9781118230725.pdf

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EarthE

TableT

CantaloupeC

(a)

(b)

(c)

FCE (gravitational force)

FCT

(d)

FTC

Earth

Cantaloupe

FCT (normal force from table)

FCE

FEC

These forces

just happen

to be balanced.

These are

third-law force

pairs.

So are these.

Figure 5-11 (a) A cantaloupe lies on a table that stands on Earth. (b) The forces on
the cantaloupeare and. (c) The third-law force pair for the cantaloupe – Earth
interaction. (d) The third-law force pair for the cantaloupe – table interaction.

F

:

F CE

:

CT

5-3 APPLYING NEWTON’S LAWS

any two bodies interact in any situation, a third-law force pair is present. The
book and crate in Fig. 5-10aare stationary, but the third law would still hold if
they were moving and even if they were accelerating.
As another example, let us find the third-law force pairs involving the can-
taloupe in Fig. 5-11a, which lies on a table that stands on Earth. The cantaloupe
interacts with the table and with Earth (this time, there are three bodies whose
interactions we must sort out).
Let’s first focus on the forces acting on the cantaloupe (Fig. 5-11b). Force
is the normal force on the cantaloupe from the table, and force is the
gravitational force on the cantaloupe due to Earth. Are they a third-law force
pair? No, because they are forces on a single body, the cantaloupe, and not on
two interacting bodies.
To find a third-law pair, we must focus not on the cantaloupe but on the
interaction between the cantaloupe and one other body. In the cantaloupe – Earth
interaction (Fig. 5-11c), Earth pulls on the cantaloupe with a gravitational force
and the cantaloupe pulls on Earth with a gravitational force. Are these
forces a third-law force pair? Yes, because they are forces on two interacting bod-
ies, the force on each due to the other. Thus, by Newton’s third law,

(cantaloupe – Earth interaction).

Next, in the cantaloupe – table interaction, the force on the cantaloupe from
the table is and, conversely, the force on the table from the cantaloupe is
(Fig. 5-11d). These forces are also a third-law force pair, and so

F (cantaloupe – table interaction).

:

CTF

:

TC

F

:

F TC

:

CT

F

:

CEF

:

EC

F

:

F EC

:

CE

F

:

F CE

:

CT

Checkpoint 5

Suppose that the cantaloupe and table of Fig. 5-11 are in an elevator cab that begins to

accelerate upward. (a) Do the magnitudes of and increase, decrease, or stay

the same? (b) Are those two forces still equal in magnitude and opposite in direction?

(c) Do the magnitudes of and increase, decrease, or stay the same? (d) Are those

two forces still equal in magnitude and opposite in direction?

F

:

F EC

:

CE

F

:

F CT

:

TC