Work, Power and Energy

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Work, Power and Energy

• Physics 11
• Mrs. Kay

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Work

• The product of the force exerted on an object and
  the distance the object moves in the direction of
  the force.
• WFcos?d
• Units J (joule) N (newton) x m (metre)
• Work is done only if an object moves

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Force vs. Displacement

• The area under a force displacement graph equals
  the work done.

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If movement is in same direction

• Holding objects doesnt count as work, because no
  movement.
• If perpendicular to movement cos(90o)0, so no
  work.
• If parallel to movement cos(Oo) 1, so WFd

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Problem

• How much work is done by a weight lifter is she
  holds a 100N barbell above her head?
• Answer
• W Fcos?d
• 100N cos(90o)(Om)
• 0 J

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Problem 2

• 2. A man carries a 50N box on his shoulder and
  walks 30m. How much work does he do on the box?
• Answer
• W Fcos?d
• 50N cos(90o) 30m
• O J

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Problem 3

• A girl exerts a force of 35N at a 35o angle to
  pull her wagon 25m down a street. How much work
  does she do on the wagon?
• Answer
• W Fcos?d
• (35N)(cos35o)25m
• 717J

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Problem 4

• A superhero stops a runaway bus by exerting a
  force of 200N against the motion of the bus for
  12m. How much work does he do on the bus?
• Answer
• W Fcos?d
• (200N)(cos 180o) (12m)
• -2400J (negative because in opposite direction
  of motion)

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Remember

• If force and motion are horizontal and in the
  same direction you can use
• WFd

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When No force is done
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Work against gravity

• When there is a vertical and horizontal
  component, the displacement is equal to the
  height or vertical component. It does not depend
  on the incline.
• Ex like a stairs or escalator question.
• Use Wmgh, where h is the height that the object
  has traveled.

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Problems

1. A 75kg firefighter climbs up a flight of stairs
   10m high. How much work is required?
2. A 900N crate rests on the floor. How much work
   is required to pull it 6m up a 35o incline?
3. A girl holds a 15kg bag above her head. How much
   work did it take for her to lift it up to 2.2m?

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Answers

• Wmgh, (75kg)(9.8m/s2)(10m)7350J
• Vertical sin(35) x 6
• 3.44m, so
• WFd(900N)(3.44m) 3097J
• Wmgh (15kg)(9.8m/s2)(2.2m)323.4J

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Practice

• Day One
• Pg 199 1-3
• Pg 202 5-7
• Pg 213 1-3 and 5
• Day Two
• Pg 213 7-11

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Power

• The rate of doing work
• PW/t
• Units watt (w) 1 Joule of energy transferred
  per second.

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Problems

• Pg.203 9-11
• Pg.214 12-13,15,18

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Forms of Energy

• Kinetic energy found in moving objects
• Potential energy stored energy. It is converted
  into kinetic energy. Can have many forms within
  it. (elastic, gravitational, chemical)
• Energy is measured in Joules (J)

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Kinetic Energy

• The kinetic energy of an object is given by the
  equation
• Ek1/2 mv2
• It is proportional to the mass and square
  velocity of the object.
• The heavier and more quickly the object is
  moving, the greater the kinetic energy

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• If there is a constant acceleration, and
  therefore a constant net force, and we assume the
  object is originally at rest, then WEk, or work
  is equal to kinetic energy

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Work-Energy Theorem

• Not all objects start at rest.
• They may begin with work exerting on them, giving
  them an original Ek.
• W-E Theorem states
• The change in the kinetic energy of an object is
  equal to the net work done on it.
• Wnet Ekf Eki Ek

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W-E theorem

• If net work is positive then Ek increases.
• Ex Pitch a ball forward, the ball and force are
  in same direction so net work is positive.

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• If net work is negative then Ek decreases.
• Ex catch the ball with mit, the balls motion if
  forward, but the mitt exerts a force opposite to
  the motion on the ball because the ball is slowed
  to zero Ek. The mitt exerts negative work on the
  ball.

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Potential Energy

• As you throw up a block, it loses kinetic energy
  (b/c it is slowing down), however it is gaining
  gravitational potential energy

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Gravitational Potential Energy

• Any time an object is thrown up, it must come
  down. Its downward motion is losing kinetic
  energy, but gaining potential energy
• Epmgh (is only valid if g is constant)
• Enet Ek Ep

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Elastic Potential Energy

• Energy can be stored in bending or stretching
  objects (like springs or elastics)
• k springs constant
• x distance spring is stretched

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Practice

• Pg.221 1-3 (Ek)
• Pg.224 5-7 (Ep)
• Pg. 237 1-4 (Problems)