NEWTON’S THIRD LAW
THE LAW OF
INTERACTION
TEST YOURSELF
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•
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Hammer hits nail,____________
Hand punches wall, ___________
Book pushes down on table top,________
Bat hits ball, _____________
Car pushes road, _____________
NEWTON’S COMMON SENSE
• Newton realized
that nature was
symmetrical. That is,
things occurred in
pairs.
• He said that forces
also occurred in
pairs
F 1,2 = -F 2,1
• For every action there is an equal and opposite
reaction
EARTH AND ROCK
• The earth pulls on a
1kg object with how
much force?
• Does the 1 kg
object pull on the
Earth? If so, how
much?
A NUMERICAL EXAMPLE.
•
•
For every action there is an equal
and opposite reaction
Lady bug
•
Mass = 3.5 g
•
Velocity = 4.8 m/s, south
•
F supplied by the car = 10 N
Nissan Titan
Mass = 2289 Kg (5000 lb.)
Velocity = 24.6 m/s (55 mph),
North
F supplied by the
bug__________N
IF THE FORCES ARE EQUAL AND OPPOSITE,
THEN WHY DON’T THEY CANCEL EACH
OTHER OUT?
• The reason they
don’t cancel out is
because they act
on different objects.
• For example: foot
kicks ball. Ball kicks
foot.
GUN EXAMPLE
• A gun with a mass
of 1.35 kg fires shot
with a mass of 45 g.
If the recoil force is 16.2 N, what are the
accelerations of the
gun and the shot?
SOLUTION
(ASSUME THE SHOT IS MOVING TO THE RIGHT
AND THIS IS THE POSITIVE DIRECTION)
• Shot
• Fnet = ma
• Fnet/m = a
• +16.2 N/.045kg = a
• a = 360 m/s2
• Gun
• Fnet = ma
• Fnet/m = a
• -16.2 N/1.35 kg = a
• a = - 12 m/s2
Would you want to shoot a huge (massive)
bullet with a small (less massive) gun?
NEWTON’S 3RD LAW IN ZERO
GRAVITY
• What would it be like??
(5 min)