Transcript Test 6: Chapter 5 Work & Machines

Test 6: Chapter 5
Work & Machines
Honors Physical Science
Vocabulary Self-Awareness Chart
 Directed Reading Packet p.22-24

Assignment:

Work – transfer of energy that occurs
when a force makes an object move

For work to occur, an object must
move.
◦ (No movement – No work!)

The motion of the object must be in
the same direction as the applied force
on the object.
Section 1: Work
Work and energy are related, since
energy is always transferred from the
object doing the work to the object on
which the work is done.
 Work is done on an object only when a
force is being applied to the object and
the object moves.
 Calculating work – work equals force (in
Newtons) times distance

◦W=Fd

Complete the Math Skills Activity on p.128
in your notes.
Math Skills Activity
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Power – amount of work done in a certain
amount of time; rate at which work is done
Calculating power – power equals work
divided by time
◦ P = W/t


Power is measured in watts (W).
Since work and energy are related, power
can also be calculated by dividing energy
by time.
◦ P = E/t.
POWER

Complete the Math Skills Activity on p.130
in your notes.
Math Skills Activity
1. What must you ask to determine if work is
being done?
2. How are work and energy related?
3. A person pushed a bowling ball 20 meters. The
amount of work done was 1,470 joules. How
much force did the person exert?
4. It took five minutes to move a refrigerator. You
did 3,150 joules of work in the process. How
much power was required to move the
refrigerator?
5. How much power is required to push a car for
10 seconds if the amount of work done during
that time is 5,500 joules?
Practice Questions:

Packet p.29
Assignment:


Device that makes doing work easier is a
machine.
Machines increase applied force and/or
change direction of applied force to make
work easier.
 Same amount of work can be done by applying a
small force over a long distance as can be done
applying a large force over a short distance, since
work equals force times distance.
 Increasing distance reduces the amount of force
needed to do the work.
 Some machines change the direction of the
applied force to do the work.
Section 2: Using Machines

Machines help move things that resist being
moved.
Force applied to machine is the effort
force. (FE)
 Resistance force – (FR) force applied by
machine to overcome resistance
 Amount of energy the machine transfers
to the object cannot be greater than the
amount of energy transferred to the
machine.

◦ a. Some energy transferred is changed to heat due
to friction.
◦ b. An ideal machine with no friction would have the
same input work and output work.
Mechanical advantage (MA) is the number
of times a machine multiplies the effort
force.
 It is calculated by MA equals resistance
force divided by effort force.

◦ MA = FR/FE
Mechanical Advantage

Efficiency – measure of how much of the
work put into a machine is changed into
useful output work by the machine
◦ Calculating efficiency – efficiency equals
(output work divided by input work) times
100%.

Efficiency = (Wout/Win) x 100
◦ Efficiency of a machine is always less than
100%.
◦ Lubricants can make machines more
efficient by reducing friction.
Efficiency
1. Why machine’s efficiency less than 100%?
2. A claw hammer is used to pull a nail from a
board. If the claw exerts a resistance force of
2,500 N to the applied effort force of 125 N,
what is the MA of the hammer?
3. Give an example of a machine you have
used recently. How did you apply effort force?
How did the machine apply resistance force?
4. Suppose you want to use a machine to lift a
6,000 N log. What effort force will you need if
your machine has a mechanical advantage of
25? …15? …1? Show your calculations.
Practice Questions:
A machine that does work with only one
movement is a simple machine.
 Six types: lever, pulley, wheel & axle,
inclined plane, screw, and wedge
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Section 3: Simple Machines

Lever – bar that is free to move about a
fixed point called a fulcrum.
◦ Effort arm is the part of the lever on which
effort force is applied.
◦ Resistance arm is the part of the lever that
exerts the resistance force.
◦ Three Classes of Levers
◦ Calculating the ideal mechanical advantage (IMA) of a
lever –
 IMAlever = lengtheffort arm/lengthresistance arm
Lever
The fulcrum is located between the effort
and resistance arms;
 multiplies force;
 changes direction of force;
 Ex. Screw driver, pry bar, neck and head

First class lever
The resistance force is located between
the effort force and the fulcrum;
 always multiplies force;
 Ex. Wheel barrow; foot when standing on
toes

Second class lever
The effort force is between the resistance
force and fulcrum;
 doesn’t multiply force but does increase
the distance over which the force is
applied;
 Ex. Baseball bat, forearm when doing
curls
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Third class lever
Grooved wheel with a rope, simple chain,
or cable running along the groove is a
pulley, which is a modified first class lever.
 Three types – fixed, movable, block and
tackle

Pulley

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A fixed pulley is attached to something that
doesn’t move;
force is not multiplied but direction is changed;
IMA = 1.
Fixed Pulley
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A movable pulley has one end of the rope fixed
and the wheel free to move;
multiplies force;
IMA = 2.
Movable Pulley

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Block and tackle – system of pulleys consisting of
fixed and movable pulleys;
IMA = number of ropes supporting the resistance
weight.
Block and Tackle
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
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Wheel and axle – machine with two
wheels of different sizes rotating
together;
modified lever form
IMA = radius of wheel divided by the
radius of the axle
Gears are a modified form of the wheel
and axle.
Ex. Pencil sharpener, door knobs, bicycle
tire
Wheel and Axle
Inclined plane – sloping surface that
reduces the amount of force required to
do work
 IMA = length of slope/divided by height of
slope
 Less force is required if ramp is longer
and less steep.

Inclined Plane
Screw – inclined plane wrapped around a
cylindrical post
 Examples – jar tops, light bulb base
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Screw
Inclined plane with one or two sloping
sides is a wedge.
 Examples – knife blade, axe

Wedge
Compound machine – uses a combination
of two or more simple machines
 Example –

◦ Can opener (lever- handles; wedge –blade;
wheel and axle – when you turn the handle);
◦ Scissors – wedge and lever
Compound Machine
What are the two basic machines, and
what other machines can these be
modified to become?
2. A 6-m ramp runs from the porch to the
ground level sidewalk. The porch is 2m
off of the ground. What is the ideal
mechanical advantage of the ramp?
1.
Practice Questions:
Copy and complete the lab on p.147 in
your textbook.
 Turn in the completed lab.

Lab Activity
Complete packet and get stamped.
 In your standardized test practice
workbook, make sure all pages are
completed for chapters 2-5.

Assignment: