Work, Power, Energy and Motion

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Work, Power, Energy and Motion
nuclear forces
chemical forces
moving electrons
moving train cars
gravitational force
Electrical force
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Work and Force Go Together Too
Let work be defined as work force
displacement
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Work Done by a Constant Force
In the SI system, the units of work are joules
A complication As long as this person does not
lift or lower the bag of groceries, he is doing
no work on it. The force he exerts has no
component in the direction of motion.
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Work Done by a Constant Force
The work done by a constant force is defined as
the distance moved multiplied by the component of
the force in the direction of displacement
W Fx cos ?
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Hookes Law Lab
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Work on a spring
Work can also be done on a spring when it is
compressed or stretched the figure below shows
work yielding kinetic energy.
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Work on a spring
The force required to compress or stretch a
spring is where k is called the spring
constant, and needs to be measured for each
spring. W ½ kx2
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Concept Question 1
Is it possible to do work on an object that
remains at rest?
1) yes 2) no
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Concept Question 1
Is it possible to do work on an object that
remains at rest?
1) yes 2) no
If there is no displacement, there is no work
done.Work requires that there be a force acting
over a distance.
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Concept Question 2

• 1) Friction does nowork at all
• Friction doesnegative work
• Friction does positive work
• Work is not defined for friction as friction acts
  on the floor and the box

A box is being pulled across a rough floor at a
constant speed. What can you say about the work
done by friction?
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Concept Question 2

• 1) Friction does nowork at all
• Friction doesnegative work
• Friction does positive work
• Work is not defined for friction as friction acts
  on the floor and the box

A box is being pulled across a rough floor at a
constant speed. What can you say about the work
done by friction?
Work is defined for all forces. The friction
acts opposite to the direction of displacement
and is therefore negative here.
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Concept Question 3
Can friction ever do positive work?

• 1) yes
• no

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Concept Question 3
Can friction ever do positive work?

• 1) yes
• no

Consider the book on your car seat. If you
accelerate slowly the book does not slide on the
seat but stays stationary with respect to the
car. Friction is causing the book to move along
with the car and hence force and displacement are
in the same direction. Work is positive.
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Concept Question

• 1) one force
• two forces
• three forces
• four forces
• No forces are doing any work

A box is being pulled up a rough incline by a
rope connected to a pulley. How many forces are
doing work on the box?
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Concept Question

• 1) one force
• two forces
• three forces
• four forces
• No forces are doing any work

A box is being pulled up a rough incline by a
rope connected to a pulley. How many forces are
doing work on the box?
Any force not perpendicular to the motion will do
work N does no work, T does positive work, f
does negative work, mg does negative work
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Power
Power is the rate at which work is done. It can
be thought of as work per second. Power Work /
sec. Power Work / time P W / t
Question Can you name 2 units for power?
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Power
Power has the same trade-offs as work. A motor
produces the same amount of power.
So, you can make a robot thats fast, but weak.
Or you can make a robot thats slow, but
strong. P Fv
The total power in must equal the total power out
(with an exception)... Work / Power Worksheet
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Kinetic Energy and the Work-Energy Principle
Because work and kinetic energy can be equated,
they must have the same units kinetic energy is
measured in joules.
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Kinetic Energy, and the Work-Energy Principle
Energy is defined as the ability to do work.
Mechanical energy is the energy associated with
motion, i.e. masses which move. Sometimes the
motion is ongoing (non-zero velocity) and
sometimes it just involves a change of position
(motion occurred between time t0 and t1).
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Kinetic Energy
Defined
Let work be defined as work force
displacementKinetic energy must work. Hows my
deriving?
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Kinetic Energy, and the Work-Energy Principle
If we write the acceleration in terms of the
velocity and the distance, we define that the
work done here is We define the kinetic energy
(6-2)
The work-energy principle
(6-3)
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Kinetic Energy, and the Work-Energy Principle
We define the work done to be equal to the change
in the kinetic energy
Heres why work and kinetic energy go together

• If the net work is positive, the kinetic energy
  increases.
• If the net work is negative, the kinetic energy
  decreases.CHAPTER 11 5, 17, 36, 38 41

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Concept Question

• 1) quarter as much
• half as much
• they are equal
• twice as much
• four times as much

Two stones, one twice the mass of the other, are
dropped from a cliff. Just before hitting the
ground, what is the kinetic energy of the heavy
stone compared to the light one?
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Concept Question

• 1) quarter as much
• half as much
• they are equal
• twice as much
• four times as much

Two stones, one twice the mass of the other, are
dropped from a cliff. Just before hitting the
ground, what is the kinetic energy of the heavy
stone compared to the light one?
Consider the work done by gravity to make the
stone fall distance d DKE Wnet F d
cos? DKE mg d Thus, the stone with
the greater mass has the greater KE, which is
twice as big for the heavy stone.
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Concept Question
Is it possible for the kinetic energy of an
object to be negative?

• 1) No.
• Yes.
• Maybe.

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Concept Question
Is it possible for the kinetic energy of an
object to be negative?

• 1) No.
• Yes.
• Maybe.

In Newtonian mechanics, the answer is no. The
kinetic energy is always positive since the
velocity squared and the mass are always
positive.Curiously, in quantum mechanics and
relativity there are no such constraints. We
believe it to still be true, but we are open to
the possibility that it may not.