Energy can change from one form to another without a net loss or gain.

Energy Notes• Definition of energy: The ability to cause change. Every change involves energy.

• Energy can be stored, transferred and used.

 

• Definition of work: W = Fd (Work equals force times distance)

 

• Work is done when a force causes something to move, and, as a result, energy moves from place to place or changes from one form to another.

 

• SI Unit for Work and Energy: Joules (J)

 

• Difference between kinetic energy and potential energy

 

Kinetic energy Potential energy

 

The energy of motion The energy of position or stored energy

 

Energy Notes• What is the equation for KE? KE = 1/2mv2 Kinetic energy = ½ mass x velocity2

• How does KE increase or decrease?

 

Increase or decrease the velocity of the object (change speed).

Double your speed, Quadruple the KE

 

Prove it: Calculate the KE of a 2500 kg car traveling at 20 m/s and at 40 m/s

 • KE at 20 m/s KE at 40 m/s• (500,000 J) (2,000,000 J)

 

More mass, same speed, more KE.

Double the mass, double the KE

Prove it: Calculate the KE of a 100 kg cart and a 200 kg cart, each traveling at 15 m/s•  • 100 kg cart at 15 m/s 200 kg cart at 15 m/s• (11,250 J) (22,500 J)

 

 

Mechanical energy is the energy due to the position of something or the movement of something.

Total mechanical energy is the total amount of energy related to the motion of an object! TME = KE + PE

PE can be gravitational PE or elastic PE (or both)

9.3 Mechanical Energy

9.7 Conservation of Energy

Same energy transformation applies

The 2 J of heat can be called non-useful work (work that is not part of the object’s total mechanical energy). This “loss” of TME really represents a transfer from TME to DE)

10 J of PE does 8 J useful work on the arrow and 2 J of non-useful work on the molecules that compose the bow and string and arrow. The arrow has 8 J of KE.

Dissipated energy (DE) is amount of energy transferred away from the total mechanical energy. More DE means less TME, which means less speed!

Gravitational Potential Energy

•Energy is stored in an object as the result of increasing its height. •Work is required to elevate objects against Earth’s gravity. •Example: Water in an elevated reservoir and the raised ram of a pile driver have gravitational potential energy.

9.4 Potential Energy

If an object is moving, then it is capable of doing work. It has energy of motion, or kinetic energy (KE).

• The kinetic energy of an object depends on the mass of the object as well as its speed.

9.5 Kinetic Energy

Kinetic Energy

KE increases with speed

Energy of motion

Elastic Potential Energy—potential to do work

•Energy stored in a stretched or compressed spring or material. •When a bow is drawn back, energy is stored and the bow can do work on the arrow. •These types of potential energy are elastic potential energy.

9.4 Potential Energy

Definitions• Energy (the ability to cause change)

• can be transferred, stored or used

• Work (net force x distance moved)

• method of energy use, transfer and storage

• Power (work per unit time)

• the rate at which work is done or energy is transferred

Chapter 9.1

Work

Energy is the ability to cause change.

Work is done when a net force acts on an object and the object moves in the direction of the net force (force causes displacement).

Work involves a transfer of energy between something and its surroundings.

9.1 Work

9.1 Work

Two categories of work: 1) Work done against another force

a) Transfer of energy to change position or store energy—work against another forceEx: Pulling back the string on a bow stores energy.

b) Transfer of energy (one form to another)Ex: Rubbing your hands together transfers one

form (kinetic) of energy to another (thermal).

2) Work done to change the speed of an objectEx: Bringing an automobile up to speed increases its (kinetic) energy.

Did the weightlifter do work on the barbell and weights? •Yes, when he first lifted them above his head.

Is the weightlifter currently doing work on the barbell and weights?

No, the barbell and weights are not moving.

•Explain two ways that the work done by the weightlifter be increased.

9.1 Work = force × distance

1) Increase the weight on the ends of the barbell

2) Increase the distance over which the weightlifter pushes the barbell and weights.

Work has the same units as energy

Joules Newton x meter

J N x m

9.1 Work

•One joule (J) of work is done when a force of 1 N is exerted over a distance of 1 m (lifting an apple over your head).

If we lift two loads, we do twice as much work as lifting one load the same distance, because the force needed is twice as great.

If we lift one load twice as far, we do twice as much work because the distance is twice as great.

9.1 Work

While the weight lifter is holding a barbell over his head, he may get really tired, but he does no work on the barbell.

Work may be done on the muscles by stretching and squeezing them, but this work is not done on the barbell.

When the weight lifter raises the barbell, he is doing work on it.

9.1 Work

think!Suppose that you apply a 60-N horizontal force to a 32-kg package, which pushes it 4 meters across a mailroom floor. How much work do you do on the package?

9.1 Work

think!Suppose that you apply a 60-N horizontal force to a 32-kg package, which pushes it 4 meters across a mailroom floor. How much work do you do on the package?

Answer:

W = Fd = 60 N × 4 m = 240 J

9.1 Work

When is work done on an object? When is work not done on an object?

9.1 Work