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Velocity differs from speed in that we also
know the direction of the moving object.
Velocity is both speed and direction.
Velocity is a vector quantity.
Speed is a scalar quantity.
Constant speed doesn’t mean constant
velocity… the opposite is true though.
FORCE = Any push or pull which can cause
something to move (change its speed or
direction)
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A change in position in relation to an object.
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A moving sidewalk is going forward at 10m/s. If you are at
rest in the airport what would you measure the velocity of a
person on the sidewalk to be if the person is
a) standing still on the sidewalk?
 b) walking with a speed of 2 m/s forwards?
 c) walking with a speed of 2 m/s backwards?
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What would the person on the sidewalk measure your
velocity to be if they aren’t walking on the sidewalk?
If you are on another moving sidewalk traveling the same
direction at 10m/s what would you measure the velocity of
the person to be if neither one of you are walking on the
sidewalk?
If you are on another moving sidewalk traveling the opposite
direction at 10 m/s what would you measure the velocity of
the person to be if neither one of you is walking on the
sidewalk?
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An object or point from which motion is
determined.
The most common frame of reference is
ARISTOTLE
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Greek scientist (Born 384
BCE)
Natural motion
Unnatural motion
Believed that it was
“natural” for heavy objects to
fall faster than light objects
Motion continues so long as there
is only an applied motion (force)
to an object. Removing the motion
(force) stops the object.
Aristotle’s ideas lasted
almost 2000 years…
GALILEO
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Italian scientist (15641642 CE).
Leaning Tower of Pisa
Said that a force is
required to change the
motion of an object
1st scientist to formulate
idea of inertia
Inertia
A property of matter
 The tendency of an object to resist any change in its
motion
 The greater the mass the greater the inertia
 The greater the speed the greater the inertia
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Sir Isaac Newton (1642-1727)
Mathematician and physicist
Discovered many things:
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Laws of motion
Optics
Gravity
Calculus
Newton’s first law of motion
The Law of Inertia
An object at rest . . .
and an object in motion . . .
at . . .
unless acted upon . . .
An object in
motion stays in
motion at constant
velocity
An object at rest
stays at rest
unless acted
upon by an
outside force.
Describe where the
outside force is and
how it changes the
motion
or
Describe how the
object is not affected
by an outside force
Physics of elevators
Bombs away!
Dropping a bomb
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Forces are vector quantities with direction
Balanced forces are equal in size and opposite
in direction
Unbalanced forces are not equal in size and/or
opposite in direction.
Free body diagrams are used to show all forces
acting on an object
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Sum of all the forces on an object ( F)
It is the net force that changes the state of
motion of an object.
Net Force
5N
10 N
5N
10 N
5N
5N
10 N
15 N
is the same as
is the same as
is the same as
5N
0N
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Net force on each box?
ΣF = 15 N
ΣF = -9 N
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A body can have many forces acting on it
and still have a zero net force.
And no net force
means no change in
motion!
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 F = 0 when an object is at a constant velocity
(stopped or moving)
If ΣF = 0 then there is no “outside force” so no
change in motion
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Equilibrium for an object at rest
Object hanging from spring scale
Book on table:
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Support or normal force
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Objects traveling at
constant velocity
have  F = 0 because
of Newton’s 1st law –
an outside force is
needed to change
the motion of an
object
Computer
simulation
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When the pellet fired into the spiral tube
emerges, which path will it follow? (Neglect
gravity).
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When the ball at the end of the string swings
to its lowest point, the string is cut by a
sharp razor.
Which path will the ball then follow?
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Correct your friend who says, "The race-car driver
rounded the curve at a constant velocity of 100
km/h."
If a huge bear were chasing you, its enormous mass
would be very threatening. But if you ran in a
zigzag pattern, the bear's mass would be to your
advantage. Why?
Consider a ball at rest in the middle of a toy wagon.
When the wagon is pulled forward, the ball rolls
against the back of the wagon. Interpret this
observation in terms of Newton's first law.
Push a shopping cart and it moves. When you stop
pushing, it comes to rest. Does this violate
Newton's law of inertia? Defend your answer.
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Consider a pair of forces, one having a magnitude
of 20 N, and the other 12 N. What maximum net
force is possible for these two forces? What is the
minimum net force possible?
The sketch shows a painting staging in mechanical
equilibrium. The person in the middle weighs 250
N, and the tensions in each rope are 200 N. What is
the weight of the staging?
A different staging that weighs 300 N supports two
painters, one 250 N and the other 300 N. The
reading in the left scale is 400 N. What is the
reading in the right hand scale?
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Nellie Newton hangs at rest from the ends of the
rope as shown. How does the reading on the scale
compare to her weight?
If you toss a coin straight upward while riding in a
train, where does the coin land when the motion of
the train is uniform along a straight-line track?
When the train slows while the coin is in the air?
When the train is turning?
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If the speedometer of a car reads a constant speed of
50 km/hr, can you say that the car has a constant
velocity?
A space probe may be carried by a rocket into outer
space. What keeps the probe going after the rocket
no longer pushes it?
Why do you lurch forward in a bus that suddenly
slows? Why do you lurch backward when it picks
up speed?
When a car moves along the highway at constant
velocity, the net force on it is zero. Why, then, do
you continue running your engine?
As you stand at rest on a floor, does the floor exert
an upward force against your feet? If so, what
exactly is this force?
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Harry the painter swings year after year from his
bosun's chair. His weight is 500 N and the rope,
unknown to him, has a breaking point of 300 N.
Why doesn't the rope break when he is supported
as shown at the left above? One day Harry is
painting near a flagpole, and, for a change, he ties
the free end of the rope to the flagpole instead of to
his chair as shown at the right. Why did Harry end
up taking his vacation early?
A horizontal force of 100 N pushes a box across a
floor at a constant speed.
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What is the net force acting on the box?
What is the force of friction on the box?