Transcript Work, Energy, and Power

Work, Energy, and
Power
Identify each of the situations below
that represent work being done.
A book in free fall
 Pushing on a wall
 Lifting a box
 Carrying a box across the room
 Pushing a box across the room
 Holding a weight over your head
 Reading a book
 Writing notes

Work
When a force acts upon an object to
cause a displacement of the object, it is
said that work was done upon the object.
 There are three key components to work
- force, displacement, and cause.
 In order for a force to qualify as having
done work on an object, there must be a
displacement and the force must cause
the displacement. (the force must be in
the same direction as the displacement)

Identify each of the situations below
that represent work being done.
A book in free fall
 Pushing on a wall
 Lifting a box
 Carrying a box across the room
 Pushing a box across the room
 Holding a weight over your head
 Reading a book
 Writing notes

Vector or Scalar?
Work is a vector quantity- it has a
magnitude and a direction.
 Since work is a vector quantity, it can
have a negative value.
 Negative work occurs when the force
doesn't cause the displacement but rather
hinders it or when the force moves the
object in the negative direction.

Calculating Work
The SI unit for work is the joule, J.
 A joule is equal to a force of 1 newton applied
over a distance of 1 meter, or
1 newton-meter.
 Formula: work = force x distance
 W=Fxd

Sample Problem #1
How much work is done by a person who
uses a force of 27.5N to move a grocery
buggy 12.3m?
 W = 338 J

Sample Problem #2
55, 000J of work is done to move a rock
25m. How much force was applied?
 F = 2200 J

Sample Problem #3
You and 3 friends apply a combined force
of 489.5N to push a piano. The amount of
work done is 1762.2J. What distance did
the piano move?
 d = 3.6 m

Sample Problem #4

John pulls a 4.5 kg sled across level snow
with a force of 225 N on a rope that is 35o
above the horizontal. If the sled moves a
distance of 65.3 m, how much work does
John do?
Sample Problem #5

Sharon is pushing a lawn mower with a
force of 88.0 N along a handle that makes
an angle of 41.0o with the horizontal.
How much work is done in moving the
lawn mower 1.2 km to mow the lawn?
Sample problem #6
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A 4200 N piano is to be slid up a 3.5 m
frictionless ramp at a constant speed. The
plank makes an angle of 30.0o with the
horizontal. Calculate the work done by
the person sliding the piano up the ramp.
Sample Problem #7
Marcus slides a 60.0 kg crate up a ramp that is
2.0 m long and attached to a platform 1.0 m
above the floor level. A 400.0 N force, parallel
to the ramp, is needed to slide the crate up the
ramp at a constant speed.
a) How much work does Marcus do in sliding the
crate up the ramp?
b) How much work would be done if Marcus simply
lifted the crate straight up from the floor to the
platform?

Power
Power is the rate of doing work.
 The SI unit for power is the watt, W.
 A watt is equal to 1 Joule/second.
 Formula: power = work / time
P=W/t

REVIEW
In physics, work is defined as
force
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times time
divided by time
times distance
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The unit of work is the
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Joule
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___ is the rate at which work is
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Power
Energy
Force
Friction
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The unit of power is the
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When we carry an object across a room,
without lifting it or setting it down, we do no
physical work on it.
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2. False
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How much work is done on a 10 N
block that is lifted 5 meters off the
ground?
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2J
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You did 170 J of work lifting a 140 N
backpack. How high did you lift it?
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30 m
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1.2 m
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You lift a 45 N bag of mulch 1.2 m and carry
it a distance of 10 m to the garden. How
much work was done?
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4. 540 J
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A mouse carries a 1.0 g ant on his back
across the floor for a distance of 10 m.
How much work was done?
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1. 10 J
2. 98 J
3. 0.098 J
4. None of the above
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What power is expended if you
lift a 10 kg boulder 1 m in 2 s?
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Is work a scalar or vector
quantity?
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Vector
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It is not possible for a person to
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False
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Energy
Energy is the ability to produce change in
itself or the environment.
 The energy can take many forms including
thermal, chemical, energy of motion.
 When the position of a moving object
changes over time, the change in position
indicates that the object has energy. The
energy resulting from a change in motion
is called kinetic energy.
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Kinetic Energy
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The equation for kinetic energy is:
KE = ½ mv2
m is the mass of the object and v is
the velocity of the object.
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What is the relationship between kinetic
energy and velocity? Between kinetic
energy and mass?
Work
Work is the process of changing the energy of
the system.
 When work is done on an object, a change in
kinetic energy results.
 Two conditions must be met for work to occur.
a. The object must move through a distance
b. A force must act upon the object in the
direction the object moves.
