Newton's Third Law of Motion Study Notes
Newton’s Third Law of Motion
Prepared by: Engr. Jhoneil M. Viernes, LPT
Definition of Newton's Third Law of Motion
Statement: “For every action, there is an equal and opposite reaction.”
Explanation of the Law
Meaning: For every force acting on an object, there is an equal force acting in the opposite direction.
Example: Gravity is pulling a person down, while the seat is pushing up against the person with an equal force.
Result: This balance of forces prevents movement, keeping the person stationary.
Real-Life Applications
Movement on Slippery Surfaces:
Scenario: If a person is standing on a skateboard or a slippery floor and pushes against a wall, they slide away in the opposite direction.
Explanation: The force exerted on the wall results in an equal and opposite force pushing back on the individual.
Pain from Stubbing a Toe:
Scenario: When a person stubs their toe against a rock.
Force Interaction: The toe exerts a force on the rock, and in turn, the rock exerts an equal force back on the toe.
Conclusion: The greater the force applied by the toe, the greater the response from the rock, leading to increased pain.
Forces and Interactions
Interaction Principle:
When one object exerts a force on a second object, the second object exerts an equal but opposite force on the first object.
Newton’s Third Law details the relationship between two forces involved in an interaction:
Action Force: The first force exerted.
Reaction Force: The second force, acting in the opposite direction.
Characteristics of These Forces:
Existence: Neither force can exist without the other.
Magnitude: Both forces are equal in strength.
Direction: Forces act in opposite directions.
Timing: Both forces occur simultaneously (at the same time).
Action and Reaction Examples
Movement of Earth and Boulder:
When a boulder is lifted, Earth is pulled upward with the same force that the boulder is pulled downward by Earth.
Jumping from a Boat:
Action: A girl jumps to shore.
Reaction: The boat moves backward due to the exerted force.
Specific Examples of Action-Reaction Forces
Paddling a Kayak:
Action: The paddle pushes water backward.
Reaction: The water pushes the paddle and the kayak forward.
Helicopter Blades:
Action: Blades exert force on air, pushing air downwards.
Reaction: Air pushes blades upwards, allowing the helicopter to rise.
Contact with Roads:
Action: The tire pushes down on the road.
Reaction: The road pushes back up on the tire.
Identifying Action-Reaction Pairs
Example 1:
Action: Earth pulls the ball down.
Reaction: The ball pulls the Earth up.
Example 2:
Action: A rocket pushes gas out.
Reaction: The gas pushes back on the rocket, propelling it forward.
Review of Newton’s Laws of Motion
Newton’s First Law: Objects in motion tend to stay in motion and objects at rest tend to stay at rest unless acted upon by an unbalanced force.
Newton’s Second Law: Force equals mass times acceleration, commonly expressed as the formula:
Newton’s Third Law: For every action, there is an equal and opposite reaction; also known as “The Law of Interaction.”