Lets examine gravitational potential
transfers to kinetic energy and back.
Gravitational Potential Energy
• Work = Force * distance
• What force do you need to overcome
gravity?
Gravitational Potential Energy
• Work = Force * distance
• What force do you need to overcome
gravity?
•
•
•
•
Work = mass * gravity * distance
The distance is the height,
So, Work = mass * gravity * height
This is called Gravitational Potential
Energy
Example
• You are holding a 2 kg ball at a height of
0.6 m. What is the Gravitational potential
energy of the ball?
• You drop the ball. What happens? By
how much will the gravitational potential of
the ball change?
Example
• You are holding a 2 kg ball at a height of 0.6 m. What is
the Gravitational potential energy of the ball?
• You drop the ball. What happens? By how much will the
gravitational potential of the ball change?
• Lets find out what the kinetic energy does.
• Find the time it takes to fall 0.6 m using gravity and the
distance.
• Find the velocity the ball hits the ground at.
• Find the kinetic energy of the ball when the ball hits the
ground.
• How does the kinetic energy of the ball compare to the
change in gravitational potential?
Conclusion
• We have seen how energy is used, transferred
between forms, and why it is useful.
• We have discovered how to find work, power,
kinetic energy, and potential energy.
• What we have not done is take a look at how we
actually use energy as a nation now, and how
and why we should make a change in the future.