WBCHSE Class 11 Physics Newton’s Third Law of Motion Notes

Newton Law Of Motion – Discussions On The Third Law

WBBSE Class 11 Newton’s Third Law Notes

Newton’s Laws 3rd Law

Newton’s third law of motion states that, for every action, there is an equal and opposite reaction. Consider two bodies A and B. They may be in contact with each other, or separated by a distance.

Suppose A exerts a force on B, denoted by \(\vec{F}_{A B}\). According to Newton’s third law, B will exert a force on A simultaneously, that is equal in magnitude but opposite in direction. This force is denoted by \(\vec{F}_{A B}\).

Thus, we have \(\vec{F}_{A B}=-\vec{F}_{B A} \quad therefore\left|\vec{F}_{A B}\right|=\left|\vec{F}_{B A}\right|\)

If \(\vec{F}_{A B}\) is called the action, \(\vec{F}_{B A}\) is the reaction. Action and reaction always coexist in pairs and one exists as long as the other one is present.

Read and Learn More: Class 11 Physics Notes

Examples Of Action-Reaction Pairs:

1. Thrust: A wooden block is resting on a table surface. The block exerts a downward force \((\vec{W})\) on the table surface due to its weight. At the same time, the table also exerts an equal upward force \((\vec{R})\), on the block.
   • The downward force of the block on the tabletop is the action and the upward force of the table on the block, is the reaction. A pair of action and reaction forces of this type is called thrust.
2. Tension: A body, suspended by a wire whose upper end is fixed to a rigid support, exerts a downward force on the wire, due to its weight. The wire, at the same time, exerts an equal upward force on the suspended body.
   • This force or reaction is called the tension of the wire. If the weight of the suspended body is termed as action, the tension in the wire is the reaction.
   • When a cord (rope, cable, etc.) is attached to a body and tightly pulled (action), the cord pulls on the body with a force (reaction) directed away from the body and along the cord. This force is also called tension force.
3. Push: When a person applies a force on a large piece of stone lying on a road, the stone also exerts a force on the person which is equal in magnitude but opposite in direction. Such a pair of actions and reactions constitute a push.
4. Impact or Collision: When a moving car runs into a wall, as the collision car exerts a force on the wall, the wall also exerts an equal and opposite force on the car. The force on the wall is the action and that on the car is the reaction. Such pairs of forces are called impact or collision.
5. Attraction: When a pole of a magnet is brought near an iron piece, they attract each other. Such an attraction is called mutual attraction. Another example of attraction is the pull of the earth on a body which causes a downward motion and it is an action.
   • The body also pulls the earth towards it as a reaction. However, since the mass of the earth is very large, the reaction force cannot cause any noticeable acceleration of the earth, and it appears to be at rest.
6. Repulsion: The action-reaction pair that causes bodies to move away from each other is called repulsion. The north poles of two magnets, when brought close together, repel each other due to action and reaction forces.
7. Friction: When a body moves or tries to move on a surface, the surface exerts an opposing force on the body. This force resists the motion, or the attempt of motion, and is called friction. The force exerted by the body on the surface is the action while the force of friction on the body is the reaction.

Key Concepts of Newton’s Third Law for Class 11

There are numerous examples in nature, where action and reaction take place simultaneously.

Contact Force And Field Force: Thrust, tension, impact, etc. come into play when two bodies are in contact. These forces are called contact forces.

• On the other hand, the attraction of the earth on a body, and the attraction or repulsion between two magnetic poles show the existence of another type of force where the bodies need not be in contact.
• A magnet produces a magnetic field around it, and whenever a magnetic substance is placed in the field, it experiences a force. The forces due to fields are called field forces.

Class 11 Physics	Class 12 Maths	Class 11 Chemistry
NEET Foundation	Class 12 Physics	NEET Physics

A Few General Experiences Related to Action-Reaction:

1. When a gun is fired, the forward-moving bullet exerts a backward reaction on the gun, and the shooter feels a backward thrust.
2. When a man jumps off a boat, the boat moves back slightly. In this case, the man exerts a backward force on the boat (action), whereas the boat exerts a reaction force on the man which enables him to reach the bank.
3. When the fuel in a rocket is ignited, gas is ejected from the rocket with great force in the downward direction. Therefore an upward reaction force is generated which helps the rocket to shoot upwards with great speed.
4. Shows an arrangement for sprinkling water in a garden. There are a few narrow jets fitted to the spherical water reservoir which is pivoted such that it can rotate about an axis.
   • When the reservoir is filled with water, water flows out in the form of fountains from the jets. A reaction force is generated in the opposite direction which sets the reservoir in a spinning motion
5. In order to bowl bouncers, a fast bowler has to pitch the ball very hard on the ground (action). The ground exerts an equal and opposite force on the ball (reaction) and this bounces the ball at a desired height.

Real-Life Examples Illustrating Newton’s Third Law

Experiments Frustrating The Action-Reaction Forces: The hooks of two spring balances are attached, as shown. The free end of one spring balance is fixed to a rigid support on the wall and the free end of the other is pulled in the opposite direction, both the balances show the same reading. On increasing or decreasing the pull, the readings change by the same amount. This is because the action is equal to the reaction of the wall.

Examples Of Newton’s Third Law

A circular toy railway tracks A is set on a table so that the track can turn smoothly about an axis passing perpendicularly through its center. When a toy train B is placed on the track and set into motion, the track will also start rotating in the opposite direction. This shows the generation of an action-reaction pair of forces.

Action And Reaction Cannot Cancel Each Other: An action-reaction pair of forces cannot exist without the presence of two bodies. If the force on the second body by the first one is action, the force on the first body exerted by the second is reaction.

Newton’s Third Law Explained with Examples

Though action and reaction forces are equal and oppositely directed, they do not act, at the same time on the same body. Hence, they do not cancel each other, and no question of equilibrium arises.