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Basic Machines - Intro to machines and motion theories | Next
Moment of Force |
CHAPTER 3
THE WHEEL AND AXLE
CHAPTER LEARNING OBJECTIVES
Upon completion of this chapter, you should be able to do the following:
l Explain the advantage of the wheel and axle.
Have you ever tried to open a door when the knob
was missing? If you have, you know that trying to twist
that small four-sided shaft with your fingers is tough
work. That gives you some appreciation of the
advantage you get by using a knob. The doorknob is an
example of a simple machine called a wheel and axle.
The steering wheel on an automobile, the handle of
an ice cream freezer, and a brace and bit are all examples
of a simple machine. All of these devices use the wheel
and axle to multiply the force you exert. If you try to
turn a screw with a screwdriver and it doesn’t turn, stick
a screwdriver bit in the chuck of a brace. The screw will
probably go in with little difficulty.
There’s something you’ll want to get straight right
at the beginning. The wheel-and-axle machine consists
of a wheel or crank rigidly attached to the axle, which
turns with the wheel. Thus, the front wheel of an
automobile is not a wheel-and-axle machine because the
axle does not turn with the wheel.
MECHANICAL ADVANTAGE
How does the wheel-and-axle arrangement help to
magnify the force you exert? Suppose you use a
screwdriver bit in a brace to drive a stubborn screw.
Look at figure 3-1, view A. You apply effort on the
handle that moves in a circular path, the radius of which
is 5 inches. If you apply a 10-pound force on the handle,
how much force will you exert against the resistance at
the screw? Assume the radius of the screwdriver blade
is 1/4 inch. You are really using the brace as a
second-class lever—see figure 3-1, view B. You can find
the size of the resistance by using the formula
In that
L
=
1
=
R
=
E
=
radius of the circle through which the
handle turns,
one-half the width of the edge of the
screwdriver blade,
force of the resistance offered by the
screw,
force of effort applied on the handle.
Figure 3-1.-It magnifies your effort.
3-1
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