1. 12.2 Newton’s First and Second Laws of Motion Inertia is the tendency of an object to resist a change in its motion. Objects in motion tend to stay in motion and objects at rest tend to stay at rest, unless acted upon by an outside force. Newton’s First Law of Motion

2. 12.2 Newton’s First and Second Laws of Motion This crash sequence illustrates inertia. The test dummy continues its forward motion as the car slows and stops. Newton’s First Law of Motion

3. 12.2 Newton’s First and Second Laws of Motion This crash sequence illustrates inertia. The test dummy continues its forward motion as the car slows and stops. Newton’s First Law of Motion

4. 12.2 Newton’s First and Second Laws of Motion This crash sequence illustrates inertia. The test dummy continues its forward motion as the car slows and stops. Newton’s First Law of Motion

5. 12.2 Newton’s First and Second Laws of Motion This crash sequence illustrates inertia. The test dummy continues its forward motion as the car slows and stops. Newton’s First Law of Motion

6. 12.2 Newton’s First and Second Laws of Motion Newton’s Second Law of Motion How does Newton’s second law relate force, mass, and acceleration? According to Newton’s second law of motion, the acceleration of an object is equal to the net force acting on it divided by the object’s mass.

7. 12.2 Newton’s First and Second Laws of Motion The force of an object is directly proportional to its mass and its acceleration. Mass is a measure of the inertia of an object. F=m∙a or F=ma Newton’s Second Law of Motion

8. 12.2 Newton’s First and Second Laws of Motion The acceleration of an object is always in the same direction as the net force. When a net force acts in the direction opposite to the object’s motion, the force produces a deceleration. Newton’s Second Law of Motion

9. 12.2 Newton’s First and Second Laws of Motion 1. A boy pushes forward a cart of groceries with a total mass of 40.0 kg. What is the acceleration of the cart if the net force on the cart is 60.0 N? Newton’s Second Law of Motion

10. 12.2 Newton’s First and Second Laws of Motion 1. A boy pushes forward a cart of groceries with a total mass of 40.0 kg. What is the acceleration of the cart if the net force on the cart is 60.0 N? Answer: a = F/m = 60.0 N/40.0 kg = 1.50 m/s 2 Newton’s Second Law of Motion

11. 12.2 Newton’s First and Second Laws of Motion Weight and Mass How are weight and mass related? Mass is a measure of the inertia of an object; weight is a measure of the force of gravity acting on an object.

12. 12.2 Newton’s First and Second Laws of Motion Mass and weight are related but are not the same. Mass is the measure of the amount of material an object contains. Weight is the force of gravity acting on an object. Weight is the product mass and acceleration due to gravity. Weight and Mass

13. 12.2 Newton’s First and Second Laws of Motion W = mg is a different form of Newton’s Second Law, F = ma. The value of g in the formula is 9.8 m/s 2. Weight and Mass

14. 12.2 Newton’s First and Second Laws of Motion If an astronaut has a mass of 112 kilograms, what is his weight on Earth where the acceleration due to gravity is 9.8 m/s 2 ? Weight = Mass × Acceleration due to gravity = 112 kg × 9.8 m/s 2 = 1100 kgm/s 2 × 1100 N Weight and Mass

15. 12.2 Newton’s First and Second Laws of Motion On the moon, the acceleration due to gravity is only about one sixth that on Earth. The astronaut weighs only about one sixth as much on the moon as on Earth. The mass of the astronaut is the same on the moon and on Earth. Weight and Mass

16. 12.2 Newton’s First and Second Laws of Motion Weight and Mass Astronaut on Earth Mass = 88.0 kg, Weight = 863 N Astronaut on Moon Mass = 88.0 kg, Weight = 141 N

17. 12.2 Newton’s First and Second Laws of Motion What is Newton’s third law of motion? Newton’s Third Law According to Newton’s third law of motion, for every action there is an equal and opposite reaction.

18. 12.2 Newton’s First and Second Laws of Motion Action and Reaction Forces The force your bumper car exerts on the other car is the action force. The force the other car exerts on your car is the reaction force. These two forces are equal in size and opposite in direction. Newton’s Third Law

19. 12.2 Newton’s First and Second Laws of Motion Action-Reaction Forces and Motion A swimmer pushing against the water is an action force. The reaction force acting on the swimmer causes motion through the water. Newton’s Third Law

20. 12.2 Newton’s First and Second Laws of Motion Action-reaction forces propel the swimmer through the water. The swimmer pushes against the water, and the water pushes the swimmer. Newton’s Third Law

21. 12.2 Newton’s First and Second Laws of Motion What is needed for an object to have a large momentum? Momentum is the product of an object’s mass and its velocity. Momentum An object has a large momentum if the product of its mass and velocity is large.

22. 12.2 Newton’s First and Second Laws of Motion An object with large momentum is harder to stop than an object with small momentum. The momentum for any object at rest is zero. Momentum

23. 12.2 Newton’s First and Second Laws of Motion Mass is measured in kilograms. Velocity is measured in meters per second. Momentum is measured kilogram-meters per second. Momentum

24. 12.2 Newton’s First and Second Laws of Motion Which has more momentum, a 0.046-kilogram golf ball with a speed of 60.0 meters per second, or a 7.0-kilogram bowling ball with a speed of 6.0 meters per second? Momentum

25. 12.2 Newton’s First and Second Laws of Motion Which has more momentum, a 0.046-kilogram golf ball with a speed of 60.0 meters per second, or a 7.0-kilogram bowling ball with a speed of 6.0 meters per second? Momentum golf ball = 0.046 kg × 60.0 m/s = 2.8 kgm/s Momentum

26. 12.2 Newton’s First and Second Laws of Motion Which has more momentum, a 0.046-kilogram golf ball with a speed of 60.0 meters per second, or a 7.0-kilogram bowling ball with a speed of 6.0 meters per second? Momentum golf ball = 0.046 kg × 60.0 m/s = 2.8 kgm/s Momentum bowling ball = 7.0 kg × 6.0 m/s = 42 kgm/s Momentum

27. 12.2 Newton’s First and Second Laws of Motion Which has more momentum, a 0.046-kilogram golf ball with a speed of 60.0 meters per second, or a 7.0-kilogram bowling ball with a speed of 6.0 meters per second? Momentum golf ball = 0.046 kg × 60.0 m/s = 2.8 kgm/s Momentum bowling ball = 7.0 kg × 6.0 m/s = 42 kgm/s The bowling ball has considerably more momentum than the golfball. Momentum

28. 12.2 Newton’s First and Second Laws of Motion How is momentum conserved? A closed system means other objects and forces cannot enter or leave a system. Conservation of Momentum In a closed system, the loss of momentum of one object equals the gain in momentum of another object— momentum is conserved.

29. 12.2 Newton’s First and Second Laws of Motion Objects within a closed system can exert forces on one another. According to the law of conservation of momentum, if no net force acts on a system, then the total momentum of the system does not change. Conservation of Momentum

30. 12.2 Newton’s First and Second Laws of Motion In each collision, the total momentum of the train cars does not change—momentum is conserved. Conservation of Momentum

31. 12.2 Newton’s First and Second Laws of Motion In each collision, the total momentum of the train cars does not change—momentum is conserved. Conservation of Momentum

32. 12.2 Newton’s First and Second Laws of Motion In each collision, the total momentum of the train cars does not change—momentum is conserved. Conservation of Momentum

33. 12.2 Newton’s First and Second Laws of Motion A class studied the speed and momentum of a 0.25-kilogram ball dropped from a bridge. The graph shows the momentum of the ball from the time it was dropped until the time it hit the river flowing below the bridge. Momentum

34. 12.2 Newton’s First and Second Laws of Motion 1. Applying Concepts At what time did the ball have zero momentum? Describe this point in the ball’s motion. Momentum

35. 12.2 Newton’s First and Second Laws of Motion 1. Applying Concepts At what time did the ball have zero momentum? Describe this point in the ball’s motion. Answer: At t = 0 s; the ball has zero momentum before it is released. Momentum

36. 12.2 Newton’s First and Second Laws of Motion 2. Using Graphs At what time did the ball have the greatest momentum? What was the peak momentum value? Momentum

37. 12.2 Newton’s First and Second Laws of Motion 2. Using Graphs At what time did the ball have the greatest momentum? What was the peak momentum value? Answer: At t = 2.5 s; about 6.5 kgm/s Momentum

38. 12.2 Newton’s First and Second Laws of Motion 3. Calculating What is the ball’s speed after 1.25 seconds? (Hint: Use the graph and the momentum formula.) Momentum

39. 12.2 Newton’s First and Second Laws of Motion 3. Calculating What is the ball’s speed after 1.25 seconds? (Hint: Use the graph and the momentum formula.) Answer: (m)(v) = 3.25 kgm/s v = (3.25 kgm/s)/(0.25 kg) = 13 m/s upward The speed is 13 m/s. Momentum