1' Lecture

1
Physics 1710 Chapter 5 Laws of MotionII
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• 1' Lecture
• Newtons Laws of Motion are

• Acceleration (or deceleration) occurs if and
  only if there is a net external force.

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

4
Physics 1710 Chapter 5 Laws of MotionII
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• 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 MotionII
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• 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
Physics 1710 Chapter 5 Laws of MotionII
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• 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
Physics 1710 Chapter 5 Laws of MotionII
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• 2nd Law of Motion Implications
• Feather and Penny Demonstration

8
Physics 1710 Chapter 5 Laws of MotionII
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• 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
Physics 1710 Chapter 5 Laws of MotionII
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• 2nd Law of Motion Implications
• Book and Paper

Book wins!
CDrag v2 Book
CDrag v2 Paper
- gm
aPaper (- gm CDrag v2 Paper) /m
- gM
aBook (- gM CDrag v2 Book)/M
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Physics 1710 Chapter 5 Laws of MotionII
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• 2nd Law of Motion Implications
• Book and Paper

Tie!
CDrag v2
- gm
- gM
aBookPaper (- gM - gm CDrag v2)/(Mm)
11
Physics 1710 Chapter 5 Laws of MotionII
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• 2nd Law of Motion Implications
• Atwood Machine

12
Physics 1710 Chapter 5 Laws of MotionII
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• 2nd Law of Motion Implications
• Atwood Machine

F g (m2 m1)
a F/(m2 m1)
a g(m2 m1)/(m2 m1)
13
Physics 1710 Chapter 5 Laws of MotionII
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• 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 Newtons apple

14
Physics 1710 Chapter 5 Laws of MotionII
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• 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
Physics 1710 Chapter 5 Laws of MotionII
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• 2nd Law of Motion Implications
• Mysterious Spool Puzzle

Spool
F ?
?????????
16
Physics 1710 Chapter 5 Laws of MotionII
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• 2nd Law of Motion Implications
• Mysterious Spool Puzzle

Spool
F ?
a F/m a, always in the direction of F
17
Physics 1710 Chapter 5 Laws of MotionII
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• Summary
• Newtons 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
Physics 1710 Chapter 5 Laws of MotionII
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• Summary (contd.)
• 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.