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UNIT 3 Forces and the Laws of Motion

UNIT 3 Forces and the Laws of Motion. ConcepTest 4.2a Cart on Track I. 1) slowly come to a stop 2) continue with constant acceleration 3) continue with decreasing acceleration 4) continue with constant velocity 5) immediately come to a stop.

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UNIT 3 Forces and the Laws of Motion

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  1. UNIT 3Forces and the Laws of Motion

  2. ConcepTest 4.2aCart on Track I 1) slowly come to a stop 2) continue with constant acceleration 3) continue with decreasing acceleration 4) continue with constant velocity 5) immediately come to a stop Consider a cart on a horizontal frictionless table. Once the cart has been given a push and released, what will happen to the cart?

  3. ConcepTest 4.2aCart on Track I 1) slowly come to a stop 2) continue with constant acceleration 3) continue with decreasing acceleration 4) continue with constant velocity 5) immediately come to a stop Consider a cart on a horizontal frictionless table. Once the cart has been given a push and released, what will happen to the cart? After the cart is released, there is no longer a force in the x-direction. This does not mean that the cart stops moving!! It simply means that the cart will continuemoving with the same velocity it had at the moment of release. The initial push got the cart moving, but that force is not needed to keep the cart in motion.

  4. Thursday October 13th Introduction to FORCES

  5. TODAY’S AGENDA Thursday, October 13 • Intro to Forces • Mini-Lesson: Properties of Forces • Hw: Practice A (All) p124 UPCOMING… • Fri: Laws of Motion • Mon: Everyday Forces • Tues: More Everyday Forces

  6. Changes in Motion Types of Forces Contact Forces: Forces resulting from physical contact between objects (push or pull) Action-at-a-Distance Forces: Forces between objects that act at a distance (gravity, electrical force, magnetic force, etc. Units of Force SystemMassAcceleration Force SI kg m/s2 N = kg m/s2 2

  7. F1 F1 F2 F2 Fnet = F2 – F1 = 0 F1 F1 F2 F2 Fnet Changes in Motion Force: something capable of changing an object’s state of motion (object’s velocity) 3

  8. Size 100 106 1020 1035 Changes in Motion Distance Forces Types Range Gravitational Unlimited Electromagnetic Unlimited Weak Nuclear  10-12 m Strong Nuclear  10-15 m 4

  9. Galileo’s Experiment A ball rolls farther along the upward incline as the angle of incline is decreased. On a smooth, horizontal surface, the ball rolls a greater distance before coming to rest. How far would the ball travel on an ideal, perfectly horizontal smooth surface? Changes in Motion 5

  10. Newton’s First Law (Law of Inertia) In the absence of an unbalanced applied force (net force = 0), an object at rest remains at rest, and an object already in motion remains in motion with a constant velocity (constant speed and direction). Newton’s 1st Law If the net force acting on an object is zero, then its acceleration is zero. 6

  11. Newton’s 1st Law Mass Mass is a quantitative measure of inertia. A massive object has more inertia, or more resistance to change in motion, than does a less massive object. A car has more resistance to the change in motion (inertia) than a bicycle. 7

  12. Newton’s 2nd and 3rdLaws Newton’s Second Law The acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass. The direction of the acceleration is in the direction of the applied net force. Fnet = ma 8

  13. m Newton’s 2nd and 3rdLaws Fw = force of weight Weight Fw = mg 2m Fnet= ma F In free fall, all objects fall with the same constant acceleration (g). An object with twice the mass of another has twice as much gravitational force acting on it. But with twice the mass, the object also has twice the inertia, so twice as much force is needed to give it the same acceleration. 2F 9

  14. Forces come in pairs F1 F2 Newton’s 2nd and 3rdLaws Newton’s Third Law (action-reaction) Action-reaction forces do not act on the same object. 10

  15. Newton’s 2nd and 3rdLaws Two Possible Outcomes to Force Problems Object at Equilibrium Object is either at REST or Object is moving at constant velocity Net force is equal to zero. a = 0 FNET = 0 Object is Accelerating The net force causes the object to accelerate. Net force is non-zero. FNET = ma 11

  16. F = 20 N vi = 0 m = 5 kg Dt = 5 s vf= ? F m Find velocity of block after Dt Newton’s 2nd and 3rdLaws 12

  17. F = 10 N vi = 20 m/s m = 5 kg vf= 0 F m x Find distance block moves Newton’s 2nd and 3rdLaws 13

  18. END

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