Newton’s third law, or the principle of action and reaction, states that when a body A exerts a force on a body B, then B exerts on A a force equal in magnitude, in the same direction, but in the opposite sense. This fundamental law is part of the middle school and senior high school curriculum.
Discover FizziQ
How to measure it in class
With the FizziQ app, it is possible to demonstrate the action-reaction principle.
Steps:
- Use two smartphones with FizziQ in force sensor mode (via a connected dynamometer)
- Or use the accelerometer to detect forces during interactions
- Have two objects interact and measure the forces simultaneously
- Verify that |F_A->B| = |F_B->A|
- Study recoil propulsion: push a cart and measure the acceleration
Scientific activities on this topic
- Recoil propulsion: https://www.fizziq.org/en/activities/space-x-rocket/
- Collision studies
- Reaction force during a jump
Learn more
Precise statement:
If an object A exerts a force F_A->B on an object B, then B exerts on A a force F_B->A such that: F_B->A = -F_A->B
These two forces have the same magnitude, the same direction, but opposite senses. They are exerted on two different bodies.
Common misconceptions:
- Action and reaction do NOT cancel out because they are applied to different objects
- It is NOT a question of equilibrium (which concerns forces on the same object)
- Both forces appear and disappear simultaneously
Applications:
- Walking: the foot pushes the ground backward, the ground pushes the foot forward
- Swimming: the arms push the water backward, the water pushes the swimmer forward
- Rocket: gases are ejected backward, the rocket is propelled forward
- Magnets: if A attracts B, then B attracts A with the same force
Formula
F_A->B = -F_B->A
In magnitude: |F_A->B| = |F_B->A|
For propulsion, momentum is conserved: m1 x Dv1 = -m2 x Dv2
where:
- F_A->B: force exerted by A on B
- F_B->A: force exerted by B on A (reaction)
Application examples
- The Earth attracts the Moon with the same force that the Moon attracts the Earth (approximately 2x10^20 N)
- When you push a wall, the wall pushes you with the same force
- A cannon recoils when it fires a cannonball (same impulse)
- A helicopter pushes air downward, the air pushes it upward
FAQ
Q: Why don’t they cancel out if they are equal and opposite? A: Because they are applied to different objects. When making a force balance, we only consider forces applied to a single object.
Q: How can I verify this law with FizziQ? A: By measuring the acceleration of two interacting objects and verifying that m1a1 = m2a2 (the forces are equal in absolute value).
Q: Does the third law apply to forces at a distance? A: Yes, it also applies to gravitation and electromagnetic forces, not just contact forces.
Q: Why does the truck cause more damage than the car in an accident? A: The forces are indeed equal, but the accelerations are inversely proportional to the masses. The car experiences a greater acceleration (deceleration).
Related concepts
Newton’s Laws - Force - Momentum - Propulsion - Action-reaction - Equilibrium