Physics 2211a Kulp Class 32: Where does energy go

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Physics 2211a (Kulp)Class 32 Where does
energy go?

• 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 (conservative systems)

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Where are we?

• Last class Class 30 (work)

• Today Class 32
• Analyze a situation in which the mechanical
  energy of an object is changed by a dissipative
  force (e.g., friction) or by an external force.
• Calculate the power required to maintain constant
  acceleration of an object.
• Determine the work performed by a force that
  supplies constant power, or the average power
  supplied by a force that performs a specified
  amount of work.

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The energy model
K
U

• For an isolated system with only conservative
  forces, energy is transformed from potential to
  kinetic and back again.

• If dissipative forces are present (e.g., friction
  or drag), mechanical energy is transformed into
  thermal energy.

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The energy model
K
U
Eth

• For a system which interacts with the
  environment, we can write the energy equation

• Identify objects in the system (and external
  forces)
• Draw before and after representations (and
  during)
• Use the energy equation (above)
• Assess results

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before

• Pushing a block across a rough table at constant
  speed.

• What objects are included in the system?
• block and table
• List conservative, nonconservative, external
  forces.
• gravity, friction, push
• Specify the sign of ?K, ?U, Wdiss, and Wext.
• ?K 0, ?U 0, Wdiss lt 0, Wext gt 0
• Describe energy transformations which occur.
• there are no internal transformation of energy
• Describe energy transfers that take place.
• Wext ? Wdiss (Eth gt 0)

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before

• The block slides across a rough table and stops.

• What objects are included in the system?
• block and table
• List conservative, nonconservative, external
  forces.
• gravity, friction, none
• Specify the sign of ?K, ?U, Wdiss, and Wext.
• ?K lt 0, ?U 0, Wdiss lt 0, Wext 0
• Describe energy transformations which occur.
• K ? Wdiss (Eth gt 0)
• Describe energy transfers that take place.
• none

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• Picking up and placing a block on a high ledge.

• What objects are included in the system?
• block and earth
• List conservative, nonconservative, external
  forces.
• gravity, none, lifting force
• Specify the sign of ?K, ?U, Wdiss, and Wext.
• ?K 0, ?U gt 0, Wdiss 0, Wext gt 0
• Describe energy transformations which occur.
• none
• Describe energy transfers that take place.
• Wext ? U

during force lifts block
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Two parts! - Part I

• Tossing a block straight up.

• What objects are included in the system?
• block and earth
• List conservative, nonconservative, external
  forces.
• gravity, none, throw
• Specify the sign of ?K, ?U, Wdiss, and Wext.
• ?K gt 0, ?U gt 0, Wdiss 0, Wext gt 0
• Describe energy transformations which occur.
• none
• Describe energy transfers that take place.
• Wext ? K, U

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Part II

• Tossing a block straight up.

• What objects are included in the system?
• block and earth
• List conservative, nonconservative, external
  forces.
• gravity, none, throw
• Specify the sign of ?K, ?U, Wdiss, and Wext.
• ?K lt 0, ?U gt 0, Wdiss 0, Wext 0
• Describe energy transformations which occur.
• K ? U
• Describe energy transfers that take place.
• none

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Power

• Power is the rate of change of energy for a
  system, i.e., the rate of doing work.

• SI units for power are the Watt, where 1 W 1
  J/s.
• For a constant force, we can write

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A flying leap

• How much power must a 100-kg athlete generate for
  a high jump of 220 cm? Assume g 10 m/s2 and
  0.2 s contact time during the leap.

• P (100)(10)(2.20)/(0.2)
• (100)(10)(1.10)(10)
• 11,000 W
• 15 hp (outboard motor)

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Whats next?

• Friday, 11/10 Class 34 What is your weight on
  the moon?
• Using Newton's Law of Gravity, determine the
  force that one mass exerts on another, the mass
  of an object, or the motion of one object from
  known quantities.
• Determine the acceleration due to gravity at a
  point outside of a spherical mass.

• To-do list
• Homework 31 before class on Friday
• Read assignment (12.1 - 12.4)

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• Suppose you drop a 1-kg rock from a height of 5 m
  above the ground. When it hits, how much force
  does the rock exert on the ground?
• 0.2 N
• 5 N
• 50 N
• 100 N
• Impossible to determine

• While we can calculate the work done to stop the
  rock, we need to know how much the ground
  compresses to determine the force required.

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Presto!

• When you pull a tablecloth out from under a set
  of dishes, its important to pull the cloth as
  fast as possible because
• the force of sliding friction that the cloth
  exerts on the dishes is proportional to the time
  during which the cloth is moving.
• the work done on the dishes by the cloth is
  proportional to the time during which the cloth
  is moving.
• The weight of the dishes on the cloth is
  proportional to the time during which the cloth
  pulls on them.
• The momentum transferred to the dishes is
  proportional to the time during which the cloth
  pulls on them.

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• A spring-loaded toy dart gun is used to shoot a
  dart straight up in the air, and the dart reaches
  a maximum height of 24 m. The same dart is shot
  straight up a second time, but this time the
  spring is compressed only half as far before
  firing. How far up does the dart go this time?
  Neglect friction and assume an ideal spring.

• 96 m
• 48 m
• 24 m
• 12 m
• 6 m
• 3 m
• Impossible to determine