Newton’s Laws of Motion Ch. 6.6 Free Falling Objects Ch. 7.4 Action-Reaction—3rd Law And hw answers pg 118-123

Ch. 6.6 Free Fall Explained Misconception: “Heavier objects always fall faster than lighter objects.” Consider a 1 kg stone and a 10 kg stone… The Aristotlean view would be that the heavier stone would fall 10 times faster than the lighter stone.

Objects In Free Fall In free fall One force only (gravitational force) Unbalanced force Produces acceleration

Pg. 99 #15. Why doesn’t a heavy object accelerate more than a light object when both are freely falling? To understand this, recall that free fall occurs when only gravitational force acts on the object. Also recall that mass and weight are directly proportional, meaning that if the mass of a falling object increases, so does its weight. Because the acceleration of a falling object is dependent upon its weight to its mass ratio (w/m), its rate of acceleration can be calculated and compared to any other free-falling object of any weight. Newton’s 2nd law is used to show that no matter the weight of the freely-falling object, the acceleration produced by its weight to mass ratio always equals 9.8 m/s2 on Earth. This proves that heavy objects and light objects accelerate identically in free fall.

Newton’s 2nd Law

What about inertia? You should also understand that the heavier object has greater inertia (property of matter that resists acceleration) and will require a greater force to keep it accelerating at the same rate as a smaller object. Greater inertia, larger net force Lesser inertia, smaller net force

In Free-Fall (do not consider air resistance) Weight to mass ratios are identical (9.8 m/s2) Both land at the same time

7.3 Identifying Action and Reaction When action is A exerts force on B, the reaction is simply B exerts force on A.

7.3 Identifying Action and Reaction When action is A exerts force on B, the reaction is simply B exerts force on A.

7.2 Newton’s Third Law The dog wags the tail and the tail wags the dog. http://www.nasa.gov/audience/forstudents/brainbites/nonflash/bb_home_bolt.html

Chapter 7, Page 118 9. The cannon and cannonball have very different accelerations because equal forces on unequal masses produce unequal accelerations. The more massive cannon has more inertia and is harder to accelerate than the much less massive cannonball.

7.4 Action and Reaction on Different Masses What can you say about the action and reaction forces experienced by the cannon and the cannonball? The force the cannon exerts on the cannonball is exactly equal and opposite to the force the cannonball exerts on the cannon. cannon cannonball

The cannonball! Because it has less mass. 7.4 Action and Reaction on Different Masses F represents both the action and reaction forces; m (large), the mass of the cannon; and m (small), the mass of the cannonball. Which has the greater change in motion and why? Same force Different masses Different accelerations The cannonball! Because it has less mass.

Chapter 7, Page 118 10. The force that propels a rocket is the force of the exhaust gases pushing the rocket in reaction to the rocket pushing the exhaust gases. Rocket pushes on exhaust gas Exhaust gas pushes on rocket RECOIL

7.4 Action and Reaction on Different Masses The balloon recoils from the escaping air and climbs upward. A common misconception is that a balloon is propelled by the impact of exhaust gases against the atmosphere. Each molecule of exhaust gas acts like a tiny molecular cannonball shot downward from the balloon.

7.4 Action and Reaction on Different Masses The rocket recoils from the “molecular cannonballs” it fires and climbs upward. Gas pushes on rocket Rocket pushes on gas

Proving the 2nd Law of Motion Prove that acceleration is directly related to net force Prove that acceleration is inversely related to mass Identify dependent, independent and constant variables Establish experimental control

Ch. 7 Page 118-122 6, 7, 8, 20, 28, 29, 30, 34-38, 46, 49

Chapter 7, pg 118-123 6. No and No. The action force must be exactly the same magnitude as the reaction force. 7. The reaction force is the chair pulling upwards on the Earth. 8. Forces are equal in magnitude, but acceleration is unequal; the cannonball has less mass and greater acceleration.

Chapter 7, pg 118-123 20. inertia, acceleration, action and reaction 28. The backward force (action force) on the log moves it backwards. 29. One can exert a greater force on carpet than on a polished floor because of the greater friction. This, in turn, provides a greater reaction force to provide traction for walking. 30. Yes, the Earth does accelerate towards you, but the accelerations (changes in motion) are greatly different because of the great difference in masses.

Chapter 7, pg 118-123 34.Both pull on each other with 1000 N. 35. The earth has much greater mass (thus, more inertia), so it resists acceleration. 36. The impact force is the same for both. The car has less mass (thus, less inertia) than the van, so the acceleration for the car is greater than for the van. 37. No. Higher speed only increases the impact force for both. 38. Both receive the same impact force (action-reaction). The bug, however, undergoes a much greater deceleration due to its tiny mass.

Chapter 7, pg 118-123 46. 50 N. For every action force there is an equal and opposite reaction force. 49. More than your weight. In order to change your motion, there must be an unbalanced force. Your push force must then be greater than your weight.