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# Newton’s 3rd law of motion

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### Newton’s 3rd law of motion

1. 1. Newton’s 3 rd Law of Motion
2. 2. A. Forces <ul><li>Forces never act alone, but instead as pairs </li></ul><ul><li>Interaction (inter – between) An action between objects where each object exerts an equal & opposite force on the other. </li></ul>
3. 6. B. Newton’s 3 rd Law <ul><li>Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first object. </li></ul><ul><li>For every action there is an equal & opposite reaction. </li></ul>
4. 7. C. Action/Reaction of Different Masses
5. 8. Firing a cannon!!! BOOM!!! <ul><li>When the cannon is fired, there is an interaction between the cannon and the cannonball. </li></ul><ul><li>Their forces are equal and opposite. (look at the size & direction of the arrows in the diagram) </li></ul><ul><li>The cannon has a “kick” in the opposite direction of the cannonball. </li></ul><ul><li>Why doesn’t it move further than just a “kick”? </li></ul>
6. 9. Compare the masses of the objects
7. 10. IV. Free Fall <ul><li>Galileo studied falling objects </li></ul><ul><li>He showed that falling objects accelerate equally regardless of their mass. </li></ul><ul><li>He could never say why the accelerations were equal. </li></ul>
8. 11. Apply this to Newton’s 2 nd Law <ul><li>Remember, mass & weight are proportional. i.e. a 2kg cat weighs twice as much as a 1kg cat </li></ul><ul><li>Using Newton’s 2 nd law we consider mass </li></ul><ul><li>2x as much force acting on 2x as much mass will produce the same acceleration as a smaller force acting on a smaller mass </li></ul><ul><li>The ration of force to mass stays the same no matter the size of the object. </li></ul>
9. 13. V. Falling & Air Resistance <ul><li>Coin vs. feather </li></ul><ul><li>In a vacuum they fall with equal acceleration </li></ul><ul><li>In the presence of air, they react differently to air resistance </li></ul>
10. 14. <ul><li>Air resistance diminishes the net forces acting on the object </li></ul><ul><li>The net forces decrease very little for the coin, but a lot for the feather. </li></ul><ul><li>The air resistance builds up quickly and cancels the weight force of the feather. </li></ul>
11. 15. <ul><li>When the air resistance = weight, the net force is zero. </li></ul><ul><li>Fnet = 0, there is no acceleration </li></ul><ul><li>Terminal velocity – the speed at which the acceleration of a falling object is zero because friction balances the weight. </li></ul><ul><li>A heavier object will attain a greater terminal velocity than a lighter object. </li></ul><ul><li>It appears to fall faster. </li></ul>