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ENERGY Energy Example Problem Legend has it that Isaac Newton discovered gravity when an apple fell from a tree and hit him in the head. If a 0.20 kg apple fell ... – PowerPoint PPT presentation

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Title: Energy

  • Energy is always present, but never visible!
  • We see the evidence of energy
  • Pushing a wheelchair
  • Jumping
  • Eating
  • Movement, sound, heat, light all provide evidence
    of energy

Trebuchet Video Consider these questions
  • How is a trebuchet constructed?
  • Which simple machine do you see?
  • What is the purpose of the counterweight?
  • What type of energy does the trebuchet have when
  • What happens when the firing pin is released?
  • How would you differentiate between potential and
    kinetic energy?
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Work- Energy Correlation
  • In order for work to be done, a force must be
    applied to an object and cause it to move in the
    same direction as the force.
  • Work transfers energy from one object to another
  • What is energy?

What is energy?
  • Energy is the ability of an object to produce a
    change in itself or the environment. It is the
    ability to do work.

Types of Energy
  • Mechanical Energy(ME) - enables an object to do
  • A. Kinetic Energy (KE)
  • B. Potential Energy(PE)

Kinetic Energy
  • Kinetic energy is defined as the energy of a
    moving object
  • Examples
  • Thrown football
  • Waterfall
  • Rock falling from a cliff

Potential Energy
  • Potential energy is defined as the energy in
    matter due to its position or the arrangement of
    its parts
  • Often referred to as stored energy
  • The more potential an object has, the more
    potential energy is has
  • Has many different forms including
  • Gravitational potential energy
  • Elastic potential energy
  • Chemical potential energy
  • Electrical potential energy

Toy observations
  1. Describe your toy.
  2. Take a few minutes to determine how your toy
    works. Make a short statement about how the toy
  3. State how you think your toy uses energy.
  4. How many forms of energy does your toy display?
  5. Name the types of energy you observe.
  6. Switch toys with another table and repeat the
    above observations.

Gravitational Potential Energy
  • When something is lifted or suspended in air,
    work is done on the object against the pull of
  • This work is converted to a form of potential
    energy, called gravitational potential energy
  • Once the object falls the potential energy is
    converted to kinetic energy

Elastic Potential Energy
  • Occurs when an object resists being pulled out of
  • Examples a stretched rubber band, a spring,
    trampoline, our skin
  • The elastic potential energy in a rubber band can
    be used to do work- such as a toy plane . . . A
    rubber band untwists and causes a propeller to

Chemical Potential Energy
  • Chemical potential energy is the energy stored in
    molecules (including in us)
  • Examples food, gasoline
  • When gasoline is burned through combustion, the
    arrangement of molecules changes, and energy is
  • The released energy is used to do work

Electric Potential Energy
  • Electrical potential energy is the result of
    energy from a battery, a power plant, a
    hydroelectric dam, solar cells, or a windmill
  • Electric potential energy can be converted to
    sound, light, motion, etc.

Work and Energy
  • Work and Energy are related
  • When work is waiting to be done, we call the
    energy potential
  • When work is being done, we call the energy

Rubber Band
  • In order to make a rubber band fly, you must do
    work on it
  • While it is stretched, the rubber band has
    potential energy
  • Once you release it, it has kinetic energy
  • If you pull it back a greater distance (therefore
    doing more work) the potential is greater
  • The result is that the rubber band will go faster
    and further (more potential energy leads to
    greater kinetic energy, thus more work can be

Situations to consider
  • Is work done in the following situations?
  • Pushing against a wall?
  • No, there is no displacement, no work, so kinetic
    energy does not change
  • Pushing at constant speed?
  • No, work done by the applied force is equal to
    work done by the frictional force, so kinetic
    energy does not change
  • Free fall
  • Gravity is the only force acting, so there is a
    net force and work is done . . . Kinetic energy

Kinetic Energy
  • Because energy is a property
  • of matter, it can be quantified
  • The equation for kinetic energy is
  • Where m is the mass and v is the velocity of
    the object
  • The unit for energy is joules

Example 1
  • What is the kinetic energy of a 45 kg object
    moving at 13 m/s?

Example 2
  • The kinetic energy of a boat is calculated at
    52,000 J. If the boat has a mass of 39,000 kg,
    with what velocity is it moving?

Potential Energy
  • Remember that potential energy is the energy of
  • To quantify potential energy use the following
  • Where m is mass, g is gravity and h is the
    height in meters

Example 3
  • A 3.8 kg object is lifted to a height of 3
    meters. What is the potential energy of the

Example 4
  • A 30 kg child climbs 15 m up a tree. When he
    stops to look around, what is the childs
    potential energy?

State whether each of the following has kinetic
energy, potential energy, or both.
  • 1. If an object is at rest, it certainly does NOT
    possess this form of energy.
  • 2. Depends upon object mass and object height.
  • 3. The energy an object possesses due to its
  • 4. The amount is expressed using the unit joule
    (abbreviated J).
  • 5. The energy stored in an object due to its
    position (or height).
  • 6. The amount depends upon the arbitrarily
    assigned zero level.
  • 7. Depends upon object mass and object speed.
  • 8. If an object is at rest on the ground (zero
    height), it certainly does not possess this form
    of energy.

Work-Energy Theorem
  • Work is the transfer of energy from one object to
    another, or . .
  • The change in KINETIC ENERGY!

Total Energy
  • At any point, the total energy of an object
  • ME KE PE

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Law of Conservation of Energy
  • Energy cannot be created or destroyed.
  • The total energy in a system before an
    interaction equals the energy after an
  • This is true for a closed, isolated system (no
    external forces present)

Law of Conservation of Energy
Example 1
  • A 12 kg rock is at the edge of a 95 m cliff.
  • a. What is the rocks initial PE and KE?
  • b. If the rock falls to the ground, what is its
    final PE and KE just before it hits?
  • c. What is the rocks velocity just before it
    hits the ground?

Example 2
  • A 33 kg cart rests at the top of a hill.
  • If the cart has a PE of 4800J, what is the height
    of the hill?
  • When the cart has reached point B (h 5.0
    m), what is its KE?

Example Problem
  • A greyhound at a race track can run at a speed
    of 16.0 m/s. What is the KE of the 20.0 kg
    greyhound as it crosses the finish line?

Example Problem 2
  • Determine the kinetic energy of a 625-kg roller
    coaster car that is moving with a speed of 18.3

Example 3
  • Missy Diwater, the former platform diver for the
    Ringling Brother's Circus, had a kinetic energy
    of 12 000 J just prior to hitting the bucket of
    water. If Missy's mass is 40 kg, then what is her

Example Problem
  • Legend has it that Isaac Newton discovered
    gravity when an apple fell from a tree and hit
    him in the head. If a 0.20 kg apple fell 7.0 m
    before hitting Newton, what was its change in PE
    during the fall?