Notes
Slide Show
Outline
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Kinetic Energy and Work
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Energy
  • Types of Energy?
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Work
  • Work is energy transferred to or from an object by means of a force acting on the object. Energy transferred to the object is positive work, and energy transferred from the object is negative work.
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Work and Kinetic Energy
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Work- Kinetic Energy Theorem
  • Kf – Ki = Δ K = W
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"In the figure"
  • In the figure: Two men slide an initially stationary safe  of mass m=225 kg on a frictionless floor. F1 =12.0 N and F2 = 10.0 N
  • What is the net work done on the safe by F1 and F2
  •          during the displacement d.
  • during the displacement d, what is the work done by the weight of the safe and the normal force?
  • What is the final speed of the safe if d= 8.50 m?


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"In the figure"
  • In the figure: a box moves on a frictionless surface through a displacement
  • d = -3 i m. The wind is acting on the box by force F = (2 i – 6 j) N,
  • a) How much work does F do during the displacement?
  • b) If the box has a kinetic energy of 10 J at the beginning of the displacement, what is the kinetic energy at the end.
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Work done by a Gravitational Force
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"An initially stationary crate in..."
  • An initially stationary crate in is pulled, via a cable, a distance L = 5.70 m
  • up a frictionless ramp, to a height h of 2.50 m where it stops.


  • a) How much work Wg is done on the crate by the gravitational force Fg during the lift?
  • b) How much work WT is done on the crate by the force T from the cable during the lift?
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"An elevator cab of mass..."
  • An elevator cab of mass m=500kg Is descending with speed vi = 4.0 m/s when
  • Its supporting cable begins to slip, allowing it to fall with constant acceleration
  • a = g/5


  • a) During the fall through a distance d=12 m, what is the work Wg done on the cab by the gravitational force Fg?
  • b) During the 12 m fall, what is the work WT done on the cab by upward pull T of the elevator cable?
  • c) What is the net work W done on the cab during the fall?
  • d) What is the cab kinetic energy at the end of the 12m fall?
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Work Done by a Spring Force
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"In the figure the floor..."
  • In the figure the floor is frictionless and the applied
  • Force Fa = 4.9 N needed to hold the block stationary
  • at x1 = 12 mm.


  • How much work does the spring force do on the block if the block is pulled rightward from x0 = 0 to x2 = 17 mm?
  • Next the block is moved leftward to x3 = -12 mm. How much work does the spring force do on the block during the displacement? Explain the sign of this work.
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"A block of mass m..."
  • A block of mass m = 0.40 kg slides on a horizontal
  • frictionless table with a speed v = 0.50 m/s. It then runs
  • into and compresses a spring of spring constant
  • k = 750 N/m. When the block is momentarily stopped by
  • the spring, by what distance d is the spring compressed?
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Work Done by a General Force
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Power
  •      Pave = W/Δt     (average power)
  •      P = dW/dt ( instantaneous power)
  •          1 watt = 1 W = 1 J/s
  •       1 kW-h   (unit of work)
  •                         P = dW/dt
  •                            = F cos Φ ds /dt
  •                            = F v cos Φ
  •                               = F . v
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"F1 and F2 constant forces..."
  • F1 and F2 constant forces acting on the box as the box slides rightward across a frictionless floor. F1 is horizontal with magnitude 2.0 N; F2 is applied upward by 60o to the floor and has magnitude 4.0 N. The speed v of the box at certain instant is 3.0 m/s.
  • What is the power due to each force acting on the box at the instant and what is the net power? Is the net power changing at that instant
  • If the magnitude of F2 is, instead, 6.0 N, what is the net power, and is it changing?
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