Download Notes for Work and Energy

Work and Energy (Ch. 9)
Conceptual Physics
Work: A force applied over a distance that causes an object to gain or lose energy.
When work is done to an object, the energy of the object may change. Work can be positive or
negative. Work is measured in Joules (J).
W=Fd
W = Δ ET
Is work done when Jill holds her cat?
1. How much work is done by the film students who apply a force of of 100 N for a distance of .75
km (750 m) to roll a prop for a movie?
2. A book moving at 1m/s has 10 J of energy. It slides to a stop over a distance of 3 m. When it stops
moving to has no energy. What caused the book to stop moving? What size force did friction apply?
What is needed to perform work?
Conservation of Energy states that….
Energy: Energy is the ability to do work. Energy is an abstract quantity of extreme usefulness. An
object gains or loses energy when work is done. The unit for both Energy and work is the Joule (J).
Different forms of Energy:
Objects can have different kinds of energy. In this unit, we will concentrate on the Mechanical Energy of
the objects.
Kinds of Mechanical Energy:

Potential Energy (PE)
Gravitational Potential Energy (PEg)
Spring Potential Energy (PEs)
 Kinetic Energy (KE)
Total Mechanical Energy (ET)
An object can have PEg, PEs, and KE as part of it's total Mechanical Energy. When does an object
have mechanical energy?
 Objects have energy when they have HEIGHT,

if they are MOVING, or

if they could SPRING BACK TO SHAPE
(like a spring, rubber band or diving board).
Potential Energy:
A 257 kg pumpkin is suspended from a crane, 65 m above the ground. How much energy does the
pumpkin have before it is dropped?
A performer is tossed in the air at a parade. How much gravitational potential energy does the 60 kg
performer have when she is at a height of 5.2 m?
Kinetic Energy:
How much kinetic energy does a 105 kg snow ball have as it is rolled at a constant velocity of 2 m/s?
How much energy does a 1350 kg car have as it is driven at a constant velocity of 15 m/s?
What is the car's energy when the speed is doubled?
Total Energy
An object can have more than one kind of energy. The total mechanical energy of an object = stored
energy of the object + energy of motion
A 48 kg child (including sled) is sledding down an icy slope. How much energy does the child have
when he is going 5 m/s and is 15 m from the bottom of the hill? (ignore friction)
Think of a free falling object…….
What happens to the potential energy as an object falls to the ground?
What happens to the kinetic energy as it falls to the ground?
What happens to the total energy of the object?
If no work is done by an external force (such as a push or pull or friction) The total amount of energy an
object has remains the same. Energy can transform from one kind to another, but the total amount
remains the same.
Examples: frictionless pendulums, frictionless roller coasters, frictionless slopes, free falling objects and
projectiles (neglect air resistance)
Devyn is riding on a frictionless roller coaster. What happens to her energy as she moves through the
roller coaster? What happens to her speed as she moves through the roller coaster?
Problems:
1. Amanda throws a 0.75 kg ball at an upward velocity of 9 m/s. How high will the ball go? (4.13 m)
2. A 7 kg bowling ball is held 5 m from the ground and released.
 What is the total energy of the bowling ball at the initial position?
 What is the bowling ball's PE at the initial position?
 What is the bowling ball's total energy just before it hits the ground?
 How fast is it going just before it hits the ground?
Work and Energy
Work: A force applied over a distance that results in the transfer of energy. When work is done to an
object, the energy of the object may change. Work can be positive or negative. Work is measured in
Joules (J).
W=Fd
W = Δ ET
Demonstration: Sweep the bowling ball...
Was work done to the bowling ball?
What affect did the work have? (Did the bb gain energy? What kind?)
Demonstration: Lift the bowling ball
What force must be overcome to lift a bowling ball into the air?
Once the ball is over your head, does it have the potential to do work?
How did the bowling ball get it's energy?
How much work is done to lift a 7 kg bowling ball from the floor over your head (2 m)?
So….what happens to an object's energy when work is done on the object?
Work changes the Energy (ET) of an object.
If work is done to change both the speed and height of the object, the work changes the both Potential
energy and the kinetic energy of the object.
W = ΔPEg + ΔKE
W = (mghf - mghi) + (½mvf2 - ½mvi2)
Work = ΔPE + ΔKE
If work is done to lift an object,
the work increases the gravitational
potential energy of the object.
W = ΔPEg = mghf - mghi
Work = ΔPE + ΔKE
If work is done to change the
speed of the object, the work changes
the Kinetic Energy of the object.
W = ΔKE = ½mvf2 - ½mvi2
3. An extinguisher applies a 150 N force over 10 m on a student sitting on a wagon (m= 98 kg). The
student and wagon were initially rolling at 0.5 m/s. a. How much work does the extinguisher do?
b. What is the student (and wagon's) final velocity?
4. How much work is done to move a 1900 kg cable car from a 8 m platform to the top of a 272 m cliff?
5. A 58 kg roller coaster has a speed of 8.0 m/s on the top of a hill that is 6.4 m tall. The cart is moving
at 9 m/s at the top of a 2.4 m hill.
a. How much energy does the cart have at the top at the top of the first hill?
b. How much energy does the cart have at the top of the second hill?
c. How much work does friction do?