Download Friction

Chapter 4
Newton’s Second Law
of Motion
How does an object move when a force acts on it?
22-May-17
What is Weight?
• Weight-The force of gravity acting on an
object
• Since F=MA and weight is a force
– F=Mg where g=the acceleration due to
gravity =10 m/s/s on earth (different on
different planets)
Friction and Air Resistance
Friction and air resistance are forces opposing motion.
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Friction
Origin of friction is molecular interaction between
solid surfaces.
Friction is complicated.
Friction depends on
support force (also called
the normal force) and on
properties of the surface.
Basic properties of
friction first established
by Leonardo da Vinci.
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Friction
• Depends on the roughness of the surfaces
that are moving over each other and the
normal force between the objects
– Coefficient of friction (m) is a dimensionless
ratio that relates the roughness of the
surfaces
– Normal force=support force. We will only be
dealing with flat surfaces so Fn=weight=m*g
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Free Body Diagram
• A free body diagram outlines all of the
forces that act on an object
– Fa=Applied force
– Ff=Friction force
– Fn=Normal Force
– Fg=Weight
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What does each force mean?
• Fa=Applied force. The outside force that is
being exerted on the object in order to
make it move.
• Ff=Friction force. The retarding force that
opposes the motion of the object.
• Fg=Weight. Force of gravity on the object.
• Fn=Normal Force. The upward support
force the surface exerts on the moving
object.
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Equations
•
•
•
•
Fn=m*g (weight)
Ff=m*Fn
(friction force)
Fnet=Fa-Ff
Fnet=M*A
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Air Resistance (Drag)
Origin of drag is molecules of gas (or liquid) striking
a moving object.
Drag force depends on:
•Size (area) of the object
•Speed of the object
Larger the size or speed,
larger the drag.
Also depends on shape of object,
density of gas or liquid, etc.
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Air Resistance on a Falling Object
Gravity force on an object
(i.e., weight) is constant but
air resistance depends on an
object’s speed.
As a falling object gains
speed, the resistance force
gets larger so the net force
decreases.
Net force is sum of:
Weight (downward)
Resistance (upward)
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Demo: Drop the Sheet
A flat sheet of paper falls slowly because of
air resistance.
What happens if we place it on top of a
book, blocking the air from reaching it?
Air
Resistance
Weight
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Check Yourself
Golf ball has more mass than a ping-pong
ball. Force of gravity is greater on:
golf ball, ping-pong ball, or the same?
The two balls are the same size; when
speeds are equal, drag force is greater
on which ball?
Which ball falls faster (which has greater
acceleration)?
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Ping
pong
Ball
Golf
Ball
Drag
Weight
Terminal Speed
Speed of falling objects increases until drag force
balances weight.
When forces balance, zero acceleration
so constant velocity.
Speed for which air resistance balances
weight called terminal speed.
High terminal speed
(better open the chute!)
Low terminal speed
(large area of chute)
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Demo: Drop the Cat
Cats seem to have an uncanny ability to
survive falls from high places. For example,
cats have been known to survive falls of up
to 32 stories. By contrast, dogs rarely
survive falls of more than six stories.
Humans usually die when they fall from
such heights.
In a study of cats that had fallen from up to 32 stories, an
interesting finding emerged: while the rate of injury in cats
seemed to increase linearly depending on the length of the fall,
after seven stories, the rate of injury seemed to level off! In
other words, the survival rate and severity of injuries were no
more severe in a cat that fell seven stories than in one that fell
32 and in some cases, injuries were even less!
From: www.animalhealthcare.ca
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Demo: Drop the Cat (cont.)
After further study, the reasons for this discrepancy
became clear. When a person falls from a
building, maximum speed or "terminal velocity"
(120 mph) is reached after 32 stories. Cats, on the
other hand, achieve terminal velocity at 60 mph
after falling only five stories!
Until a cat reaches terminal velocity, it will experience
acceleration and tend to reflexively extend its
limbs, making it more susceptible to injuries.
However, when a cat reaches terminal velocity, its
vestibular system (i.e. the organs of balance)
become less stimulated, causing the cat to relax.
It will then orient its limbs more horizontally (splaylegged), thereby increasing air drag in much the
same way a parachute does. In this posture, the
force of impact also appears to become more
evenly distributed.
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