Download Newtons 3rd Law of Motion

Newtons 3rd Law of Motion
Physics
Newton’s 3rd Law
Newton's FIRST Law tells you what happens to an object if the net force on it is zero.
Newton's SECOND Law tells you what happens to an object if the net force on it is not
zero. What could Newton's Third Law possibly tell you?
In a sense, Newton's First and Second Laws tell you what forces do. Newton's Third Law
tells you what forces are.
Newton's Third Law is often stated:
For every ACTION there is an EQUAL and OPPOSITE REACTION .
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"action...reaction" means that forces always occur in PAIRS. Forces
are interactions between objects, like conversations are interactions
between people. Single, isolated forces NEVER happen. The two
forces involved are called the "action force" and the "reaction force."
"equal" means two things:
a. Both forces are exactly the same size. They are equal in
MAGNITUDE.
b. Both forces exist at exactly the same time. They both start at
exactly the same instant, and they both stop at exactly the same
instant. They are equal in time.
"opposite" means that the two forces always act in opposite
DIRECTION - exactly 180o apart.
According to Newton’s 3rd law forces are always in the form:
A pushes/pulls B
B pushes/pulls A
One of these forces is called the action force, and the other one is called the reaction force
- it doesn't matter which is which. For example:
Charlie Brown catches a ball. The ball exerts a force against his glove.
Step 1: Identify the interacting objects – A and B.
Step 2: If the action is A on B, then the reaction is B on A.
The BALL pushes/pulls the GLOVE (Action Force)
The glove pushes/pulls the ball. (Reaction Force)
or
The GLOVE pushes/pulls the BALL (Action Force)
The BALL pushes/pulls the GLOVE. (Reaction Force)
Complete the table by writing the reaction for each action.
Action
Reaction
When you walk, you push against the floor. The floor pushes against you.
The tires of a car push against the road.
The road pushes against the tire.
When swimming, you push the water
backward.
The water pushes you forward.
A dog wags its tail.
The tail wags the dog.
You push on a wall.
The wall pushes on you.
When a batter swings, the bat exerts a force The ball exerts a force on the bat.
on the ball.
When Charlie Brown kicks a football, the force his
foot exerts on the ball is exactly equal and opposite to
the force the ball exerts on his foot.
The picture to the left shows a golf ball being hit by a
golf club. Explain why the change in velocity of the
golf ball is much greater than the change in velocity of
the golf club.
If you think about Newton’s 2nd law: F=ma the answer
becomes clear. The smaller the mass, the larger the
acceleration. The golf club has a larger mass, thus
smaller acceleration.
If the action force is the racquet hitting the ball, what is
the reaction force? The reaction force is the ball hitting
the raquet.
As an elephant falls toward Earth, suppose that the action
force is the pull of Earth on the elephant. According to
Newton’s 3rd law what is the reaction force:
a. Air resistance acting on the elephant.
b. The pull of the elephant on the Earth.
c. The acceleration of the elephant.
d. Gravity.
The reason why we do not sense this acceleration is because:
a.
b.
c.
d.
The elephant’s pull on Earth is much smaller than Earth’s pull on the elephant.
Earth’s huge mass causes its acceleration to be infinitesimally small.
Earth’s acceleration is in the same direction as the elephant’s acceleration.
The elephant’s acceleration is much smaller than the Earth’s acceleration.