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Newton’s Laws

This informative guide dives into Newton's Laws of Motion, focusing on solving force problems through free body diagrams (FBDs) and identifying movement directions. Learn about inertia, the impact of net forces, and torque, which causes rotational motion. We also explore the concepts of center of mass and static equilibrium, providing real-world examples to illustrate key physics principles. By mastering these concepts, you'll be better equipped to analyze and solve complex physics problems, ensuring a solid foundation in both translational and rotational motion.

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Newton’s Laws

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  1. Newton’s Laws

  2. Solving Force Problems • Draw a FBD • Identify direction of movement • Write the expression for ΣF = F1+F2-F3… • Set ΣF expression equal to mtotanet F1+F2-F3…= mtotanet

  3. Inertia • Newton’s 1st Law • An object wants to keep on doing what it is already doing • In order to change, it needs a net force not equal to zero

  4. Inertia • If the net force is equal to zero then: • an object will stay at rest • OR • 2. an object will continue to move at a constant velocity.

  5. Exploring 1st Law: Torque • Translational motion: motion without rotation • Torque can cause rotation • If a force is applied to an object with a fulcrum, you get rotational motion (torque) • Fulcrum: point of rotation (ex. Hinge) Fulcrum

  6. Torque • Rotational Force is most effective far from the fulcrum • Torque is the combination of force and distance from fulcrum (lever arm = r) r r A B r = 0 Which lever arm has greatest torque? C

  7. Torque • Only the perpendicularpart of the force is applied to the torque. • This requires a touch of trig. F r

  8. A

  9. E

  10. Rotational Stability • 1st Law, rotational style: • “In the absence of a net torque, an object not rotating will continue to not rotate and an object rotating will continue to rotate at a constant rotational velocity” • This means that if a rotational system is in balance (not moving), the net torque is zero.

  11. Rotational Stability • 1st Law, rotational style: • Convention: CW is negative, CCW is positive • Which dog has a positive torque?

  12. Center of Mass • Where the weight force of an object is located COM

  13. r = 0 Center of Mass • Objects are balanced when the COM is at the fulcrum • WHY? • b/c the lever arm = 0 COM

  14. Dr Carlson's Science Theatre

  15. Center of Mass • The Weight force of the COM can add a torque if it is not at the fulcrum. What is applying the torque on the left side? The center of mass! Fcm Fw

  16. Center of Mass • Objects in freefallwill rotate about their center of mass

  17. Center of Mass • Concentration of mass • Where the weight force of the object is located • Weight force can add a torque if it is not at the fulcrum.

  18. A father wants to let his 55 lb. daughter play on a seesaw alone. Assume she sits at the end of the 10.0-foot long board. If the board has a weight of 100 lbswhere must he slide the board so that the board’s weight will balance her weight? Center of mass r 5ft - r 10ft FCM = 100lbs Fgirl = 55lb

  19. A uniform plank 16.0-m long, weighing 300 N, rests symmetrically on two supports 8.00 m apart, as shown. A woman weighing 800 N walks toward the right. How far beyond point B can she walk before the plank tips? rcm= 4m 16m 8m = 300N FWoman = 800N Center of mass FCM

  20. A uniform plank 16.0-m long, weighing 300 N, rests symmetrically on two supports 8.00 m apart, as shown. A woman weighing 800 N walks toward the right. How far beyond point B can she walk before the plank tips? rcm= 4m FWoman FCM To the right of fulcrum B

  21. 4. In the previous problem, how far from the right end of the plank should support B be placed so that the woman can walk just to the right end of the plank without causing it to tip? rcm= FWoman FCM

  22. Mass: 70 kg Weight: 700 Newtons (157 pounds) Mass: 70 kg Weight: 112 Newtons (25 pounds) Mass is Same!!

  23. Lab: pg 375-377 • Write givens and equations • Box answers & include units • Mass needs in kg • r (distance to fulcrum) in m

  24. Mass vs. Weight You can convert mass into weight using Newton’s Second Law On Earth, a = 9.8m/s2

  25. Static Equilibrium • An object is in Static equilibrium if: • it isat rest (not moving) • the sum of the torques acting on the object are zero • the sum of the forces acting on the object are zero

  26. A uniform plank 16.0-m long, weighing 300 N, rests symmetrically on two supports 8.00 m apart, as shown. Draw a monkey weighing 50 N on top of support B. Fmonkey Center of mass • Label the center of mass • Now assume support B is the fulcrum • 2. Label the length of the lever arm to the center of mass Fweight

  27. What is the length of the lever arm to the monkey? • 0 m • Does the Monkey create a torque? No! rCM= 4m Fmonkey Fweight

  28. Problems pg 385 Where do you start? • Draw a diagram! • Label CoM and draw all force vectors • Set net torque = 0, (for #10 ) • Measure lever arms (r) • Useτ= Fr and F=ma to solve for unknown.

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