Newton

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Newtons 3 laws of motion
Isaac Newton 1642 --- 1727
Physics 100 Chapt 3
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1st Law Law of Inertia

• A body at rest tends to stay at rest, a body in
  motion tends to keep moving along at a constant
  speed and in a straight-line path unless
  interfered with by some external forces.

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example
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Motorcycle crash dummy
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Another example(watch the ladder)
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2nd Law Fma

• The acceleration of a body is directly
  proportional to the net force acting on it and
  inversely proportional to its mass.The direction
  of the acceleration is in the direction of the
  applied force.

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Directly proportional to Force
a
a
Small force Small acceleration
Large force Large acceleration
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inversely proportional to mass
a
a
Beach ball
Bowling ball
small mass Large acceleration
Large mass Small acceleration
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Inertial mass

• Inertial mass, mi, is the resistance to changes
  in the state of motion

Objects with large mi are hard to get moving (
once started, hard to stop),
Objects with small mi easier to get moving (
easier to stop),
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Units of mass (we cant avoid them!)

• Mass basic unit 1kilogram 1kg
• mass of 1 liter of water

10cm
This much water!
10cm
10cm
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Net force
Tip-to-tail method for adding vector
Net force is the vector from the tail of the 1st
to the tip of the 2nd. (0 in this case).
Slide tail of one to tip of the other (keep
directions fixed)
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Tip-to-tail method
Net force points down the hill
Slide tail of one to tip of the other (keep
directions fixed)
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Newtons 2nd law ? Fma
a ? F
a is proportional to F
direction of a direction of F
a ? F
a is inversely proportional to m
a ? 1/m
a ? F/m
combine
multiply both sides by m
set proportionality constant 1
a F/m?
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Units of Force
Fma
m s2
kg
Unit of force 1 Newton 1N 1 kg m/s2
1 pound 1lb 4.5 N
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What is your mass?

• Weight force of
• Earths gravity on you

Suppose I jump off a tqble
Fma
Wmg
ag
m
W g
W
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Two different aspects of mass
Force of gravity is proportional to
gravitational mass
mgg
Weight W m g
Newtons 2nd law
Inertia resistance to changes in state is
proportional to inertial mass
F m
a
mi
Experiment shows mg mi
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My mass
!!!!!
My weight in pounds W 190 lbs
Convert to Newtons W 190 lbs x 4.5 N/lb
855 N
855 N 10 m/s2
m W/g 85.5 kg
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Newton3rd Law action-reaction

• Whenever one object exerts a force on a second
  object, the second object exerts an equal in
  magnitude but opposite in direction force on the
  first.

reaction the canoe pushes me forward
action I push on the canoe
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Action ??Reaction
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I push on the bus
v 0
F
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But I accelerate
v
Newton The bus exerted an equal but
opposite force on me.
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Look again
All forces come in pairs!
-F
F
This force causes me to accelerate backwards
This force tries to accel. the bus forward
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Air-filled balloon
reaction air pushes on balloon
action balloon pushes on air
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recoil
action gun exerts force F2 on bullet making it
accelerate
reaction equal but opposite force on the gun
F1 Produces a recoil
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Rocket propulsion
reaction rocket gets pushed in the
opposite direction
action rocket engine pushes exhaust gasses out
the rear