Download Time for Work

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Describing Motion

Position (x), Velocity (v), and Acceleration (a)

An object continues in its state of motion until acted on by an
outside force (Newton’s First Law).

Acceleration is equal to the outside force applied divided by
the inertia of the object (Newton’s Second Law).

An object’s change in momentum equals the outside force
multiplied by the duration of the force (Impulse).

An isolated system’s momentum does not change, so
momentum can be transferred or shared between objects
(conservation of momentum).
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Time for Work
Work in Physics
https://youtu.be/RSssClMtrx0
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What Is “Work” In Physics?
 Work
is done when a net force acts on
an object over some distance.
 Work
is the product of the force on an
object and the distance through which the
object is moved.
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Impulse vs. Work
Impulse
 Impulse describes a
force applied over
some time.
Work
 Work describes a
force applied over
some distance.
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Doing Work
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Working hard or hardly working?
 Work
is done lifting the barbell.
 If
the barbell was lifted twice as
high, it would take twice as much
work.
 How
much work does the body
builder do when he holds the
barbell above his head?
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Working hard or hardly working?
 Work
is done lifting the barbell.
 If
the barbell was lifted twice as
high, it would take twice as much
work.
 Work
is not done if the barbell
does not move.
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Team Questions: Is work done on the
object in the following situations?
1.
A teacher applies a force to a wall
and becomes exhausted.
2.
A book falls off a table and free falls
to the ground.
3.
A rocket accelerates through space.
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Units for Work
 The
unit for work is the Newton-meter (N•m), also
called the joule (J).
 One
joule (J) of work is done when a force of 1
Newton is exerted over a distance of 1 m (e.g.,
lifting an apple over your head).
Suppose that you apply a 60-N horizontal force to a 32-kg
package, which pushes it 4 meters across a mailroom
floor. How much work do you do on the package?
Suppose that you apply a 60-N horizontal force to a 32-kg
package, which pushes it 4 meters across a mailroom
floor. How much work do you do on the package?
What would happen to the amount of work if:
(a) the distance applied doubled?
(b) the magnitude of the force tripled?
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Work Changes An Object’s “Energy”
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Work Changes An Object’s “Energy”

When work is done by an archer in drawing back a
bowstring, the bent bow acquires the ability to do work on
the arrow.
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Work Changes An Object’s “Energy”

When work is done by an archer in drawing
back a bowstring, the bent bow acquires the
ability to do work on the arrow.

When work is done to raise the heavy ram of
a pile driver, the ram acquires the ability to
do work on the object it hits when it falls.
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Work Changes An Object’s “Energy”

When work is done by an archer in drawing back a
bowstring, the bent bow acquires the ability to do work on
the arrow.

When work is done to raise the heavy ram of a pile driver,
the ram acquires the ability to do work on the object it hits
when it falls.

When work is done to wind a spring mechanism, the spring
acquires the ability to do work on various gears to run a
clock, ring a bell, or sound an alarm.
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Work Changes An Object’s “Energy”
 In
these examples, something is acquired by the
objects that gives them the ability to do more
work.
 The
property of an object or system that enables it
to do work is called energy.



Like work, energy is measured in joules.
Energy due to an object’s motion or position is known as
mechanical energy.
Just like an impulse changes the momentum of a
system, doing work on a system changes the system’s
energy.
+ Work is force over a distance and it changes
an object’s mechanical energy.