Can we influence motion?

1. Can we influence motion? We have learned how to describe motion Can we learn how to (willfully) change motion?

2. MOTION Are we moving? As the Earth spins? Around the sun? What is natural motion?

3. Newton’s First Law of Motion. (Law of Inertia) If no net force acts on an object than: If the object is at rest, it remains at rest (speed = 0). If the object is in motion, it continues to move in a straight line with the same speed.

4. Newton’s First Law The net forceis the vector sum of all the forces acting on a body. Inertiais a measure of an object’s resistance to changes in its motion.

5. Seat Belts: An Application of Newton’s First Law Slide 4-16

6. Compare a 2 kg. bar of gold (A)to a 2 kg. box of feathers(B) • Which has more inertia? • C) The same • Which has more mass? • Which has more weight? • Which has more volume?

7. Examples • Difference between mass and weight • Mass is a measure of inertia. Quantity of matter • Weight is the force on an object due to gravity. • Where does it hurt more to kick a cinder block A)In this room? B)In space?`

8. Question • You put your notebook on the front seat of your car. When your car stops, the notebook slides off, forward. Why? A) A net force acted on it. B) No net force acted on it. C) It remained at rest. D) It didn't move, but only seemed to. E) Gravity briefly stopped acting on it.

9. Question • You put your notebook on the front seat of your car. When your car stops, the notebook slides off, forward. Why? A) A net force acted on it. B) No net force acted on it. C) It remained at rest. D) It didn't move, but only seemed to. E) Gravity briefly stopped acting on it.

10. A force is a “push” or “pull” on an object. Forces are vectors How can a force be measured?

11. One way is with a spring scale. By hanging masses on a spring we find that the spring stretch  applied force. The unit of force is the newton (N).

13. Force is a vector T =T cos θ θ T Net (vector) sum of forces

14.  Tension ForceT Slide 4-21

15.  Normal Force N Slide 4-22

16. FORCE OF FRICTION • Kinetic Friction: A force which acts as an object slides across a surface and that always opposes motion. • Static Friction: Force that keeps an object “stuck” on a surface and prevents motion relative to the surface. It points in the direction necessary to prevent motion

17.   Friction fk and fs Slide 4-23

18. Friction • The force of friction is determined by the Normal force. The greater the Normal force the greater the force of friction • fk = μk N • fs = μs N μ is the coefficient of (kinetic or static )friction The direction is always to oppose motion

19.  A Short Catalog of Forces: Weight w Slide 4-19

20. Example Problem Block A hangs from the ceiling by a rope. Another block B hangs from A. Identify the forces acting on A. Slide 4-27

21. Example Problem A block is dragged uphill by a rope. Identify all forces acting on the block. Slide 4-26

22. Newton’s 2nd Law The acceleration of a body is directly proportional to the net force acting on the body and inversely proportional to the body’s mass. Mathematically:

23. An object’s mass is a measure of its inertia. The more mass, the more force is required to obtain a given acceleration. • If F is fixed and m is doubled, a is • Doubled • Halved • Remains the same

24. An object’s mass is a measure of its inertia. The more mass, the more force is required to obtain a given acceleration. • If F is fixed and m is doubled a is • Doubled • Halved a = F/m • Remains the same • What should F be if a is to be the same? • What if m is halved?

25. Example Problem A ball, hanging from the ceiling by a string, is pulled back and released. Identify the forces acting on it just after its release. Slide 4-28

27. Question • A constant force is acting on an object. Which of these graphs best represents the velocity of the object? A)B)C)

28. Question • A constant force is acting on an object. Which of these graphs best represents the velocity of the object? a=F/m A)B)C)

29. Question • A single force is acting on an object. Which type of motion is not physically allowed? • A) the object speeds up • B) the object is at rest • C) the object slows down • D) the object's motion changes direction

30. Question • A single force is acting on an object. Which type of motion is not physically allowed? • A) the object speeds up • B)the object is at rest • C) the object slows down • D) the object's motion changes direction

31. Question • A force F acts on block1 with mass m and block 2 with mass 2m. Which graph correctly represents the accelerations of the blocks? • A)B)C)

32. Question • A force F acts on block1 with mass m and block 2 with mass 2m. Which graph correctly represents the accelerations of the blocks? • A)B)C) a=F/m a = F/2m = a/2

33. Example: A box slides across a rough surface. If the coefficient of kinetic friction is 0.3, what is the acceleration of the box?

34. Free Body Diagrams (FBD): • Drawn for problems when forces are involved. • Must be LARGE so that they are readable. • Draw an idealization of body in question (a dot, a box,…). You will need one free body diagram for each body in the problem that will provide useful information for you to solve the problem. • Indicate only the forces acting on the body. Label the forces appropriately. Do not include the forces that this body exertson any other body.

35. Free Body Diagrams (continued): • A coordinate system is a must. • Do not include fictitious forces. Remember that ma is itself not a force! • You may indicate the direction of the body’s acceleration or direction of motion if you wish, but it must be done well off to the side of the free body diagram.

36. An airplane heads E. The forces acting on it are: • Gravity 16kN down • Lift 16kN up • Thrust 1.8 kN E • Drag 0.8 kN W • Ftot = ? • Ftot = 1.0 kN E

38. Example: A box slides across a rough surface. If the coefficient of kinetic friction is 0.3, what is the acceleration of the box?

39. y N Fk x w Example: A box slides across a rough surface. If the coefficient of kinetic friction is 0.3, what is the acceleration of the box? FBD for box: Apply Newton’s 2nd Law:

40. Example continued: (1) (2) From (1): Solving for a:

41. EQUILIBRIUM An object is in equilibrium if the net force on it is zero. The net force is just the vector sum of all of the forces acting on the body, often written as F.

42. EQUILIBRIUM If a = 0, then F = 0. This body can have: Speed = 0 which is called static equilibrium, or speed  0, but constant and same direction, which is called dynamic equilibrium.

43. EquilibriumExamples • Stopped car • Car moving at constantspeed in straight line

44. Equilibrium • A hawk weighing 8 N. glides due N at a constant speed. What is the force acting on the hawk due to the air? Equilibrium; net force = 0 W = 8 (down) Fair = ? Fair= 8(up)

45. A chest W =750 N, is pushed horizontally with F = 450N and moves across the floor with constant velocity. What is the contact force C of the floor on the chest? • Equiibrium: 3 Forces W, F, C

46. F2 + W2 = C2 C= 870 Nθ = Tan F/W =450/750

47. Example Problem A 5 kg ball, hanging from the ceiling by a string, is pulled back and held steady by a wire. What is the tension in the wire? 60o Slide 4-28

49. Newton’s Third Law When 2 bodies interact, the forces on the bodies from each other are always equal in magnitude and opposite in direction. Or, forces come in pairs. Mathematically: To every action there is always an equal and opposite reaction

50. Newton’s Third Law Action and reaction always act on different objects Twoexternal forces acting on the same object can be equal and opposite but they are not an action reaction pair