html5-img
1 / 29

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.].

kiersten
Download Presentation

Physics 1710 Chapter 5: Laws of Motion—III

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  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.

More Related