Physics 2211a Kulp Class 30: How is energy transferred

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Physics 2211a (Kulp)Class 30 How is energy
transferred?

• W. D. Kulp
• william.kulp at physics.gatech.edu
• Office W501b
• Office Hours MF 11-12 AM, Tu 9-10 AM, by appt.
• www.physics.gatech.edu/academics/Classes/fall200
  6/2211/a/

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Where were we?

• Last class Class 29 (collisions)

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Where are we?

• Last class Class 29 (collisions)

• Today Class 30
• Calculate analytically or graphically the work
  done on an object.
• Determine the change in kinetic energy or motion
  that results from performing an amount of work
  on an object.
• Use the scalar (dot) product operation to
  calculate the work done on an object.
• Given an object with a change in speed or kinetic
  energy, calculate the work performed by the net
  force (or by each of the component forces).
• Apply the work-energy theorem to determine the
  change in an objects kinetic energy and analyze
  the motion of the object.

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Energy and Newtons Laws

• From Newtons second law, using the chain rule
  of calculus, we found expressions for gravity and
  springs

• If we consider this process generally, we find

Well focus on calculating work, but remember
energy!
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The Work-Kinetic Energy Theorem

• A cart attached by a string to a hanging mass is
  released from rest. Assume the string and pulley
  are massless.

• Does ?K increase or decrease?
• increases
• What is the sign of ?K?
• positive
• What forces act on the cart?
• tension, normal force, gravity, friction
• For each force is the work positive, negative,
  or zero?
• positive, zero, zero, negative
• Is the Wnet positive, negative, or zero? Does
  this agree with ?K?
• positive, yes

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The Work-Kinetic Energy Theorem

• A cart attached by a string to a hanging mass is
  pushed away from the hanging mass. The string
  and pulley are massless.

• Does ?K increase or decrease?
• decrease
• What is the sign of ?K?
• negative
• What forces act on the cart?
• tension, normal force, gravity
• For each force is the work positive, negative,
  or zero?
• negative, zero, zero
• Is the Wnet positive, negative, or zero? Does
  this agree with ?K?
• negative, yes

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The Work-Kinetic Energy Theorem

• Consider an object sliding or rolling down a
  ramp.
• Neglect the dissipative effect of friction.

• Does ?K increase or decrease?
• increase
• What is the sign of ?K?
• positive
• What forces act on the object?
• normal force, gravity
• For each force is the work positive, negative,
  or zero?
• zero, positive
• Is the Wnet positive, negative, or zero? Does
  this agree with ?K?
• positive, yes

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The Work-Kinetic Energy Theorem

• An object is pushed up a ramp.
• Neglect the dissipative effect of friction.

• Does ?K increase or decrease?
• decrease
• What is the sign of ?K?
• negative
• What forces act on the cart?
• normal force, gravity
• For each force is the work positive, negative,
  or zero?
• zero, negative
• Is the Wnet positive, negative, or zero? Does
  this agree with ?K?
• negative, yes

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The Work-Kinetic Energy Theorem

• A ball attached to a string is moving in a
  horizontal circle at a constant speed.

• What forces act on the ball?
• tension, gravity
• For each force is the work positive, negative,
  or zero?
• zero, zero

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The scalar (dot) product

• We found that work is not just force times
  distance from analyzing a ball in a circular
  orbit. We need the component of force in the
  direction of the displacement.

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(Newtonian method)

• A 200 g ball is lifted upward on a string. It
  goes from rest to a speed of 2.0 m/s in a
  distance of 1.0 m. What is the tension (assumed
  to stay constant) in the string?

• T (0.2)(4/2 9.8) 2.4 N

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(Work-energy method)

• A 200 g ball is lifted upward on a string. It
  goes from rest to a speed of 2.0 m/s in a
  distance of 1.0 m. What is the tension (assumed
  to stay constant) in the string?

• T (0.2)(4/2 9.8) 2.4 N

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to the rescue!

• A 1000 kg car is rolling slowly across a level
  surface at 1.0 m/s, heading toward a group of
  small children. The doors are locked, so you
  cant get inside to use the brakes. Instead, you
  run in front of the car and push on the hood at
  an angle of 30 below horizontal. How hard must
  you push to stop the car in a distance of 2.0 m?

-290 N
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• A 1.0 kg block moves along the x-axis, passing x
  0 m with velocity 2.0 m/s. It is then
  subjected to the force on the graph.

• Which of the following is true
• The block gets to x 5 m with a speed greater
  than 2.0 m/s.
• The block gets to x 5 m with a speed of exactly
  2.0 m/s.
• The block gets to x 5 m with a speed less than
  2.0 m/s
• The block never gets to x 5 m.

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• A 1.0 kg block moves along the x-axis, passing x
  0 m with velocity 2.0 m/s. It is then
  subjected to the force on the graph.

• Calculate the blocks speed at x5 m.
• 2.2 m/s
• 2.5 m/s.
• 3.2 m/s
• 3.7 m/s

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Whats next?

• Friday, 11/3 Class 31 When is energy conserved?
• Analyze a physical situation using the
  connections among force, work, and energy.
• Determine the change in mechanical energy due to
  a nonconservative force.
• Calculate the force associated with a potential
  energy.

• To-do list
• Homework 29 before class on Friday
• Read assignment (11.5 - 11.6)

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• A child throws a tennis ball at 12 m/s directly
  at the front of a school bus which is moving at
  35 m/s towards him. If the collision is elastic,
  find the speed with which the ball bounces from
  the bus.
• 82 m/s
• 59 m/s
• 58 m/s
• 47 m/s

v2 12 m/s
v1 35 m/s
V
m1 gtgt m2
V
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• When you do positive work on a particle, its
  kinetic energy
• increases.
• decreases.
• remains the same.
• need more information about the way the work was
  done

• What does it mean when work is negative?