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Newton’s Laws

Newton’s Laws. Solving Force Problems. Draw a FBD Identify direction of movement Write the expression for ΣF = F 1 +F 2 -F 3 … Set ΣF expression equal to m tot a net F 1 +F 2 -F 3 …= m tot a net. Inertia. Newton’s 1 st Law An object wants to keep on doing what it is already doing

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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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