Chapter 4 Sec 1 Download

Transcript
Name
Date
Work and Energy
Before You Read
Before you read the chapter, respond to these statements.
1. Write an A if you agree with the statement.
2. Write a D if you disagree with the statement.
Before You
Read
Work and Machines
• Holding a heavy object motionless involves
a lot of work.
• Energy is lost when an object is
motionless.
• A machine is a device that creates energy.
• A light bulb transforms electrical energy
into light and thermal energy.
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
Construct the Foldable as directed at the beginning of this chapter.
Science Journal
Diagram a bicycle and identify the parts you think are machines.
Accept all reasonable answers.
Work and Energy 43
Name
Date
Work and Energy
Section 1 Work and Machines
Skim Section 1 of your text. Write three questions that come to
mind from reading the headings and the illustration captions.
1. Accept all reasonable responses.
2.
3.
Review
Vocabulary Define the word force.
force
a push or pull exerted on an object
New
Vocabulary Use your book or a dictionary to define these terms.
machine
simple machine
compound machine
efficiency
application of a force through a distance
a device that makes doing work easier
does work with just one movement of the machine
combination of two or more simple machines
a measure of how much work put into a machine is turned into
useful output by the machine
mechanical advantage
the ratio of the output force to the input force
Academic
Vocabulary Look up the words per and cent in a dictionory.
percent
44 Work and Energy
Then explain why 68 percent is the same as 68/100.
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
work
Name
Date
Section 1 Work and Machines
What is work?
I found this information
on page
.
SE, p. 106
RE, p. 56
(continued)
Create three sketches showing the following situations involving
work. Accept all reasonable responses.
A force is not doing
A force is doing
A force is not doing work, because the
work.
work, because there force does not point to
is no motion.
the direction of motion.
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
Sketch should
show a force acting
on some object that
moves in the
direction of the
force.
What is a
machine?
Sketch should
show a force
acting on an object
that does not move.
Complete the concept map relating simple and compound
machines.
I found this information
on page
.
SE, p. 109
RE, p. 57
Sketch should
show a force acting
acting at right
angles to the
direction of motion.
Machines
Simple Machines
Compound Machines
does work with just one
movement of the machine
combination of two or
more simple machines
Sketch a nail and a screw. Explain which one uses less force
and why.
Answer: To drive a nail you have to exert a lot of energy on the
hammer to force the nail into the wood. The screw uses less
energy because the screw takes smaller movements to reach the
same point (depth).
Work and Energy 45
Name
Date
Section 1 Work and Machines
Efficiency
I found this information
on page
.
SE, p. 110
RE, p. 58
(continued)
Evaluate the efficiency of two identical-looking conveyor belts.
Belt A can move a 10 newton weight one meter in 3 seconds. Belt
B can move a 10 newton weight 2 meters in 3 seconds. (one joule
! 1 newton meter) The input work for both belts is 20 joules.
Fill in the missing numbers below.
What do you know?
A
Belt B
Weight (newtons)
10
10
Distance (meters)
1
2
Time (seconds)
3
3
Joules = Newton/meter
10 Nm
6 Nm
Joules
10 J
5J
Input work (Win)
20 J
20 J
Output work (Wout)
10 J
5J
Efficiency (%) !
Wout " Win # 100
50 %
25 %
CONNECT IT
A child sits at the top of a slide at a playground. He wiggles
forward slightly, and then slides all the way down with no further effort. Explain the
source of the force acting on the child, and how you would calculate the work being
done.
Accept all reasonable responses. The force of gravity is acting on the child, causing him
to slide. The work being done on the child is the vertical distance he slides times the
force exerted, which is the height of the slide (in meters), times his weight (in newtons).
46 Work and Energy
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
Belt