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10/28/2013

10/28/2013. And so it comes to this, the mind blowing Sir Isaac Newton and his laws of motion Bullseye Lab. Newton’s First Law: Inertia.

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10/28/2013

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  1. 10/28/2013 • And so it comes to this, the mind blowing Sir Isaac Newton and his laws of motion • Bullseye Lab

  2. Newton’s First Law: Inertia An object at rest tends to stay at rest and an object in motion tends to stay in motion with the same speed and in the same direction unless acted upon by an unbalanced force.

  3. First Law: Application Center of gravity (mass)

  4. 10/29/2013 • Homework ?’s • That’s some good inertia • What is this Force? (not that force) • Newton’s 1st and 2nd Law Demos • Bullseye Lab

  5. What is a Force?

  6. First Law - Inertia Law of Inertia – every object continues in a state of rest, or of motion in a straight line at a constant speed, unless it is compelled to change that state by forces exerted upon it.

  7. First Law:

  8. Inertia Concepts Mass – the more mass an object has, the greater its inertia and the more force it takes to change its state of motion. Mass is the measure of the inertia of an object.

  9. Second Law: F = ma The acceleration of an object as produced by a net force is directly proportional to the magnitude of the net force, in the same direction as the net force, and inversely proportional to the mass of the object.

  10. What is a "newton?" F = ma Mass = kg Acceleration = m/s2 Force = kg ·m/s2 Newton (N) = kg ·m/s2

  11. 10/30/2013 • Bullseye Lab • Shooting the Moon • Newton’s Third Law

  12. Second Law of Motion Newton was the first to realize that the acceleration produced when we move something depends not only on how hard we push or pull, but also on the object’s mass. The acceleration produced by a net force on an object is directly proportional to the magnitude of the net force and is inversely proportional to the mass of the object.

  13. Problem #8 on page 124

  14. p. 125 #21

  15. 11/1/13 • Finish Bull’s Eye Lab • Shooting the Moon • Normal Force • Tug of war physics, oh…okay • Vectors in multiple directions!

  16. How much force is needed to keep a 20-N stone from falling? 20 N 20 N F = 20 N

  17. 3. What applied force accelerates a 20-kg stone straight up at 10 m/s2? 9.8 m/s2 Fnet = Fup + Fdown Fup Fnet Fup = Fnet - Fdown Fup = manet - madown Fup = m(anet - adown) Fdown

  18. Fup = 20kg[10m/s2-(-9.8m/s2)] Fup = 20kg[19.8 m/s2] 9.8 m/s2 Fup = 396 N

  19. 4. A rocket weighs9800 N a) What is its mass? 4 m/s2 W = mg m= W/g m = 9800n/9.8m/s2 m = 1000 kg W = 9800 N

  20. 4. A rocket weighs9800 N b) What force gives it a vertical acceleration of 4 m/s2? 4 m/s2 Fnet = Fup + Fdown Fup = Fnet - Fdown Fup = manet - Fdown W = 9800 N

  21. 4 m/s2 Fup = (1000 kg·4 m/s2) - (-9800 N) Fup = 4000 N + 9800 N Fup = 13800 N

  22. Third Law: Action - Reaction

  23. Third Law: Action - Reaction • Whenever a first body exerts a force F on a second body, the second body exerts a force -F on the first body. F and –F are equal in magnitude but opposite in direction. • The law of action reaction

  24. Interaction Pairs Two forces that are in opposite directions have equal magnitude. You push your friend, this does not cause your friend to exert a force on you. The forces exist together or not at all.

  25. Free Body Diagrams

  26. 11/5/13 • Third Law Thought Experiment • Shooting the Moon • Normal Force • Tug of war physics, oh…okay • Vectors in multiple directions!

  27. Magnetic cart moves its selfOrBlow your own sail Thought Experiment

  28. Plane Free Body Diagram

  29. Normal Force The perpendicular contact force exerted by a surface on another object.

  30. 11/6/13 • Thought Experiment • Drag Force • Tension on strings • Tug of war physics, oh…okay • Vectors in multiple directions!

  31. According to legend, a horse learned newton's laws. When the horse was told to pull a cart, it refused, saying that if it pulled the cart forward, according to Newton's third law, there would be an equal force backwards: thus there would be balanced forces, and the cart would not accelerate. How would you reason with this horse?

  32. Goals for Chapter 5 • To study conditions that establish equilibrium. • To study applications of Newton’s Laws as they apply when the net force is not zero. • To consider contact forces and the effects of friction. • To study elastic forces (such as spring force). • To consider forces as they subdivide in nature (strong, electromagnetic, weak, and gravitational).

  33. Dog Fight • Susan is holding her dog, its’ mass is 8.0 kg, when Allen decides that he wants it and tries to pull it away from Susan. • If Allen pulls horizontally on the dog with a force of 10 N and Susan pulls with a horizontal force of 11 N in the opposite direction, what is the horizontal acceleration of the dog?? • Why doesn’t the dog bite one of them?

  34. Forces on Ropes and Strings Tension: • A specific name for the force exerted by a string or rope.

  35. Forces on Ropes and Strings Tension: • A specific name for the force exerted by a string or rope.

  36. Superhero Tension Situations

  37. 11/6/13 • Thought Experiment • Drag Force • Tension on strings • Tug of war physics, oh…okay • Vectors in multiple directions!

  38. Problem 4 on page 152

  39. Two dimensional equilibrium – Example 5.2 • Both x and y forces must be considered separately. • Follow worked example 5.2 on page 130.

  40. Two dimensional equilibrium – Example 5.2

  41. p.153 # 14a

  42. # 13 on 153

  43. 11/12/13 Newton’s Laws in the real world • Drag Force • Inclined Planes • Friction • Tug of war EQUALIBRIUM • Vectors in multiple directions!

  44. Drag Force • Is it true that particles in the air around an object exert forces on it? • Yes, a huge force, but they all balance, and there is no net effect. • What if the object is moving through the air? • It experiences a drag force • Drag Force: the force exerted by a fluid on an object moving through a fluid. • There is a direct relationship between the magnitude of the drag force and the surface area of a moving object.

  45. Terminal Velocity The constant velocity that is reached when the drag force equals the force of gravity.

  46. Inclined Plane Force Components

  47. 11/13/2013 • Newton’s Laws in static and dynamic situations • Sample problems • Contact Forces Friction • Circular motion lab

  48. Problem in the back of the room

  49. What is Friction? • Friction is the force resisting the relative lateral (side to side) motion of solid surfaces, fluid layers, or material elements in contact. • So far we have neglected friction, but since it is all around us, it is worth treating.

  50. Two Main Types of Friction • Push a book across a desk, it experiences a type of friction that acts on all moving bodies. • KINETIC FRICTION (Fk) a force that is exerted by one surface against another when the two surfaces rub against each other because one or both of the surfaces are moving.

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