Reaction force is a force that is equal in magnitude and opposite in direction to an action force. It is an essential concept in Newton’s third law of motion, which states that for every action, there is an equal and opposite reaction. In simpler words, when you push something, that object pushes back on you with the same amount of force, but in the opposite direction. This is what we call reaction force. Understanding reaction force is important in various fields, from physics and engineering to sports and everyday life.
Some examples of reaction force are listed below:
- Walking: Your foot pushes against the ground, and the ground pushes back, enabling you to move forward.
- Swimming: You push water backwards with your arms and legs, and the water pushes you forward.
- Driving a car: The wheels exert a force on the road, and the road pushes back, allowing the car to move.
- Bouncing a ball: You apply a force in the downward direction, and the ball pushes back, causing it to bounce up.
- Sitting in a chair: The chair pushes up, supporting your weight, while your weight pushes down.
Reaction force = -Action force
Table of Contents
Simple Definition of Reaction Force
The reaction force is the equal and opposite force that acts in response to an applied force, as per Newton’s third law of motion.
Daily Life Examples of Reaction Force
- Hammering a nail: The hammer applies a force to the nail, and the nail exerts an equal force back, allowing you to drive the nail into a surface.
- Pushing a door: You apply a force to the door, and the door pushes back on you with the same amount of force, allowing you to enter or exit a room.
- Shooting a gun: The gun exerts a force on the bullet, and the bullet exerts an equal force back on the gun, causing the recoil.
- Jumping on a trampoline: You apply a force to the trampoline mat, and the mat pushes back with an equal force, propelling you upward.
- Kicking a football: Your foot applies a force to the ball, and the ball pushes back with an equal force on your foot, causing the ball to move in the opposite direction.
Formula of Reaction Force
There is only one formula related to the reaction force, which is:
Reaction force = – Action force
This formula is based on Newton’s Third Law of Motion, which states that for every action, there is an equal and opposite reaction. In the case of the reaction force, it is always equal in magnitude and opposite in direction to the applied force, which is known as the action force. Therefore, the negative sign in the formula indicates the opposite direction of the reaction force compared to the action force.
Formulas Related to Reaction Force
| Formula | Mathematical Form | Description with respect to Reaction Force | Examples |
| Newton’s Third Law of Motion | -F = +F | Every action has an equal and opposite reaction. The reaction force is equal in magnitude and opposite in direction to the action force. | – |
| Weight formula | Fg = mg | An object’s weight is its mass times the acceleration due to gravity. The reaction force acting on the object is equal to its weight, but in the opposite direction. | An object resting on a surface, a person standing on the ground. |
| Friction force formula | f = μN | Friction between two surfaces is the coefficient of friction multiplied by the normal force. The reaction force is equal in magnitude and opposite in direction to the force of friction. | Brakes of a car, walking on a rough surface. |
| Elastic force formula | F = -kx | The force exerted by a spring or elastic material is proportional to its displacement from equilibrium. The reaction force is equal in magnitude and opposite in direction to the elastic force. | Bouncing a ball, stretching a rubber band. |
| Centripetal force formula | Fc = ma | The force required to keep an object moving in a circular path is equal to the mass of the object multiplied by its centripetal acceleration. The reaction force is equal in magnitude and opposite in direction to the centripetal force. | A car turning a corner, a rollercoaster rid |
Multiple Choice Questions
1. Which of the following is the best definition of reaction force?
A. The force that an object exerts on another object.
B. The force that opposes the motion of an object.
C. The force that an object exerts in response to a force applied to it.
D. The force that causes an object to accelerate.
Answer: C. The force that an object exerts in response to a force applied to it.
2.According to Newton’s third law of motion, how is reaction force related to action force?
A. They are equal in magnitude and opposite in direction.
B. They are equal in magnitude and direction.
C. They are opposite in magnitude and direction.
D. They are not related.
Answer: A. They are equal in magnitude and opposite in direction.
3.Which of the following is an example of reaction force?
A. A ball being thrown in the air.
B. A car accelerating on a road.
C. A rocket launching into space.
D. A person pushing against a wall.
Answer: D. A person pushing against a wall.