Download 2.3-2.5 More forces and Frame of Reference

5.1-5.5 More forces
The greatest invisible
force of all
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Think very carefully about gravity:
how do you know it exists?
Newton described it,
calculated, but didn’t tell us
what it was…
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If you think about it, our concept of
gravity is very abstract
In order to explain why objects fall to
the earth without an apparent force
acting on it, gravity as a force was
developed to explain the behaviour of
objects that seemed to defy Newton’s
first law
Weight vs. mass
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Weight:
The force of gravity acting on an
object as measured by the base unit
Newtons
Mass:
The amount of matter within a given
object as measured with the base unit
grams
More of Newton’s
observations
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By observation of the motion of planetary
bodies, Newton formulated the behaviour
of gravity between two objects
Planets maintained circular orbits around
the sun, suggesting that the sun
generated gravity to keep them there
But the discovery of moons around other
planets also illustrated that planets had a
gravitational pull
I pull you, pull me?
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This suggested that objects affected each
other
The larger mass of the sun and its ability
to override the gravitational pull of the
earth suggested that the gravitational pull
of an object was dependent on the mass
of the object involved
The famous apple
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It is suggested that the famous apple
that started it all might have led
Newton to consider the consequences
of gravity at large distances
If gravity could cause an apple to fall
from a shorter branch, and taller ones
above it – could it affect other objects
much further away – like the moon?
Diagrammatically…
m2
m1
F m2 on m1
F m1 on m2
R = distance
between centre of
masses
Force is dependent on:
Both mass 1 and mass 2
 Distance between masses
 Therefore:
 F = Gm1m2
r2
Where: G = 6.67 x 10-11 Nm2/kg2
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Not so Normal force
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We described normal force sometime
ago, but let’s see how much you
remember
What is the definition of NORMAL
FORCE?
Normal force
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Normal force is the force that a
surface pushes back on an object that
bears weight on it
For example: your textbook sitting on
your desk pushes down on your desk,
and the desk pushes back on the book
The force pushing back on the book is
known as the normal force (FN)
For example: which one of
these situations does
normal force exist?
BANK
Hanging sign
Holding something
against the ceiling
Friction: joining the x and
y axis
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When looking at an object that moves
across a surface, we can draw a FBD
that includes gravity and normal force
in the picture
But how does it really
affect motion?
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If you think logically about it, gravity,
and normal force don’t contribute
directly to the horizontal motion of an
object on a surface – since the motion
is ultimately peripindicular to the axis
of motion
This follows Newton’s second law
Gravitynormal force
friction
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The connection between the y-axis
forces in any object sitting on a
surface is friction
Since friction depends on a portion of
normal force – which is determined by
gravity – it is important to know how
they relate to one in another in
different questions
Try to remember…
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What is friction?
How do we know it exists?
What factors affect friction?
How does friction affect motion?
What is friction?
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Friction is generated by the interaction
of the irregular surfaces of different
objects that are in contact with each
other
Imagine a surface with golf-ball sized
bumps interacting with one that has
tennis-ball sized bumps
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Irregular surfaces have raised and sunken
surfaces
Two irregular surfaces in contact that try to slide
across each other will resist motion because the
raised surfaces fall into the sunken surfaces
Therefore, additional energy is needed to move
the push the raised surfaces out of the sunken
surfaces
How do we know it exists?
Think about the process of pushing an
object across the floor
Why does the box come to
stop without any visible
force acting in the opposite
direction?
Which one of Newton’s
Laws does this violate?
Friction comes into play
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Therefore, another force that is not
inherently visible must be present to
explain this – or else the phenomenon
of sliding objects coming to a stop
would violate Newton’s first law of
inertia
What factors affect friction?
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What do your car brakes do to your
car?
Why do car brakes work differently in
the rain and snow?
What does this tell you about the
nature of friction?
Friction stops cars
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At least, let’s hope so or else it means
that you’re slamming into something
else
When you hit the brakes on your car,
you are actually stopping your wheels
from turning
But you aren’t directly controlling the
forces that cause you to come to a stop
Stopping the wheels
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Means that you are essentially
allowing the rubber on your wheels to
come into contact with the pavement
The rest is dependent on the friction
between the asphalt and the rubber
So why do rain and snow affect
braking?
Change in surfaces
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When it rains or snows, the nature of
the road surface changes
The bumpy surfaces on the asphalt
are not as accessible anymore –
because they are covered by another
substance with a different surface
Therefore, the contact surfaces
change, changing friction
Friction depends on
surfaces
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Therefore, the amount of friction you
can generate is dependent on the
types of surfaces that are in contact
How does friction affect
motion?
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So how much frictional force is
generated to oppose motion?
Think about moving two identical
cardboard boxes across the same floor
One has a greater mass than the other
Which one is easier push across the
floor?
Why?
Remember – I didn’t ask
you to lift them up
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Newton’s second law states that the
heavier one would be harder to move
Is that related directly to the mass if
we are looking at horizontal motion?
Why is a heavier object harder to slide
across the floor?
Normal force
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Remember – the equation for the
calculation of friction includes normal
force
Therefore, frictional force is a fraction
of normal force
The amount of friction generated is
affected by the normal force acting on
the box
Normal not gravitational
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Be aware: normal force is NOT always
equal to gravitational force!
You must be careful when calculating
questions and be aware of this!
How does friction affect
motion?
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Think about the objects in these two
situations – they are both at rest
In which situation is the force of
friction at work?
Static friction vs. kinetic
friction
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Static friction:
Prevents the initiation of movement
Kinetic friction:
Prevents motion
Which one always has a greater value?
(Think about accelerating a car vs.
keeping a car moving at a constant speed
– which one requires more force?)
Static friction is always greater
The coefficient of friction
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What exactly is µ as a measurement?
To summarize:
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Friction:
Always opposes motion
Is dependent on the surfaces in
contact and on normal force
Static friction is greater than kinetic
friction
Let’s see what you know
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A force is applied to an object that sits
on a surface, but it doesn’t move.
Is friction present?
What is the value of the applied force
compared to the static frictional force?