Newtons Laws of Motion

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Newtons Laws of Motion
Newtonian mechanics (aka Classical
mechanics) Forces The cause of acceleration
(must be vectors) Arise from interactions between
objects Contact forces Long range forces Four
Fundamental Forces Gravity, Electromagnetism,
Weak Nuclear Force, Strong Nuclear Force Units
Newtons Net Force on an object vector sum of
all forces acting on that object Superposition of
forces (demo)
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Newtons first law Inertia A body acted on by no
net force moves with constant velocity (which may
be zero) and zero acceleration. or An object at
rest will remain at rest and an object in motion
will remain in motion (at constant velocity)
unless acted upon by an external force. Inertia
the resistance of an object to changes in its
motion. Equilibrium no net force Inertial
frame of reference a frame of reference in
which the law of inertia holds A non-accelerating
frame of reference vP/A vP/B vB/A
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Newtons second law Mass is a measure of
inertia Relative mass relative inertia mB/mA
vA/vB Force and acceleration the 2nd Law ? F
ma Force units newton (N) 1 N 1Kg 1m/s2
Components ? Fx max ? Fy may ?
Fz maz
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A horizontal force of 10.0 N is applied to a 2.00
kg block which is initially at rest on a smooth
(i.e. frictionless) level surface. Find the
acceleration, speed and displacement of the block
after it has gone for 3.00 s.
n
FA
w
A mass of 2.00 kg is propelled across a rough
horizontal surface with an initial speed 3.00m/s
and comes to a rest in a distance of 1.50 m.
What is the magnitude and direction of the force
of friction which slows the mass?
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Weight the force exerted by earth (via gravity)
on an object In free fall, gravity is the only
force acting on the object F ma mg w Weight
(mass)(local acceleration of gravity) g varies
from 9.78 to 9.82 m/s2 over the surface of the
earth g is a vector field! Inertial Mass vs
Gravitational mass F mInerta mgravg w It is
believed that mInert mgrav for all matter ?
all things fall at the same rate
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Some notes on units cgs units 1 dyne 1g
cm/s2 english units 1 pound 1slug ft/s2 (g 32
ft/s2) 1 kg weighs 2.2 pounds english
engineering units pounds mass versus pounds force
Watch for implied quantities! (weight vs
mass!) Example 4.7 A 1.96E4 N car traveling in
the x-direction makes a fast stop. The
x-component of the net force acting on it is
1.50E4 N. What is the acceleration?
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Newtons third law action/reaction When an
object exerts a force on a second object, the
second object exerts a force on the first of
equal magnitude, but opposite direction. The
action and the reaction are forces on two
different bodies! For every action there is an
equal but opposite reaction. Forces arise from
interactions!
Forces on an Object
Action/Reaction
reaction force of table on computer
reaction force of table on computer
force of computer on table
weight of computer
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• Using Newtons laws
• Define coordinate system
• note direction of acceleration
• use coordinates consistently ( dir for r, v, F,
  a)
• Apply laws to a single body
• Draw a free body diagram
• indicate all forces acting on an object in simple
  diagram
• Identify known and unknown quantities
• components with correct sign
• Use Newtons laws to relate quantities
• Check units consistency in your answer

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A 200 kg mass is suspended from a chain. what is
the weight of the mass? what is the tension in
the chain?
A 200 kg mass is suspended from a 10 kg
chain. what is the weight of the mass? what is
the tension at the bottom of the chain? what is
the tension in the top of the chain? what is the
tension in the middle of the chain?