Unit 4: Work, Power, and Energy

I. Work

• When a force acts upon an object to cause a displacement of the object, it is said that work was done upon the object.

• There are three key ingredients to work: force, displacement, and cause.

• In order for a force to qualify as having done work on an object, there must be a displacement and the force must cause the displacement.

• In order to work to be done, some of the force must act in the same direction as the object moves. If there is no movement, there is no work.

Is Work Being Done?

1) A teacher applies a force to a wall and becomes exhausted.

2) A book falls off a table and free falls to the ground.

3) A waiter carries a tray full of meals above his head by one arm straight across the room at constant speed.

4) A rocket accelerates through space.

Answers

1) No. The wall is not being displaced.2) Yes. The force of gravity is causing the book to

free fall.3) No. Although the waiter is exerting an upward

force on the tray and the tray is being displaced horizontally across the room; the force is not causing the displacement.

4) Yes. The force of the gases being expelled from the rocket are causing the displacement of the rocket.

Work Equation

Work = Force x Distance

W = Fd

Units of Work: N m∙

1 N m = ∙ 1 Joule (J)

Guided Practice

• Rita mows lawns on the weekends. If she uses 15N of force to move a lawn mower a distance of 10 meters, how much work does Rita do?

150 J• How much work did Joe do when he used 30N

of force to pull a table a distance of 3 meters?90 J

Independent Practice

1) How much work is done by a force of 60N that moves an object 6 meters?

2) In preparing for a classroom activity, two students work together to push a desk a distance of 4.2 meters. The combined force they used to complete the task was 20N. How much work did they do together?

3) Five students exert a combined force of 500N to lift a heavy crate a distance of 2 meters off the ground. How much work did the students exert together in lifting this crate?

4) Nancy stores her holiday decorations in the garage rafters. She uses a ladder to lift a 50N box 3 meters off the ground. How much work does she do?

5) A cat weighing 40N jumps 2 meters onto a fence. How much work does the cat do?

Answers

1) 360 J2) 84 J3) 1000 J4) 150 J5) 80 J

II. Power

• Power: the rate at which work is done.Power =

P = Units of Power: Joule/second = 1 Watt (W)

• For historical reasons, the horsepower is occasionally used to describe the power delivered by a machine. One horsepower is equivalent to approximately 750W.

Guided Practice

1) It takes 2 seconds to lift a crate using 750 J of work. How much power was required to complete this job?

375 W2) You run up several flights of stairs in 1.5

minutes. If the work you do is equal to 900 J, how much power do you use?

10 W

Independent Practice

1) Two students decide to go rowing on the weekend. They row the boat for 14 minutes and together do 168,000 J or work. How much power did they exert?

2) How much power would a forklift need to raise a 500N load 1.5 meters high in 10 seconds? How much power would be needed to do the same work in 5 seconds?

3) You do 3000 J of work to slide a box across the floor in 30 seconds. Calculate your power.

4) You apply a 250N force for 20 seconds to slide a box 10 meters across the floor. Calculate your power.

5) You do 800 J of work to slide a box 5 meters across the floor. How much force did you use? If it took 6 seconds to slide the box, what was the power used?

Answers

1) 200W2) Work = 750 J; Power = 75W; Power = 150W3) 100W4) Work = 2500 J; Power = 125W5) Force = 160N; Power = 133W

• Work and Power Practice Worksheet• Work and Power Study Guide 1• Formative Assessment

• More Work and Power Practice Worksheet• More Work and Power Study Guide

(p. 441-442: 1-4, 11, 12, 28, 29, 32)

III. Energy

• Energy: the ability to do work. In other words, energy is transferred by a force moving an object through a distance.

Major Forms of Energy

1) Mechanical Energy: the energy associated with the motion and position of everyday objects.

2) Thermal Energy: the total potential and kinetic energy of all the microscopic particles in an object.

3) Chemical Energy: the energy stored in chemical bonds.4) Electrical Energy: the energy associated with electric

charges.5) Electromagnetic Energy: a form of energy that travels

through space in the form of waves.6) Nuclear Energy: the energy stored in atomic nuclei.

• Many forms of energy can be classified into two general types:–Kinetic Energy–Potential Energy (includes gravitational and

elastic)

Kinetic Energy

• Kinetic Energy: the energy of motion.• The kinetic energy of any moving object

depends upon it mass and speed.Kinetic Energy = ½(mass)(velocity)2

KE = ½ mv2

Units: kg m∙ 2/s2 = 1 Joule (J)• Kinetic Energy is directly related to the mass

and speed. Double the mass, double the KE; double the speed, quadruple the KE.

Potential Energy

• Potential Energy: energy that is stored as a result of position or shape.

• There are two forms of potential energy: gravitational potential energy and elastic potential energy.

• Elastic Potential Energy: the potential energy of an object that is stretched or compressed.– Examples: rubber band, spring, basketball, shock

absorber

• Gravitational Potential Energy: potential energy that depends upon an object’s height.

• An object’s gravitational potential energy depends on its mass, its height, and the acceleration due to gravity.

Potential Energy = (mass)(gravity)(height)PE = mgh

Units: kg m∙ 2/s2 = 1 Joule (J)• GPE is directly related to the mass and height

of an object. Double either the mass or height, and you double the GPE.

Guided Practice1) A 0.10 kg bird is flying at a constant speed of 8.0

m/s. What is the bird’s kinetic energy?3.2 J

2) A 70 kg man is walking at a speed of 2 m/s. What is his kinetic energy?

140 J3) A 1400 kg car is moving at a speed of 25 m/s.

How much kinetic energy does the car have?437,500 J

4) A 50 kg cheetah has a kinetic energy of 18,000 J. How fast is the cheetah running?

27 m/s

5) A diver at the top of a 10 meter high diving platform has a mass of 50 kg. How much potential energy does she has relative to the ground?

4900 J6) A ball with a mass of 2 kg is thrown vertically

into the air with an initial speed of 1 m/s. It’s initial height is 10 meters. How much gravitational potential energy does it possess relative to the ground?

196 J

• Kinetic Energy and Potential Energy Practice Worksheet

• Energy Study Guide

Energy Conversion and Conservation• On October 9, 1992, people from Kentucky to New

York reported a bright streak of white light shooting across the night sky. Most observers, having seen “shooting stars” before, expected this one to quickly burn out and disappear. However, that did not happen. The shooting star, or meteor, continued streaking across the sky. After a few seconds, pieces of the meteor broke off, creating a series of smaller streaks of light. Eventually, the streaks disappeared from view. Although the event was interesting, most witnesses probably soon forgot about it.

• However, the meteor was not soon forgotten by the owners of a red automobile in Peekskill, New York. Unfortunately for them, a large chunk of the meteor made it through the Earth’s atmosphere and struck their parked car. The car was badly damaged. Luckily, no one was in the car at the time, so no one got hurt.

• As the Peekskill meteor traveled through the atmosphere, some of its kinetic energy was converted into light and heat. The light made the meteor visible in the sky. The heat caused a large portion of the meteor to vaporize in the atmosphere. Upon impact, much of the meteor’s remaining kinetic energy went into smashing the metal body of the car.

• The Peekskill meteor clearly shows that energy can change forms.• Energy can be converted from one form to another.• Energy Conversion: the process of changing energy from one form

to another.• Examples:– Light bulbs convert electrical energy into thermal energy and

electromagnetic energy.– When lighting a match, your muscles use chemical energy to

move your hand to strike the match against a rough area on the matchbox. Friction between the match and the matchbox converts some of the match’s kinetic energy into thermal energy. The thermal energy triggers a chemical reaction on the match tip, releasing some of the match’s stored chemical energy. The stored chemical energy is then converted into thermal energy and electromagnetic energy in the flame.

Conservation of Energy

• When energy changes from one form to another, the total energy remains unchanged even though many energy conversions may occur.

• The Law of Conservation of Energy: states that energy cannot be created nor destroyed. Energy can be converted from one form to another. In a closed system, the energy you begin with is the energy you end with.

• Example:• You are pedaling a bicycle along a flat route.

When you stop pedaling, the bicycle will eventually come to a stop. The moving bicycle had kinetic energy. Where did it go?

• The bicycle slowed down and stopped because of frictional forces (ground and air) acting over a distance. The work done by the friction changes the kinetic energy into thermal energy.

Energy Conversions

• One of the most common energy conversions is between potential energy and kinetic energy.

• The gravitational potential energy of an object is converted to the kinetic energy of motion as the object falls.

• This is what happens when an avalanche brings tons of snow from the top of a mountain to the valley below.

• Energy Conversion in Pendulums:• A pendulum consists of a weight swinging back and

forth from a rope or string.• Pendulums were used in the first truly accurate

clocks first designed by Dutch scientist Christiaan Huygens in 1656.

• Kinetic energy and potential energy undergo constant conversion as a pendulum swings. At the highest point in its swing, the pendulum is momentarily motionless as it changes direction. At this point, the weight at the end of the pendulum has zero kinetic energy and maximum potential energy.

• As the pendulum swings downward, potential energy is converted to kinetic energy. At the bottom of the swing, the pendulum has maximum kinetic energy and zero potential energy.

• Eventually frictional forces slow down the pendulum. In a clock, a spring mechanism or hanging weights provide energy to keep the pendulum swinging despite the effects of friction.

• Energy Conversion in the Pole Vault:• In the pole vault, an athlete uses a flexible pole to propel

himself over a high bar. In order to start the jump with as much kinetic energy as possible, the pole-vaulter sprints down the runway as fast as he can. At the end of his sprint, he plants the end of a long pole at the base of the high bar and propels himself into the air. The pole-vaulter’s kinetic energy is partially converted into elastic potential energy as the pole bends. The pole springs back into shape, propelling the pole-vaulter upward, hopefully high enough to clear the bar.

• As the pole-vaulter soars, his kinetic energy decrease while he gains gravitational potential energy. Once the highest point is reached, his GPE begins to convert back to kinetic energy as the pole-vaulter falls back to the ground.

• Energy Conversion in a Roller Coaster:• A roller coaster goes through a series of exchanges

between potential and kinetic energy.• Potential energy builds as the coaster climbs and

kinetic energy builds as the coaster plunges.

http://www.pbslearningmedia.org/asset/mck05_int_rollercoaster/

• Energy Conversion Study Guide• More Kinetic Energy and Potential

Energy Practice Worksheet• More Energy Study Guide

(p. 469-470: 1-8, 11-13, 16, 18, 23)