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CHAPTER
2
Chapter Review
SUGGESTED ANSWERS
WHAT DO YOU REMEMBER?
1.
Screw - auger
Wedge - knife
Inclined plane - ramp
Lever - crow bar
Wheel and axle - steering wheel
Pulley - clothesline pulley, flagpole pulley
2. (a)
4; (b) 2; (c) 1; (d) 5; (e) 3
3. (a)
Sample answer: Friction requires contact. Friction allows me to pick up an object like a baseball and
throw it with spin. A mechanical force also requires contact. I have to touch a chair to push it across
the room.
(b)
Gravity, magnetic force, and electrical force do not require contact to have an effect.
input
force
1st Class
output
force
4.
fulcrum
5. (a)
(b)
6. (a)
fulcrum
All forces have magnitude and direction.
In a diagram, direction is shown by the direction the arrow representing the force is pointing.
Magnitude is shown by the length or width of the arrow.
The unit of force is the newton.
The unit of force is named after Isaac Newton.
(c)
The symbol for this unit is N.
(b)
NEL
fulcrum
3rd Class
input output
force force
(b)
7. (a)
8.
2nd Class
output
input
force
force
Mechanical advantage is the ratio of the amount of force that is needed to perform a task without a
machine to the amount of force that is required to perform the same task using the machine.
Ideal mechanical advantage is what the mechanical advantage would be if no input force were lost to
friction, slippage, or distortion. Actual mechanical advantage accounts for this loss of input force.
WFd
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WHAT DO YOU UNDERSTAND?
9. (a)
“Force” has many definitions in everyday life. These everyday definitions generally have to do with
making someone or something behave in a certain way. There are also groups of people who join
together to achieve a certain goal, such as the police force or the armed forces. The scientific
definition, however, is that force is the influence that produces change in a physical quantity.
There are also many definitions of “work,” all of which concern using energy to achieve a goal.
Whether you are employed and have work, work a piece of clay to form a vase, or simply work
hard in class, you are using energy to do something. In physics “work” has a much narrower
definition. In order for work to be done, an object must be moved by a force in the direction of
that force.
“Energy” is also used in many different ways. All of them tend to indicate the ability to do work, or
the capacity to do work. If a person is described as “high energy,” he has the capacity to do a lot of
work. A person can use a lot of energy and still not perform any work.
10.
(b)
I think having different scientific and everyday definitions makes it harder to learn science, because
you have to learn new definitions of familiar words that do not match your old definitions.
Sometimes you forget which definition is being used.
(c)
People often use “mass” and “weight” interchangeably, but they should not. “Weight” and “mass”
both have precise scientific and mathematical definitions. “Mass” should only be used to refer to the
amount of matter in an object. “Weight” refers to the force of gravity acting on that object. Mass is
needed to find an object’s weight, but it is not the same as weight.
For a class 1 lever, the fulcrum is between the input force and the load. Moving the fulcrum closer to
the load increases the length of the effort arm and decreases the length of the load arm. This increases
the mechanical advantage, which can be found by dividing the effort arm length by the load arm
length.
11.Assuming
2L
that the pop in the bottle has about the same mass as water, 2 L of pop will have a mass of
1000 cm3
1g
1 kg
1 cm3 1000 g 2 kg
1L
2 kg 9.8 m/s2 19.6 N
The pop would weigh 19.6 N.
12.
Student responses will vary, but their cartoons should address in some way the physics concepts covered
in this chapter, including simple machines, mechanical advantage, work, and forces.
SOLVE A PROBLEM!
13.
100 N
MA 20 N 5
14. (a)
(b)
15. (a)
W F d 0.25 m 50 N 12.5 J
The amount of work done would be the same as the amount of work needed to move the lever with
the fulcrum in the middle. The lever does not change the amount of work that needs to be done,
only the input force needed and the distance over which the force is applied.
W F d 4500 N 1.5 m 6750 J
(b)
The output force of 4500 N is approximately 4 times larger than the input force of 1200 N. To move
the truck a distance of 1.5 m, then, the people must move approximately 4 times that distance, or 6 m.
16. (a)
Yes, a single moveable pulley has a mechanical advantage of 2, so a 75 N force would be able to move
a load that is up to 150 N. The load plus the frictional force totals only 145 N.
(b)
A single fixed pulley would not work. A single pulley has a mechanical advantage of 1, so using that
pulley with a 75 N input force would only allow movement of an object with a weight of 75 N or less.
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CREATE AND EVALUATE!
17.(a)
(b)
Sample answer: Generally a lever-style door opener will have a greater mechanical advantage than a
doorknob. This is because the lever is generally longer than the radius of the doorknob, which means
that it takes less force to use a lever-style door opener. The doorknob also has the disadvantage of
being slipperier if your hands are wet. More force is needed to create the friction necessary to turn a
doorknob than is needed to move the lever.
Sample answer: A person with special physical needs would most likely find it easier to use a leverstyle door. Less force is needed to grip the lever and to move it far enough to open the door.
18.
Sample answer: Because of the importance of the automobile to society, I would say that the wheel and
axle is the simple machine that has had the greatest effect on the environment. Along with the actual
wheels and axles of the car, there are also wheels that make up the gears, and the engine itself has wheels
that move the pistons. The automobile creates pollution from burning gasoline, and it also allows people
to transport their garbage longer distances.
19.
Students will come up with many possible devices that are used by people with a variety of special needs,
including wheelchairs, access ramps, and hand-controlled brakes. Their descriptions of these devices
should detail some aspect of the physics contained in this chapter—for example, a discussion of the
simple machines used in more complex machines or of the mechanical advantages they provide.
REFLECT ON YOUR LEARNING
20. (a)
(b)
Sample answer: I first learned about the idea of gravity in a Grade 5 science class in which we
discussed different forces and how they affected our everyday lives.
(c)
Sample answer: When we learned about gravity and magnetism originally we did not learn about
them in such a detailed way. For example, we didn’t learn that forces could be measured, and that the
amount of work done by a force could be calculated mathematically.
21. (a)
(b)
NEL
Sample answer: I already knew that there were forces like gravity and magnetism that attracted objects
and caused them to move. I didn’t realize that these attractions produced a kind of work.
Sample answer: Work is what happens when a force moves an object a certain distance. Energy is
required to create the force, and the object must move in the same direction as the force.
I want to learn how more complicated machines combine the simple machines to create mechanical
advantages. I also want to learn about the four main forces, especially strong and weak nuclear forces.
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