Newton's third law

1. Actions €trnrys
RecrGflems
The basketball is passed to you, and you
dribble down the court. The score is tied.
There's only one defender between you and
the basket. You hear the fans cheering and
chanting. They're counting down the clock!
"Ten, nine, eight, seven! . ." This is your
moment. You break left. The guard is right in
your face. You break right. "Six , flve, four. . ."
She's still in your face. Cutting left again, Vou
catch her off balan ce, but she recovers and is
coming back at you. "Three, two . . ." At the
top of the key, she's right on you! With one
second to go, Vou push off the floor and rise
hign in the air with your head, arms, and
hands well above the defender. With a flick of
your wrists, Vou launch the ball toward the
basket. The fans chant, "One!" The buzzer
sounds, and silence falls throughout the arena
as the ball arcs toward the basket.
What happened? How did you get into the ,
air above the defender? What forces were
involved in your last-second heroics?.
Nevvtonts Third Law of Motion
Had Sir Isaac Newton been watching Vou,
he would have been happy to explain that you
had just provided a wonderful demonstration
of his third law of motion. Once he stopped
chanting with the crowd, he might have said,
"Ah, ffiV young friend, don't you know that
for every action force there is an equal and
opposite reaction force?" In this case, the action
force was your feet pushing against the floor.
When you shoot a basketball, the balt pushes

2. The reaction force was the floor pushing
against your feet. Because you exerted a
force, the floor pushed back against you.
This caused you to jump high above the
defender trying to block you.
Newton's third law of motion
states that for every action force there is
an equal and opposite reaction force.
Two important things to remember about
Newton's third law are, (1) forces always
occur in pairs made up of an action
force and a reaction force; (2) the
action force and the reaction force
always act on different bodies.
When you made your spectacular
jump, Vou might have felt the action and
reaction f orces betw een your feet and the
floor. But on a hard surface like a basket-
ball court, it's difficult to see these forces
work. If you jumped on a trampoline,
though, it would be easy to observe the
trampoline pushing back on you as you
pushed on it.
Mass Matters
Suppose that the defending player had
jumped high enough to block your shot.
Let's also suppose that she had pushed
off the floor with a force exactly equal to
the force you used to push off, but that
she had less mass than you. The third law
explains that the reaction f orce of the
floor would be equal to this player's
action force. The reaction force, how-
ever, would be acting on a smaller mass.
According to Newton's second law, this
player would have a gre ater acceleration
and would, therefore, go higher than
you. If that had happ ened, she probably
would have blocked your shot. But we
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3. lf two people of different masses jump with the same force,
which one is going to jump higher? Many basketball players can
jump more than one meter straight up. About how many inches is this?
know that wasn't the case, because the
basketball is still arcing toward the basket
as if in slow motion.
The Third Law and Earthts Surface
Think again about your leap. You
already know that when your feet push
against Earth, Earth pushes back. Your
momentum is equal to your mass multi-
plied by your takeoff speed. Suppose you
pushes back, causing you to jump into
the air. Since your momentum when you
jumped is 500 kg x m/s, Earth's
momentum must be - 500 kg x m/s,
the opposite of your momentum.
Remember, momentum is conserved.
How fast does Earth move backward in
reaction to the action of your jump? You
don't even have to calculate that number
to know that Earth's backward velocity is
very, very small. No one watching your
jump shot will have any idea that Earth
has rnoved at alll Actually, all the fans
care about is whether or not the ball goes
through the hoop!
So, what do you think? You took a
shot at the last second. Are you a hero,
or is the score still tied? I
have a mass of 50 kg (110 lb) and
jumped with a speed of 10 m/s (11 yd/s).
Then your momenturn would be
P=mxv
P = 50 kg x 10 m/s
P=500kgxm/s
What is Earth's reaction to your
action? As you push against Earth, it
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