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(Chris Devlin) #1
Figure 5-7(a) A block resting on a table experiences a normal force perpendicular to
the tabletop. (b) The free-body diagram for the block.

F
:
N

Block

Normal force FN

(a)(b)

y

x

Block

Fg Fg

FN

The normal force
is the force on
the block from the
supporting table.

The gravitational
force on the block
is due to Earth's
downward pull.

The forces
balance.

104 CHAPTER 5 FORCE AND MOTION—I


either mass or weight units. (Most bathroom scales in the United States work this
way and are marked in the force unit pounds.) If the scale is marked in
mass units, it is accurate only where the value of gis the same as where the scale
was calibrated.
The weight of a body must be measured when the body is not accelerating
vertically relative to the ground. For example, you can measure your weight on a
scale in your bathroom or on a fast train. However, if you repeat the measure-
ment with the scale in an accelerating elevator, the reading differs from your
weight because of the acceleration. Such a measurement is called an apparent
weight.
Caution:A body’s weight is not its mass. Weight is the magnitude of a force
and is related to mass by Eq. 5-12. If you move a body to a point where the value
ofgis different, the body’s mass (an intrinsic property) is not different but the
weight is. For example, the weight of a bowling ball having a mass of 7.2 kg is 71 N
on Earth but only 12 N on the Moon. The mass is the same on Earth and Moon,
but the free-fall acceleration on the Moon is only 1.6 m/s^2.

The Normal Force
If you stand on a mattress, Earth pulls you downward, but you remain stationary.
The reason is that the mattress, because it deforms downward due to you, pushes
up on you. Similarly, if you stand on a floor, it deforms (it is compressed, bent, or
buckled ever so slightly) and pushes up on you. Even a seemingly rigid concrete
floor does this (if it is not sitting directly on the ground, enough people on the
floor could break it).
The push on you from the mattress or floor is a normal force. The name
comes from the mathematical term normal,meaning perpendicular: The force on
you from, say, the floor is perpendicular to the floor.

F


:
N

When a body presses against a surface, the surface (even a seemingly rigid one)
deforms and pushes on the body with a normal force that is perpendicular to
the surface.

F
:
N

Figure 5-7ashows an example. A block of mass mpresses down on a table,
deforming it somewhat because of the gravitational force F on the block. The

:
g
table pushes up on the block with normal force. The free-body diagram for the
block is given in Fig. 5-7b. Forces and are the only two forces on the block
and they are both vertical. Thus, for the block we can write Newton’s second law
for a positive-upward yaxis (Fnet,ymay) as
FNFgmay.

F


:
F N

:
g

F


:
N
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