Table of Contents
INTRODUCTION OF THIRD LAW OF MOTION
With the publication of his laws of motion in 1687, Sir Isaac Newton established a standard for how big things move when subjected to external forces. Furthermore, it was discovered that the law of action and reaction, commonly referred to as the third law of motion, was the most important one.
THIRD LAW OF MOTION
According to Newton’s third law, forces applied by two bodies in contact with one another must be equal in magnitude and directed in the opposite direction.
According the third law of motion,
FAB = – FBA
DISCUSSION OF THE THIRD LAW OF MOTION
According to the third law of motion every action has an equal and opposite reaction. Suppose two objects A and B are in contact or at some distance from each other. In this case object A exerts a force FAB on object B. According to Newton’s third law of motion, object B will exert a reciprocating force on object A, which is equal and opposite to Fab. If the force exerted by object B on A is expressed by FBA,
FAB = – FBA ……(I)
If the force FAB is called the action, the force FBA is the reaction. That is, action-reaction forces always exist in pairs—no single isolated force exists in nature. As long as the action lasts, so long does the reaction last. Without action there is no reaction.
DIFFERENT TYPES OF ACTION-REACTION
- THRUST: Suppose a wooden block is on a table. In this case the block exerts a downward force on the table due to its own weight. At the same time the table also exerts a uniform and upward force on the block. The downward force exerted by the block is called the action and the upward force exerted by the table is the reaction. Such action-reaction is called Thrust.
- TENSION: If a heavy object is suspended from the lower end by tying the upper end of a wire to a fixed point, the object exerts a downward force on the wire due to its own weight. The wire also exerts a uniform and upward force on the object, called the tension in the wire. In this example, if the weight of the object is the action, the tension in the wire is the response.
- IMPACT or COLLISION: When a car hits a wall while moving, the car exerts a force on the wall, causing damage to the wall. The wall also exerts an equal and opposite force on the car, which can damage the car as well. Here the force exerted by the car is called the action and the force exerted by the wall is the reaction. Such action-reactions are called collisions.
- ATTRACTION: When a piece of iron is brought near a magnet, the magnet attracts the piece of iron. At the same time the piece of iron attracts the magnet with an equal and opposite force. Any one of these two forces of attraction is called action and the other is reaction. This type of interaction is called mutual attraction. So, the reaction that causes two objects to move or move towards each other is called attraction. From this discussion it can be easily said that since the earth attracts an object, so the object also attracts the earth equally and oppositely. But we can understand the downward motion of a falling object. That is, the motion of the object toward the surface is understood—no motion of the earth toward the object is understood. What is the reason for this?
The acceleration of the object = gravitational force of the earth / Mass of the object
And, Acceleration of Earth = Gravity of object / Mass of Earth
According to Newton’s third law of motion, the two forces of attraction are equal and opposite. But the mass of Earth is many times more than the mass of matter. So, the acceleration of the earth is very less compared to the acceleration of the object. The acceleration of the Earth would be so negligible that the motion of the Earth towards the object is not understood.
- REPULSION: When the north pole of a second magnet is placed near the north pole of one magnet, the first pole repels the second pole. At the same time the second pole also repels the first. One of these two repulsive forces is called action and the other is reaction. So, the reaction that causes two objects to move or try to move away from each other is called repulsion.
- FRICTION: Suppose an object placed on a floor is being tried to move along the team. Or suppose an object is in motion in contact with a surface. In both cases the surface also exerts a force against the motion of the object or against its motion. This phenomenon is called friction and the force that the surface exerts on the object is called the friction force. If the force applied by an object on a contact surface is called action, then friction is reaction.
CONTACT FORCE & FIELD FORCE
Analyzing the examples, it can be seen that when two objects come in contact with each other, forces such as impact, tension, shock, collision, friction etc. can be active. These are contact forces. On the other hand, forces of attraction or repulsion act even when two objects are not in contact with each other. These are field forces. For example, when a magnet is placed in a position, a force field is produced in all directions. That force field exerts an attractive force on a piece of iron.
ACTION AND REACTION DO NOT EQUAL EACH OTHER
As we have seen in each of the above discussions, action-reaction does not exist without the presence of two objects. The force exerted by the first object on the second is called action while the reaction is the force exerted by the second object on the first. That is, action and reaction, though equal and opposite, can never be applied to the same object simultaneously. And by them no question of equality arises. So, it can be said that, Action and reaction cannot moderate each other.
ACTION AND REACTION DO NOT EQUAL EACH OTHER
Walking on the Horizontal Floor
AB is a horizontal plane. A man wants to walk across the floor towards point B. For this he has to apply a force P on the floor diagonally backwards. As this force is applied, the floor also exerts an equal and opposite reaction force R on the man. As this force is acting on the man, it is what causes the man’s motion. The vertical component N of R acts against the downward weight of the man. The man moves forward under the influence of the horizontal component F of R. So, it can be said that the force applied by the man himself is not the direct cause of his motion, but the reaction of that force determines his motion. In another way, it is not the force that the man exerts that causes his motion, but the reaction of the applied force that causes his motion.
It should be noted that the horizontal component of the force applied on the ground cannot be increased much because the frictional resistance of the soil is low on the smooth path. If you increase it too much, there is a possibility of the foot slipping. So, one cannot walk fast on a smooth road.
Flying Bird in the Sky
When a bird wants to fly in the direction OC, it exerts a force on the air in the direction OA and OB with its two wings. At the same time the wind also exerts a reaction force on the bird in the direction OE and OD. Since both of these reaction forces are acting on the bird, both of these forces cause the bird to move. If the bird exerts equal force with both wings, the direction OC is the direction of the resultant reaction. So, the bird can fly in OC direction. When the bird exerts less force with one wing than the other, the locomotor response no longer acts in the OC direction, shifting slightly in the direction where less force is applied. That is, the bird’s direction of motion changes. Obviously in a vacuum the bird cannot fly because it does not get the necessary reaction force.
Motion of Hand Pulled Rickshaw
AB is a horizontal plane. Above it a man is trying to pull a pulled rickshaw towards point B. Hence the man exerts a force P diagonally on the floor with his feet. The floor also exerts a reaction force R on the man. This force can be separated into N and F components in the vertical and horizontal directions respectively. The vertical fraction N is normalized by the weight of the man. The horizontal component F causes the man’s horizontal motion. Let us assume that the man exerts a force T on the rickshaw with the help of the handle. In response the rickshaw exerts a backward force T on the man. Also, when the rickshaw moves forward, the frictional force F’ at the contact between its wheels and the road acts backward on the rickshaw.
Hence, the resultant force on the man forwards = F-T,
and the resultant force on the rickshaw forwards = T-F‘.
If the mass of the man is m1, the mass of the rickshaw is m2 and the forward acceleration of the man and the rickshaw is a
As for the man, F-T = m1·a or, T = F-m1·a
As for rickshaw, T-F’= m2·a or, T= F’+m2·a
So, F-m1·a = F’+m2·a or, (m1+m2) a = F-F’
or, a = (F-F’)/(m1+m2)
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