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Physics 1710 Chapter 5: Laws of Motion—II

0. Physics 1710 Chapter 5: Laws of Motion—II. 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—II

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

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

  5. Physics 1710 Chapter 5: Laws of Motion—II 0 2nd Law of Motion: Implications The Newton [N] is the unit of force in SI units. [F] = [m] [ a ] [N] = [kg] [m/s/s]

  6. 0 Physics 1710 Chapter 5: Laws of Motion—II 2nd Law of Motion: Implications Consider free fall (in the absence of air resistance): Weight = F = m g fact #1 F = m a fact #2 ∴ m ag = m g ∴ ag = g

  7. 0 Physics 1710 Chapter 5: Laws of Motion—II 2nd Law of Motion: Implications Feather and Penny Demonstration

  8. 0 Physics 1710 Chapter 5: Laws of Motion—II 2nd Law of Motion: Implications When a sheet of paper and a book are dropped at the same time, what will happen? Will (1) the book accelerate faster that the sheet; (2) vice versa or (3) will they each accelerate at the same rate? Why do you think so?

  9. 0 Physics 1710 Chapter 5: Laws of Motion—II Book wins! CDrag v2 Book CDrag v2 Paper 2nd Law of Motion: Implications Book and Paper - gm - gM aPaper = (- gm+ CDrag v2 Paper) /m aBook = (- gM + CDrag v2 Book)/M

  10. 0 Physics 1710 Chapter 5: Laws of Motion—II Tie! CDrag v2 2nd Law of Motion: Implications Book and Paper - gm - gM aBook+Paper = (- gM - gm+ CDrag v2)/(M+m)

  11. 0 Physics 1710 Chapter 5: Laws of Motion—II 2nd Law of Motion: Implications Atwood Machine

  12. a a g m1 g m2 0 Physics 1710 Chapter 5: Laws of Motion—II F = g (m2 – m1) a = F/(m2 +m1) 2nd Law of Motion: Implications Atwood Machine a = g(m2 – m1)/(m2 +m1)

  13. 0 Physics 1710 Chapter 5: Laws of Motion—II 2nd Law of Motion: Implications How much force is one Newton? How much mass weighs 1 N? F = m g 1.00 N = mN (9.80 N/kg) mN = 1.00/ 9.80 kg mN = 0.102 kg Experience “Newton’s apple”

  14. 0 Physics 1710 Chapter 5: Laws of Motion—II 2nd Law of Motion: Implications What acceleration will one Newton produce on a mass of one kilogram? a = F /m a = 1.00 N /1.00 kg a = 1.00 m/s2 [ m/s2] = [N ]/[kg]

  15. Pull Away from F Toward F No acceleration 0 Physics 1710 Chapter 5: Laws of Motion—II Spool F ⃗ 2nd Law of Motion: Implications Mysterious Spool Puzzle ?????????

  16. Pull Toward F 0 Physics 1710 Chapter 5: Laws of Motion—II Spool F ⃗ 2nd Law of Motion: Implications Mysterious Spool Puzzle a = F/m; a, alwaysin the direction of F

  17. 0 Physics 1710 Chapter 5: Laws of Motion—II 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

  18. 0 Physics 1710 Chapter 5: Laws of Motion—II 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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