Newtons Third Law of Motion

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Newtons Third Law of Motion Action Reaction
Pairs
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Two students A and B are sitting at rest on two
trolleys. They are going to push against each
other.
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How to push ?

• Arms are bent and at rest.

• Arms are stretched and moved forward.

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Two scenarios

• Both A and B push against each other.
• Only B do the pushing.

How will they move ?
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exerted by B on A
exerted by A on B
Whoever pushes, the PAIR of forces appear
SIMULTANEOUSLY and they point in OPPOSITE
directions.
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Two students PULL each other. Each of them holds
a spring balance to measure the magnitude of the
pulling force.
Two scenarios

• Both A and B pull against each other.
• Only B do the pulling.

Compare the readings on the two spring balances.
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Force exerted by B on A
Force exerted by A on B
What do you note about the magnitude of the two
forces ?
They are EQUAL in MAGNITUDE.
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Force exerted by B on A
Force exerted by A on B
A and B do not move towards each other. Why ?
Is it because the two forces balance each other ?
However they are acting on difference objects
! It is meaningless to say that they balance each
other.
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Force exerted by B on A
Force exerted by A on B
Friction exerted by the chair on A
Friction exerted by the chair on B
A and B do not move towards each other. Why ?
The pulling force on A is balanced by the
friction exerted on A by the chair. The pulling
force on B is balanced by the friction exerted on
B by the chair.
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Newtons Third Law of Motion

• If object A exerts a force on B, B exerts
  simultaneously a force of the same magnitude but
  opposite direction on A.
• The two forces are known as ACTION REACTION PAIR.

• Misconception
• One of the force, described as the ACTION,
  appears first.
• The other force, described as the REACTION,
  appears later.

In fact, we cannot identity which one is action
and which one is reaction. Both forces appear
simultaneously.
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Newtons Third Law of Motion

• If object A exerts a force on B, B exerts
  simultaneously a force of the same magnitude but
  opposite direction on A.
• The two forces are known as ACTION REACTION PAIR.

• Misconception
• Action Reaction pair will produce equilibrium
  because they are equal in magnitude but opposite
  in directions.

In fact, the pair of forces, although equal in
magnitude but opposite in directions, cannot
produce equilibrium in any objects because they
act on different objects.
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A baby is sitting on a chair. Identify the action
reaction pairs.
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A baby B is placed on top of a chair C. The chair
C is placed on the surface of the earth E.
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Consider the forces acting on the baby B.
RCB is the normal reaction acting by the chair C
on B.
FEB is the gravitational force acting by Earth E
on B. It is the weight of the baby B.
The baby B is in equilibrium.
RCB FEB (equal magnitude)
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Consider the forces acting on the chair C.
RBC is the normal reaction acting by the baby B
on C.
REC is the normal reaction acting by Earth E on
C.
FEC is the gravitational force acting by Earth E
on C. It is the weight of the chair C.
The chair C is in equilibrium.
REC RBC FEC
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Consider the forces acting on Earth E by B and C.
RCE is the normal reaction acting by the chair C
on E.
FBE is the gravitational force acting by B on E.
FCE is the gravitational force acting by C on E.
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What is the reaction force of RCB ?
FEB is equal in magnitude and in opposite
direction of RCB . Is it FEB ?
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What is the reaction force of RCB ?
FEB is equal in magnitude and in opposite
direction of RCB . Is it FEB ?
No. FEB is NOT the reaction force of RCB
because the two forces act on the same object B.
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RCB and RBC form an action reaction pair.
RCB is the normal reaction acting on B by C.
RBC is the normal reaction acting on C by B.
They are equal in magnitude and opposite in
direction and they act on different objects. They
are the same kind of forces.
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RCE and REC form an action reaction pair.
RCE is the normal reaction acting on E by C.
REC is the normal reaction acting on C by E.
They are equal in magnitude and opposite in
direction and they act on different objects. They
are the same kind of forces.
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FEB and FBE form an action reaction pair.
FEB is the gravitational force acting on B by E.
FBE is the gravitational force acting on E by B.
They are equal in magnitude and opposite in
direction and they act on different objects. They
are the same kind of forces.
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FEC and FCE form an action reaction pair.
FEC is the gravitational force acting on C by B.
FCE is the gravitational force acting on E by C.
They are equal in magnitude and opposite in
direction and they act on different objects. They
are the same kind of forces.
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Newtons Third Law of Motion

• If object A exerts a force on B, B exerts
  simultaneously a force of the same magnitude but
  opposite direction on A.
• The two forces are known as ACTION-REACTION PAIR.

• Addition comments
• In fact, we cannot identity which one is action
  and which one is reaction. Both forces appear
  simultaneously.
• The pair of forces, although equal in magnitude
  but opposite in directions, cannot produce
  equilibrium in any objects they act on different
  objects.
• The pair of forces are of the same kind, e.g.
  both are normal reactions or both are
  gravitational forces.

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Applications of Newtons Third Law of Motion
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2006
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The boat / swimmer pushes the water backward,
the water pushes the boat / swimmer forward.
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The contact between the paper and the table
surface is made smooth by the plastic beads.
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Rocket Propulsion
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2004
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The rocket pushes the exhaust (gas in the case of
chemical rocket or water in the case of water
rocket) out of it with a force . According to
Newtons Third Law, the exhaust pushes the rocket
back by a force of equal magnitude but opposite
in direction. Hence the exhaust and the rocket
accelerate in opposite directions. According to
Newtons Second Law, under an unbalanced force of
the same magnitude, the magnitude of the
acceleration of the object with smaller mass is
greater than that of the object of larger mass.