Study Guide for Chapter 12
Name _______________________
1
Work and Machines
Be able to define the following vocabulary:
work: Force
times distance, force exerted on an object that causes it to
move in the same direction
Work’s unit is the joule (J) or Newton*meter (N*m)
machine: a device that makes work easier by changing the
force, the
distance, or the direction.
input force (effort):the
force you exert on the machine
output force (load): The force the machine exerts on another object
input work: The
work you put into a machine, (Input force * input distance)
output work: The work the machine does on another object
mechanical advantage (MA):
the
output force divided by input force
# of times force is increased or decreased
(MA has no unit. It can be calculated with force or distance)
IMA= ideal mechanical advantage (No work is lost due to friction, efficiency =100%)
AMA= actual mechanical advantage (MA of a machine in the real world, efficiency <100%)
efficiency: output
work divided by input work times 100%
(Given as a %)
Simple Machines:
a. inclined plane: a
flat, sloped surface (ramp)
b. wedge:
an inclined plane that moves
(knife blade)
c. screw:
an inclined plane wrapped around a screw to form a spiral
d. lever: A simple machine made of a rigid bar that pivots around a fixed point (fulcrum)
(1stclass): Fulcrum is in the middle between the input force and output force (see-saw)
(2nd class): Output
force is in the middle between the input force and
fulcrum (wheelbarrow)
(3rd class): Input
force is in the middle between the output force and
fulcrum (hockey stick)
e. wheel and axle: two
attached circular or cylindrical objects (of different
radius) that rotate around a common axis
(screwdriver)
a grooved wheel with a rope or cable wrapped around it (block
and tackle)
f. pulley :
gear: A toothed wheel designed to mesh with, and turn, another gear. Teeth are called cogs.
The two systems are the spur gear (both gears on same plane) and crown gear (90o apart)
compound machine: a
machine made up of two or more connected simple
machines
Be able to:
1. Identify when work is done on an object and that there is no work without motion.
Work: The
motion is in the same direction as the force.
No work: The
object doesn’t move or motion is not in the same
direction as the force.
2. Put the following equations into triangles and use them correctly:
work=force*distance
M.A = output force
input force
W
Example 1: A dog performs 900 N*m of work pushing its bowl around the house, a
distance of 15 m. What is the force on the bowl?
F= W
D
= 900Nm = 60N
15m
Example 2: A 750-N pole vaulter lifts himself 5.0 m high in 2.5 sec.
a. How much work did he perform?
b. What was his power?
W= F*D = 750N* 5.0m= 3750Nm
P= W
= 3750Nm
= 1500W
1
T
2.5sec
3. Give examples of the 3 ways that machines make work easier.
changing force: A screwdriver puts out more force than we put in by turning the handle.
changing the distance: A
hockey stick hits a puck farther from the
fulcrum than your hand
changing the direction: We pull down on a single fixed pulley to raise a
weight
4. Know that the work you put into a machine (input work) equals the work you get out of the
machine (output work).
Force and distance are inversely related. When one increases, the other decreases.
Find a machine’s input and output forces and directions. Draw arrows to show the input
force and output force in each case. Know that
FD = FD, (work in = work out).
5. Know that if mechanical advantage is greater than 1, you will get more force out of the
machine than you put into it. That means output force = MA * input force.
Ex: 1: You build a pulley with a mechanical advantage of 4. How much input force
would be needed to pull down on the rope to lift an 80N object?
20N
Ex. 2: You squeeze a pair of tongs with an input force of 4 Newtons. If the MA of
the tongs is 0.5, what is the output force of the tongs?
2N
6.
Understand that sometimes it’s difficult to measure force, so we use distance to calculate
MA., for example: MA of an inclined plane = length of the incline/the height of the incline
Ex. 1: What is the mechanical advantage of a 1st class lever with
input distance = 9 cm, and output distance = 18 cm.
MA = input distance = 9cm
output distance
18 cm
= 0.5
If you are using this lever to lift an object that weighs 4N, how hard will you
need to push down on the input side of the lever?
IF = OF = 4N
MA
0.5
= 8N
7. Calculate mechanical advantage for each of the 6 simple machines—for example, an inclined
plane’s mechanical advantage is the length of the incline/the height of the incline. Use your
attached formula sheet for help.
Ex. 1: Your Dad is using a food processor when making dinner. The wheel inside has a radius
of 30 cm and the axle has a radius of 5 cm. What is the mechanical advantage?
The motor turns the axle first, which turns the wheel.
Ex. 2: A lever is used to lift a heavy load. 100 N of input force pushes one end of the lever
down, and lifts a 75N load. What is the MA of this lever?
Ex. 3: A gear with 34 teeth drives a gear with 14 teeth in a model transmission. What is
the gear ratio of this gear train? Gear ratio is inversely related to MA.
Gear ratio = teeth on driven gear =
teeth on driver gear
8. Know that no machine is ideal because of friction, so that the efficiency of any real machine
is less than 100%.
Example: You are using an inclined plane. You exert a force of 3000 N over a
distance of 3 meters to raise a 6000 N fridge 1 meter. What is your efficiency, and
why is it less than 100%?
9. Identify the six simple machines, plus the gear and how they work. Remember examples of
each from real life and the labs. See binder of labs for this.
10. Draw and label the three kinds of levers. Give an example of each from the real world.
11. Know that a compound machine is made of two or more simple machines, and that the
mechanical advantage (M.A.) of a compound machine is the product of the M.A. of each part
(multiply the M.A.s).
Ex. 1: A wheel and axle, with MA = 5, are pulling a rope
attached to a pulley system with MA = 2. What is the MA of
this compound machine?