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Physics 101: Lecture 9 Work and Kinetic Energy

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Title: Physics 101: Lecture 9 Work and Kinetic Energy


1
Physics 101 Lecture 9Work and Kinetic Energy
  • Todays lecture will be on Textbook Sections 6.1
    - 6.5
  • Work
  • W F s F s Cosq
  • Kinetic Energy
  • K ½mv2
  • DK W

2
Energy is Conserved
  • Energy is Conserved meaning it can not be
    created nor destroyed
  • Can change form
  • Can be transferred
  • Total Energy does not change with time.

3
Energy
  • Forms
  • Kinetic Energy
  • Potential Energy
  • Heat later
  • Mass (Emc2) p102
  • Units Joules kg m2 / s2

4
Work Energy Transfer due to Force
  • Force to lift trunk at constant speed
  • Case a
  • Case b
  • But in case b, trunk only moves ___ distance you
    pull rope.

5
Work by Constant Force
  • Only component of force parallel to direction of
    motion does work!
  • W F Dr cos q

F
q
F
WF
Dr
F
WF
Dr
F
WF
Dr
Dr
WF
F
6
Work by Constant Force
  • Example You pull a 30 N chest 5 meters across
    the floor at a constant speed by applying a force
    of 50 N at an angle of 30 degrees. How much work
    have you (Tension) done?

W F Dr cos q
50 N
30
7
Where did the energy go?
  • Example You pull a 30 N chest 5 meters across
    the floor at a constant speed, by applying a
    force of 50 N at an angle of 30 degrees.
  • How much work did gravity do?
  • How much work did friction do?

W F Dr cos q
8
Preflight 1 2
  • You are towing a car up a hill with constant
    velocity. The work done on the car by the normal
    force is
  • 1. positive2. negative3. zero

9
Preflight 3 4
  • You are towing a car up a hill with constant
    velocity. The work done on the car by the
    gravitational force is
  • 1. positive2. negative3. zero

10
Preflight 5 6
  • You are towing a car up a hill with constant
    velocity. The work done on the car by the
    tension force is
  • 1. positive2. negative3. zero

11
Kinetic Energy Motion
  • Apply constant force along x-direction to a point
    particle m.
  • W Fx Dx
  • Work changes ½ m v2
  • Define Kinetic Energy K ½ m v2
  • W D K For Point Particles

12
Preflight 7 8
  • You are towing a car up a hill with constant
    velocity. The total work done on the car by all
    forces is
  • 1. positive2. negative3. zero

13
Example Block w/ friction
  • A block is sliding on a surface with an initial
    speed of 5 m/s. If the coefficent of kinetic
    friction between the block and table is 0.4, how
    far does the block travel before stopping?

5 m/s
14
Work by Variable Force
  • W Fx Dx
  • Work is area under F vs x plot
  • Spring F k x
  • Area ½ k x2

Force
Distance
Force
Fkx
Distance
15
Summary
  • Energy is Conserved
  • Work transfer of energy using force
  • Can be positive, negative or zero
  • W F d cos(q)
  • Kinetic Energy (Motion)
  • K ½ m v2
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