Chapter 8: Work &
Machines
Pg 191 #1-3, pg 197 #1-4, pg 202
#1-3, pg 205 #1-3
Pg 191 #1-3
1. Work is done on a ball when a pitcher
throws it. Is the pitcher still doing work on
the ball as it flies through the air?
Explain.
The pitcher is no longer doing work
on the ball as it flies through the air
because he is no longer exerting a
force on it. However, work is being
done on the ball by the Earth, which
exerts a force on the ball & pulls it
back toward the ground.
Pg 191 #1-3 (cont.)
2. Explain the difference between work &
power.
Work occurs when a force causes an
object to move in the direction of the
force, and power is the rate at which
work is done. The more work you do
in a given amount of time, or the less
time it takes you to do a given amount
of work, the greater your power.
Pg 191 #1-3 (cont.)
3. You lift a chair that weighs 50N to a height
of 0.5m and carry it 10m across the room.
How much work do you do on the chair?
50N x 10m = 500J
Pg 197 #1-4
1. Explain how using a ramp makes work easier.
Using a ramp makes work easier because it
allows you to apply a smaller input force than
you would have to apply when lifting a load
straight up. However, the smaller force has to
be exerted over a longer distance.
2. Why can’t a machine be 100% efficient?
A machine can’t be 100% efficient
because some of the work input is used
to overcome friction. Therefore, work
input is always greater than work output.
Pg 197 #1-4 (cont.)
3. Suppose you exert 15N on a machine,
and the machine exerts 300N on another
object. What is the machine’s mechanical
advantage?
MA = 300N/15N = 20
Pg 197 #1-4 (cont.)
4. For the machine described in question 3, how
does the distance through which the output force
is exerted differ from the distance through which
the input force is exerted?
Because the output force is greater than
the input force, the distance over which
the output force is exerted must be shorter
than the distance over which the input
force is exerted. This is an example of the
force-distance trade-off.
Pg 202 #1-3
1. Give an example of each of the following
simple machines: 1st class lever, 2nd
class lever, 3rd class lever, inclined plane,
wedge, & screw.
1st class lever: a screwdriver used to pry
the lid off a paint can; 2nd class lever: a
wheelbarrow; 3rd class lever: your leg as
you kick a soccer ball (your knee is the
fulcrum); inclined plane: a ramp on the
back of a moving truck; wedge: a knife;
screw: a jar lid.
Pg 202 #1-3 (cont.)
2. A 3rd class lever has a mechanical
advantage of less than 1. Explain why it is
useful for some tasks.
A 3rd class lever helps because it
increases the distance through which
the output force is exerted. For
example, when you move the handle
of a fishing pole just slightly, the other
end of the pole moves a great
distance.
Pg 202 #1-3 (cont.)
3. Look back at Figures 6, 7, & 8 in Section
2. Identify the type of simple machine
shown in each case. (If a lever is shown,
identify its class.)
Figure 6: The screwdriver is a 1st
class lever; Figure 7: The ramp is
an inclined plane; Figure 8: The
handcart is a 1st class lever.
Pg 205 #1-3
1. Give an example of a wheel & axle.
Examples include the crank on a can opener,
the reel on a fishing rod, a screwdriver, a
doorknob, the crank on an ice cream maker,
and the film-advance mechanism on an old
camera.
2. Identify the simple machines that make up
tweezers & nail clippers.
Each side of the tweezers is a 3rd class lever.
The sharpened edges of nail clippers are
wedges, and the arm that activates the clipper
is a 2nd class lever.
Pg 205 #1-3 (cont.)
3. The radius of the wheel of a wheel & axle
is 4 times greater than the radius of the
axle. What is the mechanical advantage of
this machine?
The mechanical advantage of a
wheel & axle is determined by the
ratio of the wheel radius to the
axle radius. So this machine
would have a mechanical
advantage of 4.