9781118230725.pdf

Newton’s Third Law

Two bodies are said to interactwhen they push or pull on each other — that is,

when a force acts on each body due to the other body. For example, suppose you

position a book Bso it leans against a crate C(Fig. 5-10a). Then the book and

crate interact: There is a horizontal force on the book from the crate (or due

to the crate) and a horizontal force on the crate from the book (or due to the

book). This pair of forces is shown in Fig. 5-10b. Newton’s third law states that

F

:

CB

F

:

BC

Newton’s Third Law:When two bodies interact, the forces on the bodies from each

other are always equal in magnitude and opposite in direction.

For the book and crate, we can write this law as the scalar relation

FBCFCB (equal magnitudes)

or as the vector relation

(equal magnitudes and opposite directions), (5-15)

where the minus sign means that these two forces are in opposite directions. We

can call the forces between two interacting bodies a third-law force pair.When

F

:

BCF

:

CB

Figure 5-10 (a) Book Bleans against crate
C.(b) Forces (the force on the book
from the crate) and (the force on the
crate from the book) have the same mag-
nitude and are opposite in direction.

F

:

CB

F

:

BC

BookB CrateC

(a)

(b)

C

FBC FCB

B

The force onB

due to Chas the same

magnitude as the

force onC due to B.

106 CHAPTER 5 FORCE AND MOTION—I

even if the bodies and the cord are accelerating and even if the cord runs around

amassless, frictionless pulley(Figs. 5-9bandc). Such a pulley has negligible mass

compared to the bodies and negligible friction on its axle opposing its rotation. If

the cord wraps halfway around a pulley, as in Fig. 5-9c, the net force on the pulley

from the cord has the magnitude 2T.

Checkpoint 4

The suspended body in Fig. 5-9cweighs 75 N. Is Tequal to, greater than, or less than

75 N when the body is moving upward (a) at constant speed, (b) at increasing speed,

and (c) at decreasing speed?

5-3APPLYING NEWTON’S LAWS

After reading this module, you should be able to...

5.14Identify Newton’s third law of motion and third-law force pairs.
5.15For an object that moves vertically or on a horizontal or inclined
plane, apply Newton’s second law to a free-body diagram of the
object.

5.16For an arrangement where a system of several objects

moves rigidly together, draw a free-body diagram and

apply Newton’s second law for the individual objects

and also for the system taken as a composite object.

●The net force on a body with mass mis related to the body’s
acceleration by
,

which may be written in the component versions

Fnet,xmax Fnet,ymay and Fnet,zmaz.

Fnet

:

ma:

:a

Fnet

:

●If a force acts on body Bdue to body C, then there is

a force on body Cdue to body B:

The forces are equal in magnitude but opposite in directions.

FBC

: 

FCB

:.

FCB

: FBC

:

Learning Objectives

Key Ideas