Transcript Newton`s Third Law

6.3 Newton's Third Law
pp. 142-146
NEWTON’S THIRD LAW OF
MOTION

Whenever one object exerts a force
on a second object, the second object
exerts a force that is equal in size and
opposite in direction on the first
NEWTON’S THIRD LAW OF
MOTION
In other words…
 For every action there is an equal and
opposite reaction.

NEWTON’S THIRD LAW OF
MOTION



All forces act in pairs called action-reaction
force pairs
If a force is exerted, another force occurs
that is equal in size and opposite in
direction to the first.
ACTION-REACTION FORCE PAIRS DO NOT
ACT ON THE SAME OBJECT!
6.3 Newton’s Third Law

Newton’s Third Law
(action-reaction)
applies when a
force is placed on
any object, such as
a basketball.
6.3 The Third Law:
Action/Reaction


Newton’s Third Law states
that every action force
creates a reaction force
that is equal in strength
and opposite in direction.
There can never be a
single force, alone, without
its action-reaction partner.
6.3 Action and reaction

When sorting out
action and reaction
forces it is helpful to
examine or draw
diagrams.
Here the action force is on the ________________, and
the reaction force is on the _______________.
Solving Problems
A
woman with a
weight of 500
newtons is sitting on
a chair.
 Describe one actionreaction pair of
forces in this
situation.
Solving Problems
Looking for:
1.

…pair of action-reaction forces
Given
2.

Fc = 500 N
…girl’s forceW = -500 N (down)
Fw = -500 N
Relationships:
3.

Action-reaction forces are equal and opposite and act
on different objects.
Solution
4.

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
Draw a free body diagram
The downward force of 500 N exerted by the woman
on the chair is an action.
Therefore, the chair acting on the woman provides an
upward force of 500 N and is the reaction.
6.3 Collisions


Newton’s third law tells us that any time two
objects hit each other, they exert equal and
opposite forces on each other.
The effect of the force is not always the same.
MOMENTUM


Momentum is a property of a moving
object that depends on its mass and
velocity.
The more momentum an object has, the
harder it is to stop the object or change its
direction.
6.3 Momentum

Momentum is the mass of a object times its
velocity.
 The units for momentum are kilogrammeter per second (kg·m/s).
6.3 Momentum

The law of
conservation of
momentum states
that as long as the
interacting objects are
not influenced by
outside forces (like
friction) the total
amount of momentum
is constant or does
not change.
6.3 Momentum

We use positive and
negative numbers to show
opposite directions.
The result of a
skateboarder throwing a
1-kg ball at a speed of 20 m/sec is that he and
the skateboard with a
total mass of 40 kg
move backward at a
speed of +0.5 m/sec (if
you ignore friction).
6.3 Collisions
When a large truck hits
a small car, the forces
are equal.
 The small car
experiences a much
greater change in
velocity much more
rapidly than the big
truck.

Which vehicle ends up
with more damage?
Solving Problems
If an astronaut in space
were to release a 2kilogram wrench at a
speed of 10 m/s, the
astronaut would move
backward at what
speed?
 The astronaut’s mass is
100 kilograms.

Solving Problems
Looking for:
1.

… the velocity of the astronaut (backward)
Given
2.


…velocity1 = 10 m/s; mass1= 2 kg;
...mass2 = 100 kg;
Relationships:
3.

m1 v1 = m 2 v2
Solution
4.

Draw a free body diagram.