Download Work, Power, and Energy Test Review Review - Fitzmaurice

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Work, Power, and Energy Test Review
Review:
Chapter 8 (textbook)
“Work and Power” PowerPoint presentation
“Energy. Potential and Kinetic Energy” PowerPoint presentation
“Conservation of Energy. Machines” PowerPoint Presentation
“Work and Power Problems” handout
“Work and Power Questions” handout
“Work and Power Problems # 2” handout
“Potential and Kinetic Energy Problems” handout
Potential and Kinetic Energy Bell-work
Energy Bell-work
“Conservation of Energy. Machines Problems” handouts
Conservation of Energy bell-work
Energy Stations Lab
Work.
•
•
•
•
Transfer of energy through motion
Depends on the amount of force and distance
The product of the component of the force exerted on an object in the direction of the
displacement (change in position).
Defined as force times distance.
W = F*d,
where w –work, units J (joules) or N*m
F – force, units N (Newtons)
D – distance, units m (meters)
Compare Work vs. Impulse.
Similarities:
• both have a force component exerted on an object;
• both answer the question “how long” a force acts.
Differences:
• work describes “how long” a force acts in terms of distance;
• Impulse describes “how long” a force acts in terms of time.
• Work = Force * distance W = F*d
• Impulse = Force * time
I = F*t
Problem #1.
Calculate the work done when you lift a 12 N crate a distance of 2.0 m.
d = 2.0 m W = F * d W = 12N * 2.0m W = 24 J
F = 12N
W-?
sProblem #2.
Calculate the force needed to pick up the pumpkin 470 cm if the total work done is 155 000 J.
d = 470cm
F=
F=
F = 32979 N
W = 155 000 J
F-?
Convert 470 cm to meters: 470 cm = 4.70 m
Problem #3.
What is the total amount of work done if a 54 kg puppy runs 230 meters?
d = 230m
w = m*g
w = 54 kg * 9.8
m = 54 kg
W-?
W = F*d
W = 530 N * 230 N
Power.
•
•
•
How fast work is done
The rate at which work is done
Formula: P =
Units for Power are watts (W)
Units for Work are Joules (J)
Units for time are seconds (s)
SI unit for power is Watt (W) = one Joule per second ( ).
= 530 N
W = 121900 J
One kW (kilowatt) = 1000W (Watts).
In the US we rate engines in units of horsepower.
1hp = 0.75 kW=750 W
Problem #4.
•
•
If a forklift is replaced with a new forklift that has twice the power, how much greater a
load can it lift in the same amount of time? If it lifts the same load, how much faster can
it operate?
Twice the amount of power will lift twice the load in the same time or the same load in
half time.
Problem # 5.
Susan slowly climbs the stairs at a rate of 10.0 s. How much power does she generate if the
work required to climb the stairs is 2750 J.
W = 2750 J
P=
P=
P = 280 W
t = 10.0 s
P-?
Problem # 6.
Convert 75 hp to Watt. Remember that 1 hp (horsepower) = 0.75 kW and 1 kW = 1000 W.
75 hp
0.75 kW 1000 W
1 hp
1 kW
= 56250 W
or
= 56250 W
Formula Organizer.
Formula
Work: W = F * d
Power: P =
Weight: w = m*g
Variables
W – work, F – force, d - distance
P – power, W – work, t - time
w – weight, m –mass,
g –gravitational acceleration
Units
Joule (J) or N*m
Watt (W) or
Newton (N) or
Potential and Kinetic Energy.
Mechanical Energy: the energy due to the position of something, or the movement of something.
Two main forms of mechanical energy:
Potential and Kinetic Energy.
Potential Energy is the energy of position.
Kinetic Energy is the energy of motion.
Potential Energy (PE): the energy that is stored.
It has the potential for doing work. Potential energy is found in fossil fuel, electric batteries, and the
food we eat.
Work is required to elevate objects against Earth’s gravity. The potential energy due to elevated position
is called gravitational potential energy.
Gravitational Potential Energy = weight x height
PE = m*g*h
a) The amount of potential energy possessed by an elevated object is equal to the work done in
lifting it.
Kinetic Energy (KE): the energy of motion
The kinetic energy of an object depends on the mass of the object and its speed.
Kinetic energy of an object is equal to one-half the product of its mass times its speed.
Kinetic Energy (KE) = mass x speed
KE = m*v²
The kinetic energy of a moving object is equal to the work required to bring it to that speed from rest, or
the work the object can do while being brought to rest.
Net Force x Distance = Kinetic Energy
F*d = m*v²
Work-Energy Theorem: whenever work is done, energy changes.
Work done equals the change in energy.
Work = ∆E
Work changes kinetic energy. If no change in energy occurs, then no work is done.
Problem #1.
If you do 100 J work to elevate a bucket of water, what is its gravitational potential energy relative to its
starting position?
100 J because the work you do goes into increasing the gravitational potential energy.
Problem #2.
If you do 100 J work to elevate a bucket of water, what would the gravitational potential energy be if the
bucket was raised twice as high?
PE = m*g*h, so twice the height means twice the energy.
Or W = F*d, so twice the distance means twice the work means twice the energy.
200 J
Conservation of Energy.
Energy: the ability to do work.
The Law of Conservation of Energy: Energy cannot be created or destroyed. It can be transformed from
one form to another, but the total amount energy never changes. It transforms without net loss or net
gain.
Example: Newton’s Cradle.
Example: Pendulum.
When a pendulum swings back and forth, the energy is being transformed from potential to kinetic,
back to potential, etc. At the lowest point of its motion, kinetic energy is maximum and potential energy
is minimum. The acceleration is a maximum at the end points of the swing, and a minimum (zero) at the
lowest point. Pendulum will eventually come to rest due to things like friction (air resistance, rubbing at
the pivot, etc.). If I stop the pendulum, or when I start it by giving it a push or pulling it back, I change
the energy.
KE + PE + W
i
i
= KE + PE
ext
f
f
(with external force)
KE + PE = KE + PE (without external force)
i
i
f
f
After rolling halfway down an incline an object’s kinetic energy is the same as its potential energy.
Example.