Newtons Laws of Motion

1
Newtons Laws of Motion

• If the sum of all external forces on an object is
  zero, then its speed and direction will not
  change. Inertia
• If a nonzero net force is applied to an object
  its motion will change F ma
• In an interaction between two objects, the forces
  that each exerts on the other are equal in
  magnitude and opposite in direction.

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2
Forces

• Non-Contact ---- Gravity (F G m M /r2 )
• G 6.7x10-11 m3 / (kg s2)
• Example (two people)
• F (6.7x10-11 ) (60) (60)/ (.5)2
• Weight of 100g

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3
Example Weight of Person

• Calculate the gravitational force on a 60 kg
  person near the surface of the earth.
• Earth Mass 6x1024 kg, radius 6.4 x106 m.
• F G M m / r2
• (6.7x10-11 m3 / (kg s2)) (6x1024 kg) (60
  kg)/ (6.4x106 )2 m2
• kg
  m/s2 (Newtons)
• Most common gravitational calculation involves
  object close to surface of the earth
• F (G M / r2) m
• F m

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4
Newtons 2nd Law

• Fnet m a
• What if Gravity is the only force?

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Normal Force

• Perpendicular to Surface
• Repulsive
• Reactive
• Contact

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Example (person in elevator)

• Constant velocity

M 60kg
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Example (person in elevator)

• Accelerating up at 2 m/s2

M 60kg
8
Example (person in elevator)

• Accelerating down at 3m/s2

M 60kg
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Tension

• Similar to normal force (Reactive)
• Can only pull (cant push on rope)
• Constant throughout rope

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Example Box in Elevator
a 2 m/s2

• 10kg

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Springs

• Similar to tension, normal
• Reactive
• Changes length according to tension
• F -k ?x

k50 N/m
10kg