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Newton’s Third Law of Motion states that if object A exerts a force on object B, then object B must exert a force of equal magnitude and opposite direction back on object A. According to this law, forces always occur in pairs and a body cannot exert a force without also being subjected to one. This law also illustrates a specific symmetry in nature. Three Newton’s law of motion illustrates the behavior of an object with respect to motion. An object changing its position with regard to time is referred to as motion. Force is a form of push or pulls that is acting on an object and causing it to interact with another thing. As a result of the interaction, force is generated. Forces always occur in pairs. And one object cannot exert force on another without experiencing force itself.
Read More: Force and Acceleration
Key Terms: Newton’s Law of Motion, Acceleration, Force, Action-Reaction Force, Pressure, Interaction Force Pairs
Newton’s Laws of Motion
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Newton’s Laws of Motion is the foundation of all classical mechanics. Newtonian Mechanics is one of the oldest branches in the science of motion and has also paved the way for modern physics. Three Newton’s law of Motion is as follows:
- Newton’s First Law of Motion: According to Newton's first law of motion, every object will remain at rest or in uniform motion in a straight line unless compelled to change its state by the action of an external force.It explains the inertia.
- Newton’s Second Law of Motion: Newton’s second law of motion states that when a force acts on a body, the body moves in such a way that the force and the time rate of change of momentum are equal. It explains the force.
- Newton’s Third Law of Motion: According to Newton’s third law of motion, when two bodies interact, forces are generated that are both equal in strength and directed in opposing directions. It explains the action and reaction.
Laws of Motion Video Lecture
Read More: Law of Inertia
Newton’s Third Law of Motion
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Newton’s Third Law of Motion which is also known as the law of action and reaction states that every action has an equal and opposite reaction. One of the bodies of the two decides the magnitude and the direction of the body. In other words, every action has an equal and opposite reaction.
- Every interaction involves two forces operating on the two interacting objects.
- The forces exerting pressure on the first object are the same as those exerting pressure on the second object.
- When compared to the force on the second object, the force on the first object acts in the opposite direction.
- Action and reaction forces are always equal and in opposition to one another.
- The push or pull that occurs when one object interacts with another is referred to as a force.
- Frictional, tensional, and applied forces are examples of contact-related forces, whereas gravitational, electrical, and magnetic forces are examples of distance-related forces.
If object A exerts a force (FA) on object B, then object B will exert a force (FB) of equal magnitude and opposite direction back on object A.
Interaction Force Pairs
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A body will experience force on itself while exerting force on another body. Forces always take place in pairs which determines the symmetrical nature of the law.
Action-Reaction Pair
Action Force: The action force is the initial external force applied to the body.
Reaction Force: The force a body utilizes to react in the opposite direction to an action force is referred to as reaction force.
If body A is exerting force on body B, then the force acting on body B is known as action and the opposite force is called reaction.
If Fa = force exerted by Body A on body B
Fb = force exerted by body B on A
then,
Fa = Action and Fb = reaction.
- Both bodies depend on each other for action and reaction.
- There is no cause-effect relationship between the bodies of action-reaction.
- Their action and reaction to different bodies cannot be canceled out.
- The action-reaction pair involves mutual forces which are opposite and equal between the bodies.
Read More: Effects of Force
Examples of Interaction Force Pairs
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Some of examples of action-reaction pairs are:
Birds Flying
The wings of the bird exert a force on the air pushing the air downwards whereas the air pushes the air upwards.
Fish in Water
The fins of the fish help in pushing the water backward which helps in accelerating the fish forward. The size of the force on the fish is equal to the size of the force on the water. The direction of the force of the fish forward is opposite to the direction of the force of the water backward.
Swimming
A swimmer can swim in the water as he/she pushes the pool water backward and the water pushes the swimmer forward.
Lift by Helicopters
An upward reaction force is created by pushing the air downwards. This causes the helicopter to lift and hover in position.
Rock Climbers
To pull themselves upwards, the rock climbers pull their vertical ropes downwards.
Things to Remember
- Newton’s Third Law of Motion describes specific symmetry in nature.
- A push or pull that is acting on an object and causing it to interact with another thing is called force.
- There is an equal and opposite reaction to every action.
- The initial external force that is applied to the body is known as the action force.
- Reaction force is the force a body employs to respond in the opposite direction to an action force.
- Mutual forces that are opposed and equal between the bodies are involved in the action-reaction pair.
Previous Years’ Questions
- The smaller mass flies off with a velocity… [NEET – 2019]
- A block of mass 10kg is in contact against the inner wall… [NEET – 2019]
- An object with mass 5 kg is acted upon by a… [KCET – 2019]
- A block kept on a rough inclined plane… [JEE Main – 2019]
- A man weighing 60 kg is in a lift moving down… [KCET – 2018]
- The external force required to take the same body up… [JEE Main – 2018]
- The centre of gravity of the block is found to rise… [NEET – 2016]
- Two stones of masses m and 2m are whirled in… [NEET – 2015]
- If a force of 14N is applied on the 4kg block, the contact force… [NEET – 2015]
- The force of friction between A and B is… [KCET – 2011]
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Sample Questions
Ques. A rabbit that has a mass of 4kg jumps upon a table that has a mass of 25 kg. What is the average force which is applied by the table on the rabbit as the rabbit walks across the table? Given g= 10m/s2 (3 Marks)
Ans. The applying force between the rabbit and the given table is entirely dependent on the mass of this rabbit. As a result, the mass of the table is not important. It the weight of the rabbit alone which is the force exerted by the rabbit. The table also exerts the same amount of force according to the definition of the third law of newton.
The force exerted by the table is:
Fg = FN = ma
Thus, (4*10) kg m/s2 = 40N.
Ques. Deepa and Aswani are standing on identical skateboards. Aswini and Deepa push off of each other and travel in opposite directions. If aswini (M) and Deepa (m) have equal masses, who travels farther?(2 Marks)
Ans. Neither. Deepa and Awsini will cover an equal distance as the force exerted to each will be the same.
Ma = ma
a = a
They will cover the same distance because both will have the same acceleration and we know that acceleration controls the distance of each of them.
Ques. Why a horse cannot pull a cart and run in an empty place? (1 Mark)
Ans. The horse pushes the earth under its feet for running or pulling the cart. The reaction of the earth initiates the forward movement. In an emptyspace, there is no force of reaction hence, a horse cannot pull a cart or run in an empty space.
Ques. A cat of mass 5kg jumps on the dining table of mass 30kg. As the cat walks around on the table, what is the average force that the table implies to the cat? (3 Marks)
Ans. The mass of the table is not important as the table solely depends on the mass of the cat. The force that the cat exerts on the table is solely its weight. The table also exerts a force to the cat of the same magnitude.
The force on the cat from the table is:
Fg = FN = ma
= (5kg) [10 (m/s2)]
= 50N
Ques. A wooden block of mass 2 kg rests on a soft horizontal floor. When an iron cylinder of mass 25kg is placed on top of the block, the floor yields steadily and the lock and the cylinder go down together with an acceleration of 0.1 ms-2. What is the action of the block on the floor before and after the floor yields? Take g= 10ms-2. Identify the action-reaction pairs in the problem. (5 Marks)
Ans. Before the floor yields: the block is resting on the floor. The force of gravitational action by the earth is 2*10 = 20 N and the normal force R on the floor on the block. According to the first law, the net force on the block must be zero which means R=20N. According to the third law of motion, the force implied by the floor by the block is equivalent to 20N and initiates downward movement.
Action-reaction pairs:
(i) The force of gravity that is 20N on the block by the earth = action
The force of gravity on the earth by the block is equivalent to 20N is directed downwards = reaction
(ii) Action = the force on the floor by the block
Reaction = the force on the block by the floor
After the floor yields: the block and the cylinder exert downward movement with 0.1ms-2. There are two forces on the system- the normal force R' by the floor, the force of gravity due to the earth, i.e., 270N. There are also internal forces between the cylinder and the block. According to the second law of motion, 270- R' = 27* 0.1N, i.e., R' = 267.3 N. according to the third law of motion, the action of the cylinder and block on the floor is equal to 267.3 N vertically downward.
Action-reaction pairs:
(i) Action= the force of gravity that is 270N on the system by the earth
Reaction= the force of gravity on the earth by the system which is equal to 270N upwards.
(ii) Action=the force on the floor by the system
Reaction = the force on the system by the floor
Ques. State Newton’s third law of motion. Give an example. (2 Marks)
Ans. According to Newton's Third Law of Motion, if object A applies a force to object B, then object B must apply a force to object A that is equal in strength and directed in the opposite direction.
For example The swimmer accelerates (swims) in the direction opposite to that of his push while pushing against the pool wall with his feet.
Ques. Mention the types of forces. (2 Marks)
Ans. Force is the pushing or pulling that affects an object and causes it to interact with another object.
There are two types of forces
- Contact force
- Non-contact force
Ques. A man pushes a wall with a force of 60N towards the north. What force does the wall exert on the man? (3 Marks)
Ans.
Given that,
The action force, F is 60 N.
According to Newton’s third law of motion,
Action force = – Reaction force
Therefore, the reaction force = -60 N
Hence, the reaction force is equal to 60 N towards the south.
Ques. A tennis ball of mass 600 g traveling at 20 m/s is hit by a cricket bat to return it along its original path with a velocity of 10 m/s. Calculate the change of momentum that occurred in the motion of the tennis ball by the force applied by the tennis racket. (5 Marks)
Ans. Given that,
The mass of the tennis ball, m is 600 g = 0.6 kg.
The initial velocity of the ball, u is 20 m/s.
The final velocity of the ball after hitting, v is 10 m/s.
The change in momentum = Final momentum – Initial momentum
= mv – mu
= m (v – u)
= 0.6 kg × (20 m/s – 10 m/s)
= 0.6 kg × 10 m/s
= 6 kg m/s
Therefore, the change of momentum of the tennis ball by the force applied by the tennis racket is 6 kg m/s.
Ques. What principle does Newton's third law of motion adhere to? State the principle. (2 Marks)
Ans. Newton’s third law of motion obeys the principle of conservation of momentum. When no unbalanced external force affects the system, the principle of conservation of momentum states that the sum of the beginning momentum and the sum of the system's final momentum is exactly equal.
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