WORK AND ENERGY

Chapter 4

I. WORK AND MACHINES – 4.1

A. Definition of Work1. Motion and work

a. Please Define Work:Transfer of energy when a force isapplied over a distance;

measured in joules.

2. Force and direction of motiona. Force parallel to motion

(1.) Write the Work Equation:Work (in joules) =applied force (in Newtons) x distance (in

meters)W = Fd

b. Practice Problems p. 1071.

2.

3.

4.

c. Force perpendicular to motion:Technically, no work is done onan object when the force appliedis at right angles or perpendicularto motion. The work from that forceis zero.

d. Other directions:1. Maximum work is done on an object

when the force appliedis parallel to the distance moved.

2. Remember no work is done onobject when the force appliedis perpendicular to the distance moved.

3. If work done on an object is someangle between 0o and 90o then thework done on the object must becalculated using the equation: W = F x d cos θ

3. When is work done?Work is only done when:a. Force applied and distance moved

are parallel to each other.b. Only when the object moves.

No work is done on a stationaryobject with no contact on the object.

B. Machines1. Please Define Machine:

Device that makes doing work easier byincreasing the force applied to an object,changing the direction of an applied force,

or increasing the distance over which a force can be applied.

2. Types of machinesa. Please Define Simple Machine:

Machine that does work with onlyone movement; examples includelever, pulley, wheel and axle, inclinedplane, screw, and wedge.

b. Please Define Compound machine:Machine that is a combination oftwo or more simple machines.

3. Efficiencya. Please Define Efficiency:

Ratio of the output work done bythe machine to the input work doneon the machine, expressed as a percentage.

b. Efficiency Equation1. Please write the Efficiency

Equation:e = Wout x 100

Win

c. Practice Problems p. 1105.

6.

7.

4. How are machines useful?a. Increase speed

Example: Bicycles increase speed.

b. Change direction of forceExample: Wedge-shaped blade ofan ax changes the downward forceinto the outward force to split wood.

c. Increase forceExample: A car jack increases the force but decreases the speed of the lift.

5. Mechanical Advantage:a. Please Define Mechanical Advantage:

Ratio of the input force exerted by a machine to the input force applied to the machine.b. Write Mechanical Advantage (MA)

as an Equation:MA = Fout

F in

5. Mechanical Advantage

5. Mechanical Advantagea. Practice Problems p. 112

8.

9.

II. DESCRIBING ENERGY- 4.2

A. Change Requires Energy1. Please Define Energy:

The ability to cause change, measuredin joules.

2. Work transfers energy:In Figure 6 of your text p. 114, the tennis racketdoes work on the tennis ball, applying a forceto that ball through a distance. When this happens,

the racket transfers energy to the ball. Therefore,energy can also be described as the ability to do work.

3. Systemsa. Please Define System:

A region or set of regions around which a boundary can be defined.

B. Different Forms of Energy1. There are different forms of Energy

a. Mechanical energyb. Electrical energyc. Chemical energyd. Radiant energy

2. An energy analogy3. Kinetic energy

a. Please Define Kinetic Energy:Energy a moving object has becauseof its motion; described by the massand speed of the object.

b. Write Kinetic Energy as an Equation:KE = ½ mv2

c. Practice Problems p. 11616.

17.

4. Potential energya. Please Define Potential

Energy:Energy that is stored due to the

interactions between objects.

b. Elastic potential energy1.) Please Define Elastic Potential Energy: Energy that is stored by

compressing or stretching an object.

c. Chemical potential energy1.) Please Define Chemical Potential

Energy:Energy that is due to chemical bonds.

d. Gravitational potential energy1.) Please Define Gravitational

Potential Energy:Energy that is due to the gravitational force between objects.

2.) Please write the Gravitational Potential Energy Equation:

GPE = mgh

e. Height and gravitational potential energy:

To calculate gravitational potential energy,height is measured from a reference level.This means that gravitational potential

energy varies depending on the chosen reference level.

f. Practice Problems p. 11918.

19.

III. CONSERVATION OF ENERGY – 4.3

A. The Law of Conservation of Energy1. Please Define the Law of Conservation of

Energy: That energy cannot be created nor destroyed but simply is transformed into one or more other forms of energy.

2. Conserving resources

B. Energy Transformations

1. Mechanical energy transformationsa. Please Define Mechanical Energy:

Sum of the potential energy and kineticenergy of the objects in a system.

b. Mechanical energy includes:1.) Kinetic energy of objects.2.) Elastic Potential energy.3.) Gravitational Potential energy.

c. Falling objects

1.) As the Apple falls it loses it gravitationalpotential energy and gains back kineticenergy.

2.) The form of mechanical energy changes,but the total amount of energy remainsthe same.

d. Projectile Motion

1.) As the maximum height of the projectile the object has the maximum Gravitational Potential Energy, and the lowest Kinetic Energy.

2.) When the projectile falls back down toward the Earth it picks up Kinetic Energy but loses Gravitational Potential Energy.

e. Swings

1.) When an object starts off it has the maximum Gravitational Potential Energy but loses this as the object swings down and picks up Kinetic energy. However, as the object moves through the swing, it loses Kinetic energy and gains

Gravitational Potential energy. (See Figure 13, page 123.)

2. Other Energy transformations

a. The effect of friction1.) If the mechanical energy of the swing

decreases, then some other forms of energy must increase by an equal amount to keep the total amount of energy the same.

2.) Two possible forms of energy are: a.) Friction of swing’s ropes or chain

rubbing on the point of attachment. b.) Air resistance as air pushes on the rider.

b. Transforming electrical energy

1.) Into a toaster as thermal energy.2.) Into a Television as radiant energy.3.) Into a Washing Machine as mechanical energy.

c. Transforming chemical potential energy

1.) By human’s eating food.2.) By burning gasoline in your automobile.

3. Power-how fast energy changes

a. Please Define Power: The rate at which energy is converted; measured in watts (W).

b. Please write the Power Equation:Power (in watts) = Energy (in joules)

time (in seconds)P = E

t

4. Energy conversions in your body

a. One Calorie is equal to about 4,000 Joules.b. Every gram of fat in a food, supplies a person

with about 10 Calories or 40,000 Joules of energy.c. Every gram of a carbohydrate or protein supplies a person

with about 5 Calories or20,000 Joules of energy.

IV. CHAPTER 4 REVIEW pp. 132-135.A. Please do Check Concepts 41-47,

on page 132 in your Text.B. Please do Standardized Test Practice

questions 1-9 found on page 134 in your Text.

C. I will type out a Pre-Test over Problems associated with Chapter 4

plus an Equation Sheet which you canuse on your Chapter 4 Test.