Newton's Third Law of Motion is all about action and reaction. It states that for every force one object exerts on another, there's an equal force pushing back in the opposite direction. This law explains how we walk, swim, and even how rockets fly.

Understanding this law helps us grasp how forces work in everyday life. From sitting in a chair to jumping off the ground, Newton's Third Law is always at play. It's crucial for explaining how objects interact and move in our world.

Newton's Third Law of Motion

Applications of Newton's Third Law

• States that for every action force, there is an equal and opposite reaction force
  • If object A exerts a force on object B, object B simultaneously exerts an equal force back on object A in the opposite direction
• Applies to various everyday situations (walking, swimming, jumping, sitting)
  • Walking: foot pushes backward on ground (action), ground pushes forward on foot (reaction) propelling person forward
  • Swimming: hands push water backward (action), water pushes hands and body forward (reaction) moving swimmer through water
  • Jumping: feet push down on ground (action), ground pushes up on feet (reaction) launching person upward
  • Sitting: body exerts downward force on chair (action), chair exerts upward force on body (reaction) supporting person's weight
• Friction plays a crucial role in many of these applications, allowing objects to push against surfaces effectively

Force calculations in action-reaction pairs

• Forces in an action-reaction pair are always equal in magnitude and opposite in direction
  • Represented by equation $F_{action} = -F_{reaction}$
• To calculate reaction force, determine action force and reverse its direction
  • Person with mass of 70 kg jumping up from ground exerts their weight as action force ($F_{action} = mg$)
    • $F_{action} = 70 \text{ kg} \times 9.81 \text{ m/s}^2 = 686.7 \text{ N}$
    • Ground exerts equal and opposite reaction force: $F_{reaction} = -686.7 \text{ N}$
• Other examples:
  • Box pushing against a wall with 50 N force (action), wall pushes back on box with -50 N force (reaction)
  • Bird with 0.5 kg mass flies upward exerting 4.9 N force down on air (action), air pushes up on bird with -4.9 N force (reaction)
• Normal force is a common reaction force in many scenarios, such as objects resting on surfaces

Newton's Third Law in physical systems

• Rockets propelled by expelling hot gases downward (action), gases push rocket upward (reaction)
  • Force of gases on rocket moves it forward according to $F_{reaction} = -F_{action}$
• Collisions involve objects exerting equal and opposite forces on each other
  • Object A exerts force on object B (action), object B exerts equal force back on object A (reaction)
  • Collision forces cause changes in objects' velocities and momentums obeying $F_{action} = -F_{reaction}$
  • Two cars colliding head-on each experience equal magnitude force in opposite directions
  • The impulse experienced by each object in a collision is equal and opposite
• Fluid propulsion relies on propellers/jets pushing fluid backward (action), fluid pushing vehicle forward (reaction)
  • Boat propeller pushes water backward (action), water pushes boat forward (reaction) moving it through water
  • Jet engine expels air backward (action), air pushes plane forward (reaction) generating thrust

Types of Forces in Action-Reaction Pairs

• Contact forces occur when objects physically touch (e.g., normal force, friction, tension)
• Non-contact forces act between objects without physical contact (e.g., gravity, electromagnetism)
• Both types of forces follow Newton's Third Law, creating action-reaction pairs
• Equilibrium is achieved when all forces acting on an object, including action-reaction pairs, balance out