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Physics 1710 Chapter 5: Laws of Motion—III

0. Physics 1710 Chapter 5: Laws of Motion—III. Acceleration (or deceleration) occurs if and only if there is a net external force. 1 ′ Lecture Newton’s Laws of Motion are:. a = F / m [Note this is a vector eqn.].

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Physics 1710 Chapter 5: Laws of Motion—III

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  1. 0 Physics 1710 Chapter 5: Laws of Motion—III • Acceleration (or deceleration) occurs if and only if there is a net external force. 1′ Lecture Newton’s Laws of Motion are: • a = F/m[Note this is a vector eqn.] • The force exerted by a first object on a second is always equal and opposite the the force exerted by the second on the first. F12= - F21

  2. 0 Physics 1710 Chapter 5: Laws of Motion—III 1′ Lecture –continued (30″ more) Weight is the force of gravity equal to g times the mass of the object. g =9.80 N/kg The force of friction is opposed to the motion of a body and proportional to the normal force. Free body diagrams are sketches of all the forces acting on a body.

  3. 0 Physics 1710 Chapter 5: Laws of Motion—III Laws of Motion 1st Law: An acceleration is caused by a net external force. REVIEW

  4. 0 Physics 1710 Chapter 5: Laws of Motion—III 2nd Law of Motion: a= F /m Note the vector nature of the equation: ax= Fx /m ay= Fy /m REVIEW

  5. 0 Physics 1710 Chapter 5: Laws of Motion—III Third Law of Motion: If two objects interact, the force of the 1st on the 2nd is equal and opposite to the force of the 2nd acting on the 1st body. F12 = - F21 #2 #1 F12 F21

  6. 0 Physics 1710 Chapter 5: Laws of Motion—III #2 #1 ? F21 F12 3rd Law in Action—Tug of War It’s the Law!F12 = - F21

  7. Lou GeelsAs Baron von Munchenhausen 0 Physics 1710 Chapter 5: Laws of Motion—III Arm force = Third Law of Motion, Implications : Baron von MunchenhausenDraw “free body diagram” - Reaction force Net force = 0 F12 = - F21

  8. 0 Physics 1710 Chapter 5: Laws of Motion—III • The tree is not pushing on the elephant. • The tree is pushing back, but the elephant is pushing harder. • The tree is pushing harder on the elephant that she is pushing on the tree. • The tree and elephant push equally against each other. • The tree is not pushing on the elephant. • The tree is pushing back, but the elephant is pushing harder. • The tree is pushing harder on the elephant that she is pushing on the tree. • The tree and elephant push equally against each other. But how can you have acceleration in this case? The acceleration of object #2 isa2 = F12 /m2 Not 0 = ( F12 +F21 )/(m1 + m2)

  9. 0 Physics 1710 Chapter 5: Laws of Motion—III Third Law of Motion, Implications : Tug of war Draw “free body diagram”

  10. 0 Physics 1710 Chapter 5: Laws of Motion—III Floor reaction: mg Mg Third Law of Motion, Implications : Tug of war :“free body diagram” M m Rope tension Friction:μ (Mg) Friction: -μ (mg) F12 F21 -mg -Mg Weight Resultant: Fm = F12 – μ(mg)FM = 0

  11. 0 Physics 1710 Chapter 5: Laws of Motion—III Third Law of Motion, Implications : Why does mud fly backward from the tire when an automobile accelerates in mud? For each “action” and equal but “opposite action” must happen.

  12. Fdrag Fdrag - gm - gm 0 Physics 1710 Chapter 5: Laws of Motion—III 2nd and 3rd Laws of Motion: Implications Why does air have “resistance?”

  13. No Talking! Think! Confer! 0 Physics 1710 Chapter 5: Laws of Motion—III Peer Instruction Time

  14. Fdrag Fdrag - gm - gm 0 Physics 1710 Chapter 5: Laws of Motion—III Inertia of the air in the way 2nd and 3rd Laws of Motion: Implications Why does air have “resistance?” Δmair= ρAv Δt aair = v/ Δt Fair, max = Δmairaair = (ρAv Δt)(v/ Δt) = ρAv 2 Fair = ½ D ρAv 2 = - Fdrag

  15. 0 Physics 1710 Chapter 5: Laws of Motion—III • Friction is a force that resists motion. • Friction is proportional to the “normal” force • (ie perpendicular force) and opposed to the • direction of the applied force. Why do objects stop moving?

  16. 0 Empirical Observation: Physics 1710 Chapter 5: Laws of Motion—III Ff = - μN, in opposite direction of applied force. μ is the coefficient of friction, which depends upon the nature of the two surfaces in contact and the state of motion.

  17. 0 Physics 1710 Chapter 5: Laws of Motion—III Friction is caused by “welding” of the two surfaces. Lubrication changes the nature of the contact and/or the surface, atomically.

  18. Welds 0 Physics 1710 Chapter 5: Laws of Motion—III Friction is caused by “welding” of the two surfaces.

  19. N N More contact points 0 Physics 1710 Chapter 5: Laws of Motion—III Effect of Normal Force—more contact. Ff ,max = μN

  20. 0 Physics 1710 Chapter 5: Laws of Motion—III Friction is caused by “welding” of the two surfaces. Lubrication changes the nature of the contact and/or the surface, atomically.

  21. Adhesion Ff = μN N Normal Force 0 Physics 1710 Chapter 5: Laws of Motion—III Rolling Friction:

  22. No Adhesion Ff = μN N Normal Force 0 Physics 1710 Chapter 5: Laws of Motion—III Rolling Friction: Lubrication

  23. No Talking! Think! Confer! 0 Physics 1710 Chapter 5: Laws of Motion—III Peer Instruction Time

  24. 0 Physics 1710 Chapter 5: Laws of Motion—III Friction, Application: For constant velocity F = -Ff = -(- μ N) N = m g = (5.2 kg)(9.8 N/kg) = 51. N F = (0.30) (51. N) = 15. N

  25. 0 Physics 1710 Chapter 5: Laws of Motion—III Third Law of Motion, Implications : A father (100 kg) and his daughter (50 kg) are ice skating. The push off from each other. Who pushes harder? What is there relative acceleration?

  26. M m 0 Physics 1710 Chapter 5: Laws of Motion—III Third Law of Motion, Implications : A father (100 kg) and his daughter (50 kg) are ice skating. The push off from each other. Who pushes harder? M m FDad Fkid

  27. M m 0 Physics 1710 Chapter 5: Laws of Motion—III 3rd Law of Motion says they push the same and opposite. Third Law of Motion, Implications : A father (100 kg) and his daughter (50 kg) are ice skating. The push off from each other. Who pushes harder? M m -F F aM =-F/M am =F/m

  28. Physics 1710 Chapter 5: Laws of Motion—III Summary: Newton’s Laws of Motion are: (1) Acceleration (or deceleration) occurs if and only if there is a net external force. (2) a = F/m [Note this is a vector eqn.] (3) The force exerted by a first object on a second is always equal and opposite the the force exerted by the second on the first. F12= - F21

  29. Physics 1710 Chapter 5: Laws of Motion—III Summary (cont’d.) : Weight is the force of gravity equal to g times the mass of the object. g =9.80 N/kg The force of friction is opposed to the motion of a body and proportional to the normal force. Free body diagrams are sketches of all the forces acting on a body.

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