Energy and Work

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Chapter 10

• Energy and Work

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10.1 Energy and Work

• Energy is the ability to produce a change.
• Forms of Energy Thermal, chemical, electrical,
  nuclear, molecular, and mechanical (K.E P.E),
  solar.
• Kinetic Energy is the energy of an object
  resulting from motion.

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• An object of mass m, moving at speed v0. if you
  apply a force, F, to it to accelerate it to a new
  speed, v1. The motion equation that describes
  this situation is
• v12 v02 2ad
• Add a negative v02 to both sides.
• v12 - v02 2ad, Using F/m a.
• v12 - v02 2Fd/m, multiply both sides by 1/2m.
• 1/2mv12 1/2mv02 Fd
• Describe the energy of the system that results
  from motion, and is represented by the symbol K,
  for kinetic.

Kinetic Energy K 1/2mv2
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• The process of changing the energy of the system
  is called work, and is represented by the symbol
  W.
• Substituting K and W into the equation
• K1 K0 W
• When work is done on an object, a change in
  kinetic energy results.
• Joule (J) is a unit of energy.

Work W Fd
Work-Energy Theorem ? K W
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Example Problem

• An 875-kg compact car speeds up from 22.0 to 44.0
  m/s while passing another car. What were its
  initial and final energies, and how much work was
  done on the car to increase its speed?

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• Work
• Through the process of doing work, energy can
  move between the environment and the system.
• If the environment does work on the system, then
  Wgt 0, and the energy of the system increases.
• If the system does work on the environment, then
  Wlt0, and the energy of the system decreases.

Energy transfer
Environment System
Work
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A golfer uses a club and gets a stationary golf
ball moving when he or she hits the ball. The
club does work on the golf ball as it strikes the
ball. Energy leaves the club and enters the ball.
This is a transfer of energy. Thus, we say that
the club did work on the ball.
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• Calculating Work
• W Fd holds only for constant forces exerted in
  the direction of the motion.
• If the orbit is circular, as in the motion of
  planets around the sun, the force (of gravity) is
  always perpendicular to the direction of motion
• A perpendicular force to the direction of motion,
  does not change the speed of the object, only its
  direction.

V
Planet
F
sun
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• The speed of planets doesnt change, ?K0 and
  thus W 0.
• If F and d are at right angles, then W 0. Unit
  of Work is also Joules.
• An apple weighs about 1N. When you lift an apple
  a distance of 1 m, you do 1 J of work on it.
• Example Problem
• Calculating Work
• A 105-g hockey puck is sliding across the ice. A
  player exerts a constant 4.5-N force over a
  distance of 0.15 m. How much work does the
  player do on the puck? What is the change in the
  pucks energy?

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• Finding Work done When forces change
• A graph of the force versus the displacement lets
  you determine the work done by a changing force.
  
• The graph shows the force exerted by a spring,
  which varies linearly from 0 to 20N as it is
  compressed 1.5 m.

• The work done by the force that compressed the
  spring is the area under the curve, which is a
  triangle. Work Area 1/2basealtitude
  1/2201.5 15 J.

F(N)
20
Area
d (m)
0
0
1.5