Excerpts from Ch 10-11

Work, Power,

and Energy

Work

Work in the scientific sense: The work done on an object is equal to the product of the force exerted in the direction of motion, times the object’s displacement.

Work is a vector quantity!

Work = Force x distance

W F d

More on Work…

• To lift an object requires work, but to hold an object without moving it (by definition) does not require work.

• NO MOTION = NO WORK

• The unit of Joule comes from the name of a British physicist named James Prescott Joule.

Work = Force|| x Distance

• Only the component of force parallel to the displacement does work

• In this case, the distance is the magnitude of the displacement.

xxdFW

x

F

xF

yF

Examples of Work and No Work

Hammer applies a force to move the nail in the same direction = WORK

Waiter applies a force upward while the tray moves forward = NO WORK

Practice Problems - Work

Example Problem #2 Solution

JW

W

FdW

4109.2

35825

?

35

825

W

md

NFA.)

B.)

?

35

1650

W

md

NF

JW

W

FdW

4108.5

351650

Example Problem #3 Solution

JW

W

dmgW

dFW g

2100.6

2.880.95.7

?

2.8

min30

5.7

W

md

t

kgmA.)

B.)

JW

W

dFFW persongbackpackg

3109.5

2.8)64580.95.7(

)(

Power

• Power is equal to the amount of work done per unit time.

• The unit for power is the Joule/second which is also called a Watt named after the Scottish inventor, James Watt.

interval time

donework Power

Energy…the BIG picture

• The Universe is made up of matter and energy.

• Energy is the “mover” of matter.

• There are many forms of energy.

• Conversions from one form of energy to another continually occur.

• Energy cannot be created or destroyed.

Forms of Energy

There are many forms of energy – chemical, mechanical, thermal, solar, electrical, kinetic, potential…

All are measured in Joules, the same units as work!

Potential energy (symbol PE) is stored chemical energy or energy of position.

Types of PE: elastic, gravitational and chemical.

Potential Energy

Gravitational PE

Gravitational Potential Energy (symbol GPE) can be found by multiplying the mass (in kilograms) of an object, by the acceleration due to gravity, and the height (in meters).

GPE = mgh

Kinetic energy (symbol KE) is energy of motion. Kinetic energy depends on both mass and velocity.

Kinetic Energy

KE 1

2mv2

Energy possessed by an object due to its motion or its stored energy of position.

It can be either potential energy or kinetic energy, or a combination of both.

All forms of energy are measured in joules (J).

Mechanical Energy

Mechanical Energy Conversions

…Total Energy is CONSTANT.

Calculate speed at positions

B,C and D

Work-Energy Theorem

• Relates chapter 10 Work and 11 Energy

• Note that Kinetic Energy (KE) of a moving object = the work an object can do while it is being brought to rest.

• Work = change in KE; if no change in KE, no work is done.

• Practice #61 p. 307

2

2

1vmdF

Energy cannot be created or destroyed.

Energy can be transformed from one form to another, but the total amount of energy never changes.

Ex: We can go from PE to KE to PE and back again…think about a roller coaster or a pendulum.

Law of Conservation of Energy

Energy Transfer...

This diagram shows that the boys potential energy is changing. Explain what is happening, in other words how is the energy changing? Where is he moving the fastest? Why?

What happens to energy?

The energy we often think of as lost is not truly lost but is just converted into forms that may no longer be useful to us – often transferred to thermal energy or heat…Ex: burning log in a fireplace; gasoline in an engine; electric light bulb.