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4 outline

4 outline. Newton’s 2 nd Law friction dynamics of falling objects RQ: 1, 2, 3, 4, 5, 6, 8, 11, 13, 14, 16, 18, 20, 21, 24, 26, 27, 30, 31, 33. Ex: 2, 3, 20, 31, 32, 33, 42, 43, 44. Problems: 2, 3, 5 . example uses. design crumple zones (cars & barriers) power requirements for cars

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4 outline

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  1. 4 outline • Newton’s 2nd Law • friction • dynamics of falling objects • RQ: 1, 2, 3, 4, 5, 6, 8, 11, 13, 14, 16, 18, 20, 21, 24, 26, 27, 30, 31, 33. • Ex: 2, 3, 20, 31, 32, 33, 42, 43, 44. • Problems: 2, 3, 5.

  2. example uses • design crumple zones (cars & barriers) • power requirements for cars • parachute design • elevator design • /

  3. force & acceleration • Net force  acceleration • acceleration ~ net force • e.g. The net force on a car is doubled. The acceleration of the car will then also double in size. (“~” means “directly proportional to”)

  4. 0 Newton’s Second Law: the acceleration of an object is proportional to the Net External Force acting on it, and inversely proportional to the object’s mass. acceleration

  5. Effect of a Net Force • object at rest begins to move • moving object changes its: • speed • or direction • or both. • change in velocity is in the same direction as the net force on the object.

  6. Finding Net Force Using Diagrams • Example of adding two perpendicular Forces A = 3, B = 4: • A) Walk 3 steps forward. • B) Turn left or right, walk 4 steps in this direction. • C) Walk directly between the starting and ending locations counting your steps. • Distance in “steps” in (C) is the Net of the two forces.

  7. Ex. Net Force Rightward • Car moving to right • net force is to right • Speed increases

  8. Ex. Net Force Leftward • Car moving to right • net force is directed left • Car slows down

  9. Ex. Net Force Downward • Ball is tossed to right • net force is down • object turns downward

  10. Mass and Weight • Mass is the quantity of matter. • Mass measures “inertia”. • Mass is measured in kilograms (kg) • Weight is the force on mass due to gravity. • Weight is measured in newtons (N) or pounds (lb). • Weight ~ Mass • 1 kg of mass has a weight of 2.2 lbs

  11. Comparing Accelerations of Objects in Free Fall. • Downward force is weight. • a = weight/mass • but an object with twice the mass will have twice the weight… • so the accelerations are the same… • We call this acceleration “g”. • g is about 10m/s/s downward.

  12. Friction • Objects in contact “like each other”, i.e. they form a “bond”. • They resist being moved when in contact. • Ex. A Chest sitting on a wood floor seems “glued” down. It is harder to get it moving than it is to keep it moving. • These resistance forces are called “frictional forces”.

  13. Direction of Frictional Forces • Frictional forces oppose the direction of motion of object if it is moving.

  14. Direction of Frictional Forces (cont.) • If the object is at rest there still may be a frictional force. If there is a frictional force then it is in a direction opposite to the direction of the net applied force due to other causes.

  15. Categories of Friction Sliding Friction: exists when one object slides against a second object, e.g. box along floor. Static Friction: exists when a force is applied to an object, but that force is not large enough to break the frictional bond. Air friction force ~ (speed of object)2. / 16

  16. Weight Force • Weight (N) = mass (kg) x (10N/kg) • Weight = mg • g = 10 m/s/s = 10 N/kg • Acts at all times, e.g. object is falling, sitting, etc. • Ex: A 2kg object weighs 20N. • Weight = (2kg)(10N/kg) = 20N • /

  17. Free-Fall • only force is object’s weight • air drag is negligible in size • Ex. A solid steel ball falling a short distance is in free-fall. • Ex. A falling feather is not in free-fall since air drag is equal to its weight

  18. Non Free Fall • Whenever air drag is significant compared to weight the object will fall with acceleration less than 10m/s/s. • Example: A 5kg object weighs 50N but an air drag force of 10N acts on it. • Acceleration = NetForce/mass • = (50-10)N/5kg • = 40/5 m/s/s • = 8 m/s/s • /

  19. Ex. Falling Motion 1 M = 1kg, speed is small, air friction = 0. Weight is 10N. Net force = 10N – 0 = 10 N Acceleration = Net Force/Mass = 10N/1kg = 10 m/s/s N/kg = m/s/s /

  20. Ex. Falling Motion 2 M = 1kg, speed is large and air friction is 1/3 the weight = 10N/3 =3.33N Net force = 10N – 3.33N = 6.67N Acceleration = Net Force/Mass = 6.67N/1kg = 6.67m/s/s /

  21. Ex. Falling Motion 3 M = 1kg, speed is larger and air friction is equal to the object weight = 10 N Net force = 10N – 10N = 0N Acceleration = Net Force/Mass = 0N/1kg = 0 m/s/s /

  22. Terminal Speed/Velocity • Air drag on falling objects increases… • until equal to the objects weight… • … resulting in balanced forces • acceleration = 0, speed no longer changes • This top speed is called the “terminal velocity” of the object… • … and varies from object to object.

  23. 4 summary • Newton’s 2nd Law relates net force, mass, and acceleration. It also covers the 1st Law. • frictional forces are proportional to the forces holding objects together • falling objects accelerate until air drag equals their weight

  24. p.11 practicing physics • Mass is fundamental, does not depend on location • Mass is not a vector and not a force, it is simply a number • 1kg mass “weighs” 10N on earth (2.2lbs), less on the moon • Weight = mg (earth g = 10m/s/s)

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