Review Newtons Three Laws at Work

1
Review Newtons Three Laws at Work

• Physics 1010
• Dr. Don Franceschetti
• September 11, 2007

2
Lets Disambiguate

• Velocity
• A vector, measured in m/s
• Acceleration
• A vector, (change in velocity)/time, measured in
  m/s2, zero if velocity is constant
• Force
• A push or pull, a vector, measured in newtons
  (kg m/s2)
• Net force is mass x acceleration
• Mass
• Amount of matter, a scalar, measured in kg.
• Weight
• Gravitational force on an object, measured in
  newtons.

3
Newtons Three Laws

• Inertia, Fma, action/reaction
• Need to be used together
• Allow us to determine magnitude of a force if the
  other forces are known.

4
The basic question

• Determine the forces being exerted in a given
  situation, which objects are exerting them, and
  which are action/reaction pairs.

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Situation

• You sleeping in bed
• On you
• Gravity (your weight) Rxn is you pulling on Earth
• Support force from bed. Rxn is you pushing down
  on bed (even while asleep)
• Net force is zero

6
Situation

• Book lying on table
• On book
• Gravity (weight). Rxn is book pulling on Earth
• Support force from table. Rxn is book pushing on
  table
• Net force is zero

7
Situation

• You rollerblading (neglect friction and air
  resistance)
• On you
• Gravity (weight) Rxn is you pulling on Earth
• Support force of ground. Rxn is you pushing down
  on ground.
• Net force zero
• Compare walking

8
Situation

• You pushing a box across the floor
• On box
• Gravity (weight)
• Support force of floor
• Friction of floor
• Your push

• On you
• Gravity (weight)
• Support force of floor
• Friction of floor
• Boxs push on you

9
More situations

• Driving on a level road
• Airplane in level flight
• Airplane banking
• Two weights and a pulley
• Falling raindrop
• Parachuting
• Box lowered on cable
• Biceps and forearm