# WORK AND ENERGY Chapter 4.

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WORK AND ENERGY Chapter 4

I. WORK AND MACHINES – 4.1 Definition of Work 1. Motion and work
a. Please Define Work: Transfer of energy when a force is applied over a distance; measured in joules.

2. Force and direction of motion a. 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

c. Force perpendicular to motion: Technically, no work is done on
an object when the force applied is at right angles or perpendicular to motion. The work from that force is zero.

1. Maximum work is done on an object when the force applied
d. Other directions: 1. Maximum work is done on an object when the force applied is parallel to the distance moved. 2. Remember no work is done on object when the force applied is perpendicular to the distance moved. 3. If work done on an object is some angle between 0o and 90o then the work done on the object must be calculated using the equation: W = F x d cos θ

3. When is work done. Work is only done when: a
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 stationary object with no contact on the object.

1. Please Define Machine: Device that makes doing work easier by
Machines 1. Please Define Machine: Device that makes doing work easier by increasing the force applied to an object, changing the direction of an applied force, or increasing the distance over which a force can be applied.

a. Please Define Simple Machine: Machine that does work with only
2. Types of machines a. Please Define Simple Machine: Machine that does work with only one movement; examples include lever, pulley, wheel and axle, inclined plane, screw, and wedge.

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

a. Please Define Efficiency: Ratio of the output work done by
the machine to the input work done on the machine, expressed as a percentage.

b. Efficiency Equation 1. Please write the Efficiency Equation: e = Wout x 100 Win

c. Practice Problems p

4. How are machines useful? a. Increase speed
Example: Bicycles increase speed. b. Change direction of force Example: Wedge-shaped blade of an ax changes the downward force into the outward force to split wood. c. Increase force Example: A car jack increases the force but decreases the speed of the lift.

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

Mechanical Advantage a. Practice Problems p. 112 8. 9.

II. DESCRIBING ENERGY- 4.2 Change Requires Energy
1. Please Define Energy: The ability to cause change, measured in joules. 2. Work transfers energy: In Figure 6 of your text p. 114, the tennis racket does work on the tennis ball, applying a force to 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. Systems a. Please Define System: A region or set of regions around which a boundary can be defined.

Different Forms of Energy
1. There are different forms of Energy a. Mechanical energy b. Electrical energy c. Chemical energy d. Radiant energy

a. Please Define Kinetic Energy: Energy a moving object has because
2. An energy analogy 3. Kinetic energy a. Please Define Kinetic Energy: Energy a moving object has because of its motion; described by the mass and speed of the object. b. Write Kinetic Energy as an Equation: KE = ½ mv2

c. Practice Problems p

a. Please Define Potential Energy: Energy that is stored due to the
interactions between objects.

b. Elastic potential energy
1.) Please Define Elastic Potential Energy: Energy that is stored by compressing or stretching an object.

c. Chemical potential energy 1
c. Chemical potential energy 1.) Please Define Chemical Potential Energy: Energy that is due to chemical bonds.

d. Gravitational potential energy 1
d. Gravitational potential energy 1.) 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

III. CONSERVATION OF ENERGY – 4.3
The Law of Conservation of Energy 1. 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
Mechanical energy transformations a. Please Define Mechanical Energy: Sum of the potential energy and kinetic energy 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 gravitational potential energy and gains back kinetic energy. 2.) The form of mechanical energy changes, but the total amount of energy remains the 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
The effect of friction 1.) 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
Please Define Power: The rate at which energy is converted; measured in watts (W). Please write the Power Equation: Power (in watts) = Energy (in joules) time (in seconds) P = E t

4. Energy conversions in your body
One Calorie is equal to about 4,000 Joules. Every gram of fat in a food, supplies a person with about 10 Calories or 40,000 Joules of energy. Every gram of a carbohydrate or protein supplies a person with about 5 Calories or 20,000 Joules of energy.

CHAPTER 4 REVIEW pp 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 can use on your Chapter 4 Test.