Transcript Lecture 09 - WebPhysics

Goal: To understand Energy
Objectives:
1) To learn about What energy is
2) To learn about Work
3) To understand the relationships and
differences between Potential and
Kinetic energy
4) To understand the relationships between
Work and Kinetic energy
5) To understand the transfer of energy
What is energy?
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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.
Potential and Kinetic Energy
• All energies are comprised of Potential
and Kinetic Energies.
• Kinetic Energy = energy of motion
• Potential Energy = stored energy
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
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?
Pushing the house
• You push a icehouse across a frozen lake (assume you
can neglect friction here).
• Your friend, who wants to do something else, pushes
back.
• You push with a force of 300 N forward.
• Your friend pushes with a force of 200 N backwards.
• A) What is the work that you have done if you push the
icehouse 30 m forward?
• B) What is the work that your friend has done if you push
the icehouse 30 m forward (note direction, this will have
an effect on the work even though work does not have
“direction”)?
• C) What is the net work done on the ice boat?
Push start
• Where is this work going to go?
Work represents
• The work represents either the change in kinetic
energy or the change in potential energy on an
object.
• Positive work means that the object speeds up
or that it goes up.
• Negative work means that you have stolen
energy which means the object gets slower or
goes down.
• Friction for example is an energy thief because
its force is always negative.
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!
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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.
A) What is the work you have done to the box?
B) What is the acceleration on the box?
C) How long does it take to push the box 5 m
(we have distance and acceleration…)?
• D) Using v = at, what velocity is the box traveling
when you get to the 5 m mark.
• E) 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?
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Work = mass * gravity * distance
The distance is the height,
So, Work = mass * gravity * height
This is called Gravitational Potential
Energy
Example if we have time
• 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?
Make up upwards example if time
permitted
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.