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Newton’s Laws
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1st Law – Inertia
• Newton's first law of motion is often
stated as:
• An object at rest tends to stay at rest
and an object in motion tends to stay in
motion with the same speed and in the
same direction unless acted upon by an
unbalanced force.
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2nd Law
• Newton's second law of motion can be
formally stated as follows:
• The acceleration of an object as produced by
a net force is directly proportional to the
magnitude of the net force, in the same
direction as the net force, and inversely
proportional to the mass of the object. In
terms of an equation, the net force is equated
to the product of the mass times the
acceleration.
• Fnet = m a
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3rd Law
• "For every action, there is an equal and
opposite reaction."
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1st law restated
• Inertia - the tendency of an object to
resist a change in its motion
– If at rest, wants to stay at rest
– If moving, wants to continue moving
– UNLESS ACTED UPON BY AN OUTSIDE
FORCE!
– http://www.glenbrook.k12.il.us/gbssci/phys/
Class/newtlaws/u2l2b.html
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Inertia and mass are related in that
the more massive an object is
the more resistance to change in
the object’s motion
or
INERTIA
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Assignments
• On Slide 8 do: 1-6
• On Slides 9 - 12 do: 1-9, 12, 13-21
(odds only), 36. Show formulae, work
with numbers and units, and answers
with numbers and units for 13-21 odds.
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There are many more applications of Newton's
first law of motion. Several applications are
listed below – try to provide explanations for
each application.
• blood rushes from your head to your feet
when riding on a descending elevator
which suddenly stops.
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•the head of a hammer can be
tightened onto the wooden
handle by banging the bottom
of the handle against a hard
surface.
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a brick is painlessly broken over the chest of
a physics teacher by slamming the brick with a
sledge hammer. (CAUTION: Do not attempt
this at home!)
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to dislodge ketchup
from the bottom of a
ketchup bottle, the
bottle is often turned
upside down, thrust
downward at a high
speed and then
abruptly halted.
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 headrests are placed in cars to prevent
whiplash injuries during rear-end
collisions.
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while riding a skateboard (or wagon or
bicycle), you fly forward off the board when
hitting a curb, a rock or another object which
abruptly halts the motion of the skateboard.
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2nd Law (F=ma)
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Force, F
units: newtons
Mass, m
kg
Acceleration, a
m/s 2
2nd Law
Problem:
F
m
a
Find the net force necessary for a
l.6 x 103 kg car to accelerate forward at 2.0 m/s2.
F = ma = 1.6 x 103 kg (2.0 m/s2)
F = 3.2 x 103 kgm/s2 or newton, n
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Weight = mass x free fall
acceleration
• W = mg
• On Earth, g = 9.8 m/s2
• A 1.0 kg object has a weight of
w
mg
l.0 kg x 9.8 m/s2 = 9.8 n
If you weigh 125 lb, find your weight in newtons.
125 lb 1 kg
9.8 m = 556.8 n
2.2 lb
s2
What law is this?
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3rd Law
For every action force
There is an equal and opposite reaction
force.
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ETC.
• Centripetal Force - the force that pulls an
object moving in a circle toward the center
of the circle.
1. and 2. show
centripetal force
vectors.
1.
2.
Object moving
clockwise in a
circle is always
being pulled
toward the center
of the circle.
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Kinetic and Potential Energies
• Kinetic Energy - energy of motion
• Potential Energy - energy of position
KE = 0
PE = max
KE = 0
PE = max
KE = max
PE = 0
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KE and PE
• KE = ½ m v2
Kgm2 = joule or j
v2
• PE = mgh
Kg m m = joule also
s2
g = 9.8 m/s2
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12.4 Universal Forces
• Electromagnetic Forces - act between charged
particles
– Opposites attract for both electric and magnetic forces
– Electric force is responsible for repulsion of like charged particles
• Nuclear Forces
– Strong nuclear force holds neutrons and protons together in the
nucleus
– Weak nuclear force (weaker than the strong n.f.) acts only over
very small distances and is involved in nuclear decay
• Gravitational - acts between any two objects in the
universe and acts over large distances
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