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Newton’s Laws

Newton’s Laws. Newton-1: Law of Inertia. Newton’s First Law An object subject to no external forces is at rest or moves with a constant velocity if viewed from an inertial reference frame . If no net forces act, there is no acceleration. Minimizing Friction.

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Newton’s Laws

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  1. Newton’s Laws

  2. Newton-1: Law of Inertia • Newton’s First Law • An object subject to no external forces is at rest or moves with a constant velocity if viewed from aninertial reference frame. • If no net forces act, there is no acceleration.

  3. Minimizing Friction • One way to minimize friction is to float objects on a cushion of air. You will use an air track in lab this week. • Any horizontal force exerted on the cart is the net force acting on the cart.

  4. Acceleration vs Force • Experiments show that Acceleration is proportional to force

  5. Acceleration vs Mass • Acceleration is inversely proportional to mass

  6. Newton-2 • Combining these two observations gives • and by an appropriate choice of units Or, more familiarly, Units: Mass has SI units of kg, and acceleration has SI units of m/s2. The SI unit of force is:1 newton = 1 N = 1kg m/s2.

  7. m1 Example: Accelerated Mass Anet force of 3.0 N produces an acceleration of 2.0 m/s2 on an object of unknown mass. What is the mass of the object?

  8. Newton-2 (Second Law of Motion)

  9. Combining Forces Forces add as vectors.

  10. Force Addition ACT Two forces are exerted on an object. Which third force would make the net force point to the left? (a) (b) (c) (d)

  11. Free Body Reprise • Free-body diagrams: • A free-body diagram shows every force acting on an object. • Isolate the object of interest • Choose a convenient coordinate system • Sketch the forces • Resolve the forces into components • Apply Newton’s second law in each coordinate direction

  12. Free-body Diagram Example

  13. Kinematics & Dynamics Combo You are stranded in space, away from your spaceship. Fortunately, you have a propulsion unit that provides a constant net force F for 3.0 s. You turn it on, and after 3.0 s you have moved 2.25 m. If your mass is 68 kg, find F. Kinematics Dynamics Since there is only one force, we call that direction positive x and only worry about magnitudes.

  14. Free-body ACT

  15. Example: Three Forces Moe, Larry, and Curley push on a 752 kg boat, each exerting a 80.5 N force parallel to the dock. (a) What is the acceleration of the boat if they all push in the same direction?(b) What is the acceleration if Moe pushes in the opposite direction from Larry and Curley as shown?

  16. Newton’s Third Law of Motion Forces always come in pairs, acting on different objects: If Object 1 exerts a force F on Object 2, then object 2 exerts a force –F on Object 1. These forces are called action-reaction pairs.

  17. Newton’s Third Law of Motion Some action-reaction pairs:

  18. Contact forces: The force exerted by one box on the other is different depending on which one you push. 3 kg 12 N 1 kg Boxes 1 and 2 rest on a frictionless surface. What is the acceleration in each case? What is the force between the boxes in each of the cases? 12 N 3 kg 1 kg

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