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7.3 Newton’s third law of motion
Action and reaction
If you hit on the wall
with hand, you will feel
pain. Why?
The wall exerts an equal
but opposite force on
your hand.
 The pair of forces is called the
action-reaction pair.
7.3 Newton’s third law of motion
Examples of action-reaction pairs in sports:
A football player exerts
a force on the ball,
the ball also exerts
a force on his foot.
A high jumper exerts a
force on the pole,
an opposite force lifts
him up.
7.3 Newton’s third law of motion
The swimmer exerts a
force on the platform,
an opposite force pushes
the swimmer forward.
The paddle exerts a
force on the water,
the water exerts an
opposite force on the
paddle.
7.3 Newton’s third law of motion
Newton’s third law of motion
 Whenever an object
A exerts a force on
another object B,
object A
B will exert an equal (football
player)
but opposite force
on A.
 Newton’s third law of motion:
object B
(football)
To every action there is an equal and
opposite reaction.
7.3 Newton’s third law of motion
How water rockets
work?
compressed
air and water
force acting
on the rocket
force acting on
the water
When the air forces the water out,
the water exerts an opposite force.
Video: How water rockets work
The rocket
lifts up.
7.3 Newton’s third law of motion
How a bird
lift up?
forces exerted by the air on the wing
forces exerted by
the wing on the air
How a
helicopter lift up?
forces exerted by the air on the rotor
forces exerted
by the rotor on
the air
7.3 Newton’s third law of motion
Identifying the action-reaction pair
Consider a person standing on the
ground…
What are the forces
acting on the
person and on the
ground?
Which two
forces form an
action-reaction
pair?
7.3 Newton’s third law of motion
gravitational force
exerted on the person
by the Earth
mg
R
supporting force exerted by
the ground on the person
Do these two forces form
an action-reaction pair?
Action and reaction act on
different objects.
No
7.3 Newton’s third law of motion
Consider the force on the ground…
R
R’
supporting force
exerted by the ground
on the person
downward force exerted by
the person on the ground
R and R’ form an action-reaction pair,
but not R and the weight mg.
7.3 Newton’s third law of motion
Apparent weight in a lift
When you are travelling up in a lift…
What do you feel when
the lift just begins to
move upwards?
heavier
What do you feel
when the lift is about
to stop?
lighter
7.3 Newton’s third law of motion
Imagine that you are standing on a weighing scale
in a lift …
R gives you the feeling
of your own weight.
R = the reading on
the scale
Weight
mg
Supporting
force R
Net force
= R – mg = ma
7.3 Newton’s third law of motion
The lift is at rest
The lift moves upwards
at an acceleration a
a=0
mg
a>0
(take upward
direction as
positive)
R
Net force = R – mg = 0
R = mg
mg
R
Net force = R – mg = ma
R > mg
You feel heavier.
7.3 Newton’s third law of motion
The lift moves upwards
at a constant velocity
The lift moves upwards
at a deceleration a
a=0
mg
a<0
(take upward
direction as
positive)
R
Net force = R – mg = 0
R = mg
Simulation: Apparent weight in a lift
mg
R
Net force = R – mg = ma
R < mg
You feel lighter.
7.3 Newton’s third law of motion
That’s the end of Section 7.3
Check Point
Key Ideas
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