Regents Physics

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Regents Physics

• Work and Energy

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Energy and Work

• Energy is the ability to Work
• Work is the transfer of energy to an object when
  the object moves due to an application of a force
• W Fd unit is Joules (J)
• Energy is also measured in Joules

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When is Work Done?

• Work is only done when the direction of motion is
  in the direction of the force
• So we can rewrite the equation to
• W Fcos ? d

F
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Example of Work?

• 1. A teacher applies a force to a wall and
  becomes exhausted. Is work done?
• 2. A book falls off a table and free falls to the
  ground. Is work done?
• 3. A waiter carries a tray full of meals above
  his head by one arm across the room. Is work
  done?
• 4. A rocket accelerates through space. Is work
  done?

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Power ? The Rate at Which Work is Done

• Work is done when a force moves an object in the
  direction of the force
• Work Force x distance
• Power is the rate at which work is done
• Power work (J) / time (s)
• Unit of Power is a Watt (W) J/s
• P Work / time Fd/t Fv

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For Example

• 1) How much work is done lifting a 1 kg mass 2
  meters?
• 2) What is the power rating if this work is done
  in 8 seconds?
• 3) If a cart is rolled up a track to a height of
  1 m. It is also raised to the same height. Is
  the work the same?

End
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Forms of Energy

• Energy has many forms, including
• Thermal Energy heat, is the total kinetic
  energy possessed by the individual particles of
  an object
• Internal Energy is the total of the potential
  and kinetic energies of an object
• Nuclear Energy is the energy released by
  nuclear fission or fusion
• Electromagnetic Energy is the energy associated
  with electric or magnetic fields

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Potential Energy

• The energy possessed by an object due to its
  position or condition
• If there is no energy loss due to friction, the
  work done to bring an object from its original
  position is equal to the objects change in
  potential energy
• We can see this in observing changes in
  gravitational potential energy

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Gravitational Potential Energy

• Objects gravitational potential energy as they
  are lifted to a distance above the Earths
  surface
• Work is done against gravity to lift the object
• As long as there is no loss due to friction, the
  change in potential energy is due only to change
  in height!

?PE mg?h
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Work and Energy Relationship

• If there is no friction, all the work done in
  lifting an object to a new height is equal to the
  objects increase in potential energy
• The change in potential energy depends only on
  the height, not on the path taken
• For example

W 98 J
1.0m
10 Kg
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Conservative Forces

• When work done against a force is independent of
  the path taken, the force is said to be a
  conservative force
• Gravitation is an example of this type of a force
• Notice no friction is involved

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Nonconservative Forces

• Air resistance and friction are examples of
  nonconservative forces
• The work done against a nonconservative force is
  dependent upon the path taken

1.0m
10 Kg
A
B
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Noncons. example
Wf Ffd Ff ukFN FN gets larger as the
angle gets smaller, so A requires more work
against friction than B
W 98 J Just to lift it
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Kinetic Energy

• Energy associated with motion
• Kinetic energy is gained as potential energy is
  lost

KE 1/2mv2
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Conservation of Energy

• Just like momentum, energy is also conserved
• Energy cannot be created or destroyed, it can
  only be transferred!
• The sum of the changes in a closed system must be
  equal to zero
• We must consider energy conservation under
  perfect and reality like situations

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Ideal Mechanical Systems

• The sum of the kinetic and potential energies in
  a system is called the total mechanical energy
• Ideal Mechanical System is a closed system in
  which no friction or other nonconservative force
  acts
• The sum of the kinetic and potential energy
  changes is equal to zero
• Example the pendulum

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Nonideal Mechanical Systems

• When a system is acted upon by a nonconservative
  force, such as friction, it is called a nonideal
  mechanical system
• The friction opposes the motion of two objects in
  contact with each other and moving relative to
  each other
• The frictional energy is converted into internal
  energy..an increase in temperature

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Ideal vs. Nonideal
NonIdeal
Ideal
?KE -?PE
ET PE KE Q
1/2mv2 mgh
ET mgh 1/2mv2 Q
worksheet
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• For Each Situation
• What type of Forces are Doing work on the ball?
• Is the energy of the ball conserved?
• Whats the KE prior to striking the ground?
• What is the ball final velocity?

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The cartoon strip below depicts a pile-driver
falling from a high elevation (diagram A) to a
low elevation (diagram B) before it encounters
the force of a spike which ultimately brings it
to rest (diagram C). Assume that there is no air
resistance and that the spike moves only
slightly. Fill in the blanks in the cartoon
strip.
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• A worker pushes a 50.0-kg cylinder up a
  frictionless incline at constant speed to a
  height of 3-meters.
• Of the forces acting upon the cylinder, which
  one(s) do work upon it?
• Based upon the types of forces acting upon the
  system and their classification as internal or
  external forces, is energy conserved? Explain.
• Calculate the work done on the cylinder.

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Use the following diagram to answer questions 5
- 7. Neglect the effect of friction and air
resistance. 5. As the object moves from point A
to point D across the frictionless surface, the
sum of its gravitational potential and kinetic
energies a. decreases, only. b. decreases and
then increases. c. increases and then
decreases. d. remains the same.
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6. The object will have a minimum gravitational
potential energy at point a. A. b. B. c.
C. d. D. e. E.
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7. The object's kinetic energy at point C is less
than its kinetic energy at point a. A only. b.
A, D, and E. c. B only. d. D and E.
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Regents Physics

• Springs!!

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Elastic Potential Energy

• Energy is stored in a spring when work is done
  stretching or compressing it
• This energy is called elastic potential energy

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Compression / Elongation

• The compression or elongation of a spring is the
  change in spring length from its equilibrium
  position when a force is applied to it
• The compression (elongation) of the spring is
  directly proportional to the applied
  forceprovided the elastic limit of the spring is
  not exceeded
• This gives us an equation!

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Hookes Law
Fs kx
The applied force on a spring is proportional to
the distance the spring is displaced (x) and the
spring constant (k)
k is the spring constant and is the constant of
proportionality between the applied force and
the compression/elongation of the spring Unit is
the Newton - meter
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Springs Store Energy

• Work done to compress/stretch a spring is equal
  to the stored potential energy..just like in
  gravitation!
• Thus

W Fsx ½ kx x ½ kx2 PEs ½ kx2
Animation _at_ Regents pre
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Practice Problem

• Determine the potential energy stored in the
  spring when a force of 2.50 N is used to stretch
  it 0.100 m?

Solve for k Fs kx Fs / x k k 25 N / m
Solve for PE PE ½ kx2 PE ½ (25 N/m)
(0.100m)2 PE 0.125 J
worksheet