AP Physics I.B

1. AP Physics I.B Newton’s Laws of Motion

2. 4.1 Contact and field forces

3. 4.2 Newton’s First Law (the law of inertia) – an object at rest will remain at rest, or an object in motion at a constant velocity will continue at a constant velocity, unless acted upon by a net force.

4. An unlikely trio – Mr. Evans, James Lovell and Sir Isaac Newton

5. Net force – the sum of all the forces acting on an object

6. If the net force is zero . . . • The object is not moving or . . . • The object is moving at a constant velocity therefore . . . • The object is in equilibrium

7. A net force changes velocity

8. Inertia and mass

9. 4.3 Newton’s Second Law

10. Some examples

11. The acceleration of an object is directly proportional to the net force and inversely proportional to its mass.

12. The more familiar mathematical form (a new unit)

13. Net force is a vector in the same direction as the acceleration

14. Note! If an object accelerates then the net force is NOT zero. If the net force is zero, then the object is moving with a constant velocity or it is at rest.

15. Use free body diagrams to show all of the forces acting on an object.

16. Ex. A barge has a mass of 8.0 EE 3 kg. A force of 1.00 EE 4 N pulls on the barge toward the left while a force of 7.5 EE 3 N pulls in the opposite direction. What is the acceleration of the barge?

17. Forces are vectors and may have components like any other vectors.

18. p. 121: 4-7, 9 • 0.041 s • 2900 N

19. Ex. Two groovy people pulling on a boat between two docks.

20. Ex. Find the displacement of the boat if the forces are maintained for 10.0 s and the boat has an initial velocity of 0.50 m/s.

21. 4.5 Newton’s Third Law (highly misunderstood)

22. Newton’s Third Law – when one object exerts a force on a second object, the second object exerts a force that is equal in magnitude, but in the opposite direction to that of the first

23. “This third law is confusing!” Remember – Newton’s Third Law deals with two forces and two objects, not two forces on one object.

24. Ex. Two skaters with masses of 75 kg and 45 kg respectively, face each other and push away. If the acceleration of the 75 kg skater is 0.73 m/s2, what is the acceleration of the 45 kg skater?

25. p. 121: 10-12, 14-16 • You do – easy one. • Note, only acceleration is horizontal. Ans. 1.2 m/s2, left. • Find ax and ay. Use kinematics eqns. to find east and south components of displacement. Use Py. Th. to find displacement (0.78 m 22º S of E) 16. Hint: total distance traveled by tug and asteroid is 450 m. (64 s).

26. The Four (or is it three?) Fundamental Forces • The (real) strong nuclear force (short range – holds the protons and neutrons of an atom together) • Electromagnetic forces (10-2 times the strong force) – long range, holds atoms and molecules together • Weak nuclear force (10-6 times the strong force) – short range, responsible for radioactive decay • The (really weak) gravitational force (10-43 times the strong force) long range

27. 4.7 The gravitational force – more to say about this later, but for now . . .

28. The gravitational force is always attractive and never repulsive.

29. Mass and weight

30. Finding weight the hard way

31. Ponder the lunar explorer . . .

32. Now, a much simpler method

33. 4.8 The Normal Force “As opposed to the abnormal force” The force a surface exerts on an object, perpendicular to the surface

34. Some instructive illustrations

35. Ex. The tension in a rope applies a force of 100.0 N upward to a box that has a mass of 10.0 kg. What is the acceleration of the box?

36. Apparent weight (how much your mother or father weighs)

37. Ex. A rope attached to a box (what else?) with a mass of 10.0 kg applies a force of 40.0 N above the horizontal so that the blocks slides across a frictionless floor. a) What is the horizontal acceleration of the box? b) What is the normal force on the box?

38. Ex. Two boxes with masses of 12.0 kg and 10.0 kg respectively are attached with a cord. A second cord pulls the 10.0 kg box to the right. a) Find the acceleration of each box and b) the tension in the cord between the boxes.

39. 4.9 Static and Kinetic friction

40. Friction: we love it, we hate it

41. The nature of friction

42. Static friction and the crate

43. Maximum static friction • Independent of area (if surfaces are hard and nondeformable) • Directly proportional to the normal force • Depends on the surfaces in contact • The equation is . . .

44. Kinetic friction is • Independent of area • At slow speeds, independent of speed • Directly proportional to the normal force and the coefficient of kinetic friction • The second equation is . . . (hmmm, looks familiar . . .)

45. Ex. A student attaches a rope to a box and pulls with a force of 90.0 N at an angle of 30.0º with the horizontal. The box has a mass of 20.0 kg and the coefficient of kinetic friction between the bottom of the box and the floor is 0.500. Find the acceleration of the box.

46. Ex. A sled reaches the bottom of a hill with a velocity of 4.0 m/s. It slides horizontally along the snow until it comes to a stop. What is the distance the sled slides if the coefficient of kinetic friction between the snow and the sled is 0.0500?

47. 4.10 Tension “These forces are killing me, give me an Excedrin.”

48. The nature of tension

49. 4.11 Equilibrium – the net force is zero • So the sum of the horizontal forces is zero • And the sum of the vertical forces is zero

50. Ex. A bright physics student finds her car stuck in the mud. She ties a strong rope to the back of the bumper and the other end to a tree. She pushes at the midpoint with maximum effort, which she estimates to be 3.0 EE 2 N. The car just begins to budge (from the sludge) when the rope makes an angle of 5.0º. With what force is the rope pulling on the car?