Transcript Example Chapter 5 Work and Energy

Section 1
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The product of the force on an object and the
distance through which the object is moved
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Measured in joules(J)
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Joule = N x m
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Equation:
W = Force x distance
Work is done only when components of a
force are parallel to a displacement
If the force is at an angle to the displacement
use the equation:
W= FdcosΘ
A person lifts a 4.5 kg block a vertical distance
of 1.2 m. Determine the work done by the
person.
When catching a baseball, a catcher’s glove
moves by 10 cm along the line of motion of
the ball. If the baseball exerts a force of 475
N on the glove, how much work is done by
the ball?
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How much work is done on a vacuum cleaner
pulled 3.0 m by a force of 50.0 N at an angle
of 30.0° above the horizontal?
Section 2
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The energy of an object that is due to the
object’s motion
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Measured in joules (J)
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Kinetic Energy= ½ x mass x (velocity)²
Calculate the speed of an 8.0x10⁴ kg airliner
with a kinetic energy of 1.1x10⁹ J.
Two 3.0 g bullets are fired with speeds of 40.0
m/s and 80.0 m/s, respectively. What are
their kinetic energies? Which bullet has more
kinetic energy?
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The net work done by all the forces acting on
an object is equal to the change in the
object’s kinetic energy
Equation:
◦ Net Work = ΔKE
On a frozen pond, a person kicks a 10.0 kg
sled, giving it an initial speed of 2.2 m/s.
How far does the sled move if the coefficient
of kinetic friction between the sled and the
ice is 0.10?
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Any object that is at rest has this
SI unit:
Joule, J
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The energy associated with an object due to the
object’s position relative to gravity
PEg=mass X acceleration due to gravity X height
A spoon is raised 21.0 cm above a table. If the
spoon and its contents have a mass of 30.0
g, what is the gravitational potential energy
associated with the spoon at that height
relative to the surface of the table?
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The energy available for use when a deformed
elastic object returns to its original configuration
Ep= ½ X spring constant X (distance
compressed or
stretched)²
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Represented by “k”
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Is also called the force constant
A spring with a force constant of 5.2 N/m has
a relaxed length of 2.45 m. When a mass is
attached to the end of the spring and allowed
to come to rest, the vertical length of the
spring is 3.57 m. Calculate the elastic
potential energy stored in the spring.
The staples inside a stapler are kept in place by
a spring with a relaxed length of 0.115 m. If
the spring constant is 51.0 N/m, how much
elastic potential energy is stored in the spring
when its length is 0.150 m?
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There is also chemical potential energy
This deals with the energy found in the food
you eat
In one food calorie there are 4.186 J of
chemical potential energy
Section 3
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The sum of kinetic energy and all forms of
potential energy
Mechanical Energy (ME) = Kinetic Energy (KE) +
Potential Energy (PE)
 ME = KE + ∑PE
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Energy cannot be created or destroyed. It can
be transformed from one form into another,
but the total amount of energy never
changes.
Conservation of Mechanical Energy
 MEi= MEf
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MEi = MEf
(Equation for conservation
of mechanical energy)
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ME= KE + PE
(Equation for mechanical
energy)
KE= ½ mV²
PE = mgh
(Equation for Kinetic Energy)
(Equation for Potential
Energy)
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Therefore;
½ mVi² + mghi = ½ mVf² + mghf
Starting from rest, a child zooms down a
frictionless slide from an initial height of 3.00
m. What is her speed at the bottom of the
slide? Assume she has a mass of 25.0 kg.
A 755 N diver drops from a board 10.0 m
above the water’s surface. Find the diver’s
speed 5.00 m above the water’s surface.
Then find the diver’s speed just before
striking the water.
Section 4
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Measures the rate at which work is done or
energy is transformed
Unit: watt, W
P= W
Δt

Work
Time interval
What is the average power produced by a
steam engine that does 6.8 J of work in 3.6
seconds?
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Equation:
P= FV 
Power = force X speed
Given:
m= 19 kg
V= 2.2 m/s
Solve for power.
A motor-driven winch pulls a 50.0 kg student
5.00 m up the rope at a constant speed of
1.25 m/s. How much power does the motor
use in raising the student? How much work
does the motor do on the student?