Acceleration and Momentum

1. Acceleration and Momentum Chapter 3 General Science Chapter 3

2. Review • Newton’s first law • Weight of one kilogram on Earth’s surface • Gravity General Science Chapter 3

3. Newton’s second law • A net force acting on an object causes the object to accelerate in the direction of the force. • The acceleration is affected by two things • size of force • mass of object General Science Chapter 3

4. Force • If the force on an object increases, acceleration will increase for that object and if force decreases the acceleration will decrease. IF THE MASS REMAINS CONSTANT General Science Chapter 3

5. Mass • A more massive object will take a larger force to give it the same acceleration as a smaller massed object. General Science Chapter 3

6. Equation for Newton’s 2nd law We can also use the “magic” triangle General Science Chapter 3

7. Units on force • Last chapter we learned that force has units of Newtons. • From Newton’s second law equation, General Science Chapter 3

8. Example • How much force is needed to accelerate a 70-kg rider and the 200-kg motorcycle the rider is on at 4 m/s2? • F = 270 kg x 4 m/s2 • 1080 N General Science Chapter 3

9. You try • How much force is needed to accelerate a 60-kg person and the 500-kg car the person is in at 6 m/s2? • F = 560 kg x 6 m/s2 • 3360 N General Science Chapter 3

10. You try • A 63-kg skater pushes off from a wall with a force of 300 N. What is the skater’s acceleration? • 300 N = 63 kg • a • a = 300 N ÷ 63 kg • 4.76 m/s2 General Science Chapter 3

11. Review #1 • What is Newton’s 2nd Law of Motion? • What equation do we use with the 2nd Law of Motion? • What 2 things will affect acceleration? • What are the units for Force? • True or False: A more massive object will take less force to accelerate? General Science Chapter 3

12. Acceleration due to gravity • Near Earth’s surface, gravity causes all falling objects to accelerate at a rate of 9.8 m/s2, regardless of their mass. • Acceleration due to gravity is represented by the letter g. • Does this number seem familiar? General Science Chapter 3

13. Weight • The weight of an object is the force of gravity acting on it. General Science Chapter 3

14. Reality check • Do all objects really fall with the same acceleration? General Science Chapter 3

15. Air resistance • The force air exerts on a moving object. • Acts in the opposite directions to which an object is moving. • For falling objects, air resistance pushes up while gravity pulls down. General Science Chapter 3

16. Air resistance • Depends on the speed, size, and shape of the object. • The larger the object, the more air resistance affects it. • The faster an object is moving, the more air resistance affects it. General Science Chapter 3

17. Terminal velocity • As an object falls, air resistance gradually increases until it equals the pull of gravity. • At this point, the object stops accelerating and moves with a constant velocity – called its terminal velocity. General Science Chapter 3

18. Example • A ball is dropped from the roof of a 50-m tall building. It hits the ground 3.2 s later. What is its velocity right before it hits the ground? • a = v ÷ t OR v = a • t • v = 9.8 m/s2 x 3.2 s • 31.36 m/s General Science Chapter 3

19. Review #2 • Define air resistance. • What is air resistance dependent upon? • What is terminal velocity? • What is the acceleration of gravity near the surface of the earth? • How do we determine the weight of an object? General Science Chapter 3

20. Projectiles • Anything that’s shot or thrown through the air. • Will always follow a curved path. General Science Chapter 3

21. Projectile motion • Can be split into independent vertical and horizontal motions. General Science Chapter 3

22. The horizontal part • Once the object is released, there is no force acting on it horizontally. • If there is no force, it must maintain a constant horizontal velocity. General Science Chapter 3

23. The vertical part • There is a force acting – gravity • The object is pulled downward with a constant acceleration of 9.8 m/s2. General Science Chapter 3

24. Circular motion • Objects moving in circles are constantly accelerating. • This acceleration is towards the center of the circle or curve. • It is called centripetal acceleration. • Towards the center of a circle or a curved path. General Science Chapter 3

25. Centripetal force • The force that causes an object to move in a curved or circular path. General Science Chapter 3

26. Weightlessness • To be truly weightless, an object would have to be free from gravity. • To feel weightless, something has to be in free fall along with its surroundings. General Science Chapter 3

27. Weightlessness • How does a scale measure weight? • If it can’t push back, it would read zero. • You would feel weightless. General Science Chapter 3

28. Review #3 • What is a projectile? • What are the 2 components of a projectile? • What does centripetal mean? • Why are objects weightless during free fall? General Science Chapter 3

29. Newton’s 3rd law • To every action, there is an equal and opposite reaction. • When one object exerts a force on a second object, the second object exerts a force on the first object that is equal in size and opposite in direction. General Science Chapter 3

30. Examples • Released balloon • Firing a gun • Jumping out of a boat • rockets General Science Chapter 3

31. Balanced forces? • Action-reaction pairs of forces are not balanced forces. • The forces act on different objects. • To be balanced forces, the equal and opposite forces must act on the same object General Science Chapter 3

32. Momentum • A property a moving object has because of its mass and velocity General Science Chapter 3

33. Momentum • Has units of kg∙m/s • Has direction, because velocity has direction General Science Chapter 3

34. Momentum • Which has a higher momentum, a 50-kg dolphin swimming at 16.4 m/s or a 6300-kg elephant walking at 0.11 m/s? • Dolphin – (50 kg)(16.4 m/s) = 820 kg∙m/s • Elephant – (6300 kg)(0.11 m/s) = 693 kg∙m/s • The dolphin has higher momentum. General Science Chapter 3

35. Example • Which has higher momentum, a 65-kg person running at 9 m/s or a 60-kg person running at 11 m/s? • 1st person – (65 kg)(9 m/s) = 585 kg∙m/s • 2nd person – (60 kg)(11 m/s) = 660 kg∙m/s • The 60-kg person has higher momentum. General Science Chapter 3

36. Law of conservation of momentum • The total amount of momentum of a group of objects does not change unless outside forces act on the objects. General Science Chapter 3

37. Momentum transfer • Momentum can be transferred from one object to another. • Think about playing pool • The cue ball transfers its momentum to the ball it hits. • The momentum lost by the cue ball is gained by the other ball. General Science Chapter 3

38. Example • A 5-kg object with a velocity of 6 m/s strikes a motionless 10-kg ball. The 5 kg object stops moving. What is the velocity of the 10 kg ball after the collision? • Momentum of object before collision: • (5 kg)(6 m/s) = 30 kg∙m/s General Science Chapter 3

39. Example continued • When the object stops, it loses all its momentum. • The momentum it lost is transferred to the ball. • Momentum of the ball after collision: • (10 kg)(velocity) = 30 kg∙m/s • Velocity = 3 m/s General Science Chapter 3

40. Example continued • Notice that the velocity was not conserved, but momentum was. General Science Chapter 3

41. Review #4 • What is Newton’s 3rd Law of Motion? • Define momentum. • What units does momentum have? • What is the law of conservation of momentum? General Science Chapter 3

42. Chapter 3 Review • State all 3 Laws of Motion • What 2 things affect acceleration on an object • What is air resistance dependent upon? • How are the Law of Inertia and the Law of conservation of momentum related? General Science Chapter 3