Newton’s Third Law acloutier copyright 2011


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Newton's Third Law ~ Action - Reaction
Ann C Cloutier Power Point

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Newton’s Third Law acloutier copyright 2011

  1. 1. Newton’s Third Law Action Reaction Equal and Opposite Ann C Cloutier copyright 2011 Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first object
  2. 2. Force > F is always part of a mutual action that involves another force <ul><li>Force is not something an object has like mass </li></ul><ul><li>Force is an interaction between one object and another </li></ul><ul><li>An object may possess the capability of exerting a force on another object, but it cannot possess force as a thing in itself </li></ul><ul><li>A stick of dynamite possesses energy </li></ul>
  3. 3. Equal and opposite <ul><li>Distance is balanced between objects A and B </li></ul><ul><li>Distance either way is the same </li></ul><ul><li>The same with force pairs </li></ul><ul><li>Both the Earth and Moon pull on each other with equal and opposite forces </li></ul>
  4. 4. Horse - Cart Problem Not the sharpest tool in the shed
  5. 5. In the horse in the system of a horse pulling a cart, pushes the ground the ground with a greater force than it pulls on the cart, there is a net force on the horse, and the horse-cart system accelerates <ul><li>a = F / m </li></ul><ul><li>the forces are equal & opposite stated by Newton’s Third Law, but, they do not cancel out to zero … </li></ul><ul><li>The ground provides friction = reaction force </li></ul><ul><li>Reaction by the wheeled cart is less than horse’s feet pushing against ground and the ground pushing back at horses feet </li></ul><ul><li>The ground is important to the process because it pushes forward equally on the horses feet </li></ul><ul><li>This is how the horse moves forward while the cart pulls backward on the horse </li></ul><ul><li>The system of the cart , horse and ground interact as a whole </li></ul><ul><li>The cart system interacts with the ground to move forward </li></ul><ul><li>The outside reaction (external) </li></ul><ul><li>the ground to wheels of the cart, is what pushes the system </li></ul>
  6. 6. Cannon ball undergoes more acceleration than the cannon because its mass is much smaller <ul><li>Cannonball </li></ul><ul><li>F/m = a </li></ul><ul><li>Cannon </li></ul><ul><li>F = a </li></ul><ul><li>Action and reaction on different masses </li></ul><ul><li>A boulder falls to Earth; </li></ul><ul><li>Earth falls to the boulder. </li></ul><ul><li>Equal and opposite ? Yes </li></ul><ul><li>But earth falls a shorter distance </li></ul><ul><li>than the boulder </li></ul>
  7. 7. Its not all rocket science… <ul><li>Rocket can move in the environment of deep space </li></ul><ul><li>How ? </li></ul><ul><li>By pushing molecules of exhaust like tiny cannonballs </li></ul><ul><li>Less resistance of space allows movement </li></ul><ul><li>Rockets recoil from the explosion of take off </li></ul><ul><li>Think of blowing up a balloon and releasing it untied </li></ul><ul><li>Think of a helicopter blade pushing air down (action) as it spins </li></ul><ul><li>The air forces the blades upward (reaction) </li></ul>
  8. 8. Kicking a ball… what action reaction is happening here ? <ul><li>The ball accelerates when you kick the ball </li></ul><ul><li>No other force has been applied to the ball </li></ul><ul><li>No reaction force is not acting on the ball </li></ul><ul><li>Kick it into another players leg and you get a “reaction” </li></ul><ul><li>You can not cancel the action force on the ball with your foot > the ball is banging into your shoes with equal force </li></ul>
  9. 9. The end is near