Lesson 3: Action & Reaction Forces

Newton’s 3 Laws of Motion:

  1. The Law of Inertia: An object in motion stays in motion, and an object in rest stays at rest, unless acted upon by an external force.

  2. Force & Acceleration: The force acting on an object is equal to the mass of the object multiplied by the acceleration (F = ma).

  3. Action & Reaction: For every action, there is an equal and opposite reaction.

Focus In Law 3:

“For every action, there is an equal and opposite reaction.“

  • This law, in my understanding, means that there must be 2 objects to create force. The first object is the object that is creating the force (such as pull, push or twist) onto another object. The other object reacts to that force by pushing back in the opposite direction so that only you can feel the force, but in reality, it still moves the way you want it to.

Key Notes:

  1. A force interaction can’t be made with one object- there must be two different objects. For example, a person and a box, or a rocket and air.

  2. The force doesn’t cancel even when they’re happening in different directions (which usually prevents the object from moving) because they’re happening on different objects.

  3. The same amount of force applied onto an object will be sent back the opposite direction through a different object, since the principle is “For every action, there is an EQUAL and OPPOSITE reaction.”

Example By Khan Academy:

A rocket and air are two different objects that will represent the equal and opposite action and reaction. The rocket blasts up towards space, releasing hot gases in the direction downwards to earth. At the same time, tiny molecules released in the rocket push upwards due to the equal and opposite reaction and push upwards instead without canceling the movement because the molecules is acting on the rocket and the other one is acting on the air.

Example of Newton’s 3rd Law using a human applying force against a wall to push it: