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PHYSICS UNIT 2: DYNAMICS (Explaining Motion)

PHYSICS UNIT 2: DYNAMICS (Explaining Motion). FORCES. Force : a "push" or a "pull“ unit: Newtons, N (1 N is about ¼ lb) vector - includes direction contact forces and field forces (act over a distance) net force : total effect of all forces acting on an object. FORCES. Typical Forces

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PHYSICS UNIT 2: DYNAMICS (Explaining Motion)

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  1. PHYSICS UNIT 2: DYNAMICS(Explaining Motion)

  2. FORCES • Force: a "push" or a "pull“ • unit: Newtons, N (1 N is about ¼ lb) • vector - includes direction • contact forces and field forces (act over a distance) • net force: total effect of all forces acting on an object

  3. FORCES • Typical Forces • gravity, FG:object’s weight, always directed toward center of earth (FG=mgmass × acceleration due to gravity) • normal force, FN: supporting force a surface exerts on an object, always directed upward perpendicular to the surface • tension, FT: force transmitted by a rope or chain, directed along the rope, constant throughout the rope

  4. FORCES • Free body diagrams: show just one object & the forces actingon the object (NOT forces the object is exerting on other things) example: car hitting a wall

  5. Examples • Apple on a table • Rock under water • Block on a hill • Water skier • Child pulled forward at an angle on a sled

  6. NEWTON’S LAWS OF MOTION • The Law of Inertia (1st Law): an object’s velocity stays constantunless acted upon by a net external force • inertia: resistance to change in motion (mass is a measure of inertia, more mass = more inertia)

  7. Example of Newton’s 1st Law

  8. NEWTON’S 2nd LAW OF MOTION • The Law of Acceleration (2nd Law): a net force causes an acceleration proportional to the force, in the same direction, and inversely proportional to mass. • Fnet = maFnet: sum of all forces or net force (N), • m: mass (kg), • a: acceleration (m/s2) • 1 N = 1 kg·m/s2

  9. NEWTON’S 2nd LAW OF MOTION Second • The greater the force, the greater the acceleration • The greater the mass, the greater the force needed for the same acceleration • Calculated by: F = ma • (F = force, m = mass, a = acceleration)

  10. action: hammer hits anvil reaction: anvil hits hammer NEWTON’S 3rd LAW OF MOTION • The Law of Interaction (3rd Law): for every action force from one object on another, there is an equal magnitude, opposite direction reaction force from the 2nd object back on the 1st

  11. NEWTON’S 3rd LAW OF MOTION • Law of Interaction (3rd Law) • action & reaction forces do not balanceeach other - they are on different bodies (ex: car pulling a trailer) • equal force does not mean equal acceleration - depends on mass (ex: person jumping off the ground)

  12. Examples of Newton’s 3rd law

  13. FORCES • Finding the Net Force (total of all forces on an object) • draw a free body diagram • identify & label x & y axes • separate forces into x and y parts – Fx=FcosqFy=Fsinq • add all x forces, add all y forces • equilibrium: no net force – x forces add up to zero, y forces add up to zero

  14. Example

  15. LAB 2.3 – Elevator Scene 1

  16. LAB 2.3 – Elevator Scene 2

  17. LAB 2.3 – Elevator Scene 3

  18. LAB 2.3 – Elevator Frame 1

  19. LAB 2.3 – Elevator Frame 2

  20. LAB 2.3 – Elevator Frame 3

  21. LAB 2.3 – Elevator Frame 4

  22. LAB 2.3 – Elevator Frame 5

  23. LAB 2.3 – Elevator Frame 6

  24. LAB 2.3 – Elevator Frame 7

  25. QUIZ 2.1 • Joe rolls a ball down a hill. The ball has a mass of 0.500 kg. The force pulling the ball down the hill is 6.00 N. The hill is 100.0 m long. (a) What is the ball’s acceleration? (b) How fast is the ball going at the bottom of the hill, if it started at rest at the top? (c) If the force on the ball doubled, what would happen to the ball’s acceleration? (d) If instead the mass of the ball doubled, what would happen to its acceleration? 12.0 m/s2 49.0 m/s doubles (24 m/s2) halves (6 m/s2)

  26. PHYSICS UNIT 2: DYNAMICS (Explaining Motion)

  27. NEWTON’S LAWS OF MOTION • Law of Inertia (1st Law) • objects slow & stop, or require continued force to keep moving, due to friction

  28. FRICTION • Friction Force, Ff: resistance to motion between objects in contact with each other • acts parallel to contact surface, opposite to motion • caused by uneven surfaces, molecular attraction

  29. FRICTION • static friction: resistance to starting motion (at rest) • beneficial (walking, building, eating, wheels rolling) • kinetic friction: resistance to continued motion (sliding) • undesirable (machines, moving furniture, wheels skidding) kinetic friction < static friction

  30. FRICTION • coefficient of friction, m: constant that depends on type of surfaces in contact • ms: coefficient of static friction • mk: coefficient of kinetic friction • Ff = mFN(friction force = m× normal force)

  31. FRICTION

  32. FN Ff mg FRICTION • on horizontal surface: • FN= mg (normal force = body weight) • so Ff = mmg

  33. FN Ff mgcosq mg FRICTION • on tilted surface: q • FN= mgcosq • so f = mmgcosq

  34. PHYSICS UNIT 2: DYNAMICS (Explaining Motion)

  35. QUIZ 2.2 • A 1200 kg car sits on a horizontal road. (a) How much force does Joe need to push the car at a constant speed if the coefficient of kinetic friction is 0.600? (b) How much will the car accelerate if Joe uses a force of 10,000 N? a) 7060 N b) 2.45 m/s2

  36. PHYSICS UNIT 2: DYNAMICS (Explaining Motion)

  37. vy v q vx PROJECTILE MOTION • Projectile motion: parabolic trajectory(path) • Two dimensions of motion: horizontal (x), vertical (y) vy = vsinq vx = vcosq

  38. PROJECTILE MOTION • Vertical Motion if a bullet was fired horizontally, andanother bullet was dropped from thesame height at the same time, whichwould hit the ground first? constant vertical acceleration due to gravity(2nd Law)

  39. PROJECTILE MOTION • A monkey hangs from a tree branch. A hunter aims his tranquilizer gun barrel straight at the monkey. When the hunter fires his gun, should the monkey keep holding on to the branch, or let go?

  40. PROJECTILE MOTION • Vertical Motion • position: y = h + visinqit – ½gt2 • a. for ground launch, h=0, y = visinqit – ½gt2 • b. for horizontal cliff launch, q0=0, y = h – ½gt2 • speed: vy = visinqi – gt • flight time, T: t when y=0 • ground: cliff:

  41. PROJECTILE MOTION A tank moving at constant speed fires ashell straight up into the air. Where willthe shell come back down? • Horizontal Motion constant horizontal speeddue to no horizontal force(1st Law)

  42. PROJECTILE MOTION • A snowmobile fires a flare, then slows down. Where does the flare land? If the snowmobile speeds up instead, where does the flare land?

  43. PROJECTILE MOTION • Horizontal Motion • position: x = vicosqit • for horizontal cliff launch, qi=0, x = vit • speed: vx = vicosqi • range, R: x when t = T • ground: cliff:

  44. PROJECTILE MOTION • Example: A projectile is launched from ground level with a velocity of 50 m/s at an angle of 30 degrees. What is its position and velocity 2 seconds later? What is its flight time? What is its range?

  45. PHYSICS UNIT 2: DYNAMICS (Explaining Motion)

  46. RELATIVE MOTION A plane moving at constant speed drops a flare. Describe the path of the flare. • Reference Frames: projectile motion in one reference frame can be vertical free fall in another reference frame (and vice versa)

  47. PHYSICS UNIT 2: DYNAMICS (Explaining Motion)

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