Journal #51 1/6/2010

Journal #51 1/6/2010

Calculate the answers to the following problems. Use scientific notation and sig figs in your answers.

?101.41075.2 57

?1001.110881.7 75

Journal #51 Solutions

?101.41075.2 57

?1001.110881.7 75

13101.1

21080.7

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.• 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.

x

F

xxdFW

xF

yF

Practice Problems - work

Example Problem #2 Solution

JW

WFdW

4109.2

35825

?35825

WmdNFA.)

B.)

?351650

WmdNF

JW

WFdW

4108.5

351650

Example Problem #3 Solution

JW

WdmgW

dFW g

2100.6

2.880.95.7

?2.8min305.7

Wmd

tkgm

A.)

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 timedonework Power

What is horsepower? 1 horsepower = 746

Watts The horsepower was

originally defined to compare the output of steam engines with the power of draft horses

Horsepower is not a recognized SI unit but is still widely used in many applications (such as cars).

Homework – Work and PowerP. 278 at back of chapter 10

#52-59You must show left-hand line-up (aka -

define your terms!), formulas used, all of your work, then circle your final answers!

Journal #52 1/7/10 Order the speed of the rollercoaster (from fastest to

slowest) at the positions labeled. If you believe the speed would be the same at two points, indicate that as well.

AB

C

F

E

D

Journal #52 Solution Fastest to slowest: C, F, B, A & D same, then E is

slowest. What rule can we make up?

AB

C

F

E

D

Making the graph

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 energyThere 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 Potential

Gravitational Potential Energy 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 12mv 2

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

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

F d 12mv 2

Calculate speed at positions B,C and D.

More Energy Transfer High speed winds are used to

do work on the blades of a turbine at the so-called wind farm.

Mech. Energy from the air gives the air particles the ability to apply a force to the blades.

As the blades spin, their energy is subsequently converted into electrical energy (a non-mechanical form of energy) and supplied to homes and industries in order to run electrical appliances.

Energy Transfer...This diagram

shows that the boys potential energy is changing. Explain what is happening, in other words how is the energy changing?

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 coaster or pendulum

Law of Conservation of Energy

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