Newton's Third Law - University Physics Volume 1
Learning Objectives
- State Newton's third law of motion
- Identify action and reaction forces in different situations
- Apply Newton's third law to define systems and solve problems of motion
Newton's Third Law of Motion
- Definition:
- Whenever one body exerts a force on a second body, the first body experiences a force equal in magnitude and opposite in direction to the force it exerts.
- Mathematically: ( F_{AB} = -F_{BA} )
- Nature's Symmetry:
- Forces occur in pairs
- No single force can exist in isolation
- Often referred to as action-reaction where the force exerted is action and the force experienced is reaction.
Practical Examples
- Swimmer in a Pool:
- Swimmer pushes against the wall, wall exerts equal and opposite force on swimmer.
- Forces do not cancel as they act on different systems (swimmer vs. wall).
- Professor Pacing:
- Exerts force backward on the floor; the floor exerts forward reaction force causing acceleration.
- Car Acceleration:
- Ground pushes forward on drive wheels in reaction to wheels pushing backward.
- Rocket Thrust:
- Expels gas backward at high velocity; gas exerts forward reaction force (thrust).
- Helicopters:
- Create lift by pushing air down, experiencing upward reaction force.
- Birds & Airplanes:
- Wings exert force on air to move forward and gain lift.
- Octopus Propulsion:
- Ejects water to propel forward, similar to jet skis.
- Mountain Climber:
- Pulls down on rope, rope pulls up on climber.
Features of Newton's Third Law
- Magnitudes: Action and reaction forces are equal in magnitude but opposite in direction.
- Different Bodies: Forces act on different bodies and do not cancel each other out.
Examples in Daily Life
Example 5.9: Forces on a Stationary Object
- Scenario: Package sits on a scale.
- Forces Involved: Scale force (S) and weight force (w).
- Conclusion: For non-accelerating package, scale measures force equal to weight.
Example 5.10: Getting Up to Speed
- Scenario: Professor pushes cart with equipment.
- Calculation: Acceleration determined by external forces (150 N push, 24 N friction).
- Results: Acceleration of 1.5 m/s².
Example 5.11: Force on the Cart
- Scenario: Determine force professor exerts on the cart.
- Calculation: Using System 2, external forces are professor's push minus friction.
- Conclusion: Calculated force is 53 N, less than initial 150 N exerted on the floor.
Check Your Understanding
- Practice Problem: Two blocks with force applied, explore acceleration.
Additional Resources
- Video examples illustrating action and reaction forces.
These notes provide a detailed summary of Newton's Third Law, demonstrating its application through real-world examples and practical calculations. Understanding these principles is crucial for solving complex motion-related problems in physics.
References