Chapter 5: Work and EnergyChapter 5: Work and Energy

Today’s ObjectivesToday’s Objectives

What do you think?What do you think?

List five examples of things you have done List five examples of things you have done in the last year that you would consider in the last year that you would consider workwork..

Based on these examples, how do you Based on these examples, how do you define work?define work?

WorkWork In physics, work is the magnitude of In physics, work is the magnitude of

the force (the force (F) F) times the magnitude of times the magnitude of the displacement (the displacement (dd) ) in the same in the same direction as the force.direction as the force.

WW = = FF••dd

Work Done by a Constant ForceWork Done by a Constant Force

What are the SI units for work?What are the SI units for work?• Force units (N) Force units (N) displacement units (m) displacement units (m)

• NN••m are also called joules (J).m are also called joules (J).

How much work is 1 joule?How much work is 1 joule?

• Lifting an apple weighing about 1 N from Lifting an apple weighing about 1 N from the floor to the desk, a distance of about the floor to the desk, a distance of about 1 m, is equivalent to 1 J of work.1 m, is equivalent to 1 J of work.

Question?Question? How much work is done in pushing How much work is done in pushing

this Barack Obama Ice Statue with this Barack Obama Ice Statue with 20 N of Force a distance of 10 20 N of Force a distance of 10 meters?meters?

Answer: Work = Fd

(20 N) x (10 m) = 200 Nxm

Question 2Question 2

No work on it. Why?!

How much work is done carrying a 10 kg bag a distance of 10 meters?

Work only depends on Force that is in the same direction as the movement!!!

WorkWork

What is the component of What is the component of FF in the direction of in the direction of dd??• FF cos cos

If the angle is 90°, what is the If the angle is 90°, what is the component of component of FF in the direction in the direction of of dd??• FF cos 90° = 0 cos 90° = 0

If the angle is 0°, what is the If the angle is 0°, what is the component of component of FF in the direction in the direction of of dd??• FF cos 0° = cos 0° = FF

How would you calculate the work in this case?

More AccurateMore Accurate EquationEquationWW = = FF••dd••cos(cos())

Work Done by a Constant ForceWork Done by a Constant Force

Work done by forces that oppose the direction of motion, such as friction, will be negative.

Centripetal forcesCentripetal forces do no work, because they are always perpendicularperpendicular to the direction of motion.

How muHow much work is done by friction if the vacuum is 25 kg andthe kinetic coefficient of friction is .4 when the vacuum is draggedacross a rug 4 m?

Work is a ScalarWork is a Scalar

Work can be Work can be positive or positive or negative but negative but does not have does not have a direction.a direction.

What is the What is the angle between angle between FF and and dd in each in each case?case?

Now Now what do you think?what do you think?

Based on the physics definition, list Based on the physics definition, list five examples of things you have done five examples of things you have done in the last year that you would in the last year that you would consider consider work.work.

(Show your List to Mr. P. before you leave.)(Show your List to Mr. P. before you leave.)

• Energy is traditionally defined as the ability to do work.

• Energy appears in many forms. Light, Electricity, Nuclear Energy, Heat, and Mechanical Energy are all examples.

• Mechanical Energy is energy due to position or movement.

2 Types of Mechanical Energy2 Types of Mechanical Energy

Kinetic Energy – Energy due to MotionEnergy due to Motion

Any moving object has the ability to do work Any moving object has the ability to do work on another object. Therefore every moving on another object. Therefore every moving object has Kinetic Energy.object has Kinetic Energy.

M = mass

V2= velocity squared

2 Types of Mechanical Energy2 Types of Mechanical EnergyPotential EnergyPotential Energy - energy due to position.

Any object can have potential energy due to its position or because of its surroundings.

Familiar examples of potential energy:Familiar examples of potential energy:

• A wound-up spring

• A stretched elastic band

• An object raised to some height over the ground

Water MillWater Mill

Gravitational Potential EnergyGravitational Potential Energy

• Work is done in order to lift an Work is done in order to lift an object up off the ground:object up off the ground:

• W = Fd = (mg)hW = Fd = (mg)h

• By doing work on the object, By doing work on the object, we have given it energy.we have given it energy.

• We therefore define the We therefore define the gravitational potential energygravitational potential energy::

P.E. = mghP.E. = mgh

m: massm: mass g: 9.8 g: 9.8 m/sm/s22 h: heighth: height

Potential EnergyPotential EnergyThis potential energy can become kinetic energy if the object is dropped.

Potential energy is a property of a system as a whole, not just of the object (because it depends on external forces).

We usually measure h from the ground. So at the ground, h = 0, which means the P. E. is zero.

Potential EnergyPotential Energy

Potential energy can also be stored in a spring Potential energy can also be stored in a spring when it is compressed; the figure below shows when it is compressed; the figure below shows potential energy changing into kinetic energy.potential energy changing into kinetic energy.

Weekly Homework

Due on TuesdayPg. 184 #’s: 1, 7, 8, 9, 12, 19, 21, 23, 33

Recap – so far…Recap – so far…

Work = Fd(cos Work = Fd(cos )) Energy is the ability to do work.Energy is the ability to do work. Kinetic Energy – Energy due to Kinetic Energy – Energy due to

motion – K.E. = 1/2mvmotion – K.E. = 1/2mv22

Potential Energy – Energy due to Potential Energy – Energy due to position – (gravitational P.E. = mgh)position – (gravitational P.E. = mgh)

Units for work and energy: joules (j) Units for work and energy: joules (j)

Kinetic Energy, and the Work-Kinetic Energy, and the Work-Energy PrincipleEnergy Principle

The amount of work that can be done on an object, is equal to the amount of kinetic energy or potential energy it gains or loses:

• If the net work is positive, energy increases.

• If the net work is negative, energy decreases.

Work = change in energyWork = change in energy

Kinetic Energy, and the Work-Energy Principle

Because work and kinetic energy can be equated, they must have the same units: kinetic energy is measured in joules.

Mechanical Energy and Its Mechanical Energy and Its ConservationConservation

Energy cannot be created or destroyed. It can only transfer from one form to another or be passed from one object into another.

This is the Law of conservation of This is the Law of conservation of energy.energy.

bowling ball pendulumbowling ball pendulum

Bowling Ball Pendulum – You Tube

Pendulum Analysis QuestionsPendulum Analysis Questions

Where does the bowling ball in the Where does the bowling ball in the pendulum have the highest velocity? pendulum have the highest velocity?

How does the original height of the How does the original height of the pendulum compare to its final height pendulum compare to its final height before it begins to come back?before it begins to come back?

Is energy Is energy conserved in the in the pendulum? Why or why not?pendulum? Why or why not?

Conservation of Energy in a Conservation of Energy in a PendulumPendulum

Energy Conservation Example 2Energy Conservation Example 2Roller Coasters!!!Roller Coasters!!!

Another Example:

http://surendranath.tripod.com/Applets/Dynamics/Coaster/CoasterApplet.html

Energy Conservation Example 3Energy Conservation Example 3

Other Forms of Energy; Energy Other Forms of Energy; Energy Transformations and the Transformations and the Conservation of EnergyConservation of Energy

•Work is done whenever energy is transferred from one object to another.

•Work is also done whenever energy changes forms.

•Accounting for all forms of energy, we find that the total energy neither increases nor decreases. Energy as a whole is conserved.

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PowerPower Power is the rate at which work is Power is the rate at which work is

done. done.

Power = WorkPower = Work**/Time/Time **(force x distance)(force x distance)

The unit of power is the The unit of power is the wattwatt..