Newton's third law of motion states that for every action, there is an equal and opposite reaction. Whenever one object exerts a force on another object, the second object exerts an equal and opposite force back on the first object. When a cannon fires, the cannonball exerts a forward force on the cannon while the cannon exerts a backward force of equal magnitude on the cannonball, causing the cannon to recoil. Although the cannon exerts an equal reaction force, it does not move back very far due to its greater mass compared to the cannonball.

1. Newton’s 3 rd Law of Motion

2. A. Forces Forces never act alone, but instead as pairs Interaction (inter – between) An action between objects where each object exerts an equal & opposite force on the other.

6. B. Newton’s 3 rd Law Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first object. For every action there is an equal & opposite reaction.

7. C. Action/Reaction of Different Masses

8. Firing a cannon!!! BOOM!!! When the cannon is fired, there is an interaction between the cannon and the cannonball. Their forces are equal and opposite. (look at the size & direction of the arrows in the diagram) The cannon has a “kick” in the opposite direction of the cannonball. Why doesn’t it move further than just a “kick”?

9. Compare the masses of the objects

10. IV. Free Fall Galileo studied falling objects He showed that falling objects accelerate equally regardless of their mass. He could never say why the accelerations were equal.

11. Apply this to Newton’s 2 nd Law Remember, mass & weight are proportional. i.e. a 2kg cat weighs twice as much as a 1kg cat Using Newton’s 2 nd law we consider mass 2x as much force acting on 2x as much mass will produce the same acceleration as a smaller force acting on a smaller mass The ration of force to mass stays the same no matter the size of the object.

13. V. Falling & Air Resistance Coin vs. feather In a vacuum they fall with equal acceleration In the presence of air, they react differently to air resistance

14. Air resistance diminishes the net forces acting on the object The net forces decrease very little for the coin, but a lot for the feather. The air resistance builds up quickly and cancels the weight force of the feather.

15. When the air resistance = weight, the net force is zero. Fnet = 0, there is no acceleration Terminal velocity – the speed at which the acceleration of a falling object is zero because friction balances the weight. A heavier object will attain a greater terminal velocity than a lighter object. It appears to fall faster.