Chapter 3 Forces and Motion

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Chapter 3 Forces and Motion
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Chapter 3 Forces and Motion
Force a push or a pull that changes an objects
motion or its shape Newton The SI unit of
force and its symbol is N written as a capital
letter because its named after Isaac Newton
For example, a force of 12 N. Friction a
special force that always opposes motion as
moving surfaces rub against each other.
Gravity is a special force that always pulls
downward
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Video Forces and Motion
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Video constant motion
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Isaac Newton
Newton was born in 1642 the year Galileo
died. Newton was dissatisfied with the classical
science (Aristotle) he studied at Cambridge after
he read Galileos writings about motion.
School closed for 2 years during the great
plague, so Newton fled to his families
countryside home where he now had plenty of time
to think about motion, force and inertia. Newton
thought up his three laws of motion and the
mathematics to support it (calculus, the
mathematics of change).
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Video Newtons 3 Laws of Motion
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Newtons 1st Law of motion
(often called the law of
inertia)
Newtons law of inertia how the first two
conditions of motion that occur without forces
Ø     no motion where v 0 (an object at rest)
Ø     constant speed motion where vi vf (an
object in motion)
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Newtons First Law of Motion
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Net Force the algebraic sum of all forces
acting on an object.
Net force 12 N 5 N 7N
Net Force 15N 6 N 9N
Net forces can also act vertically Upward
forces positive and downward negative.
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EquilibriumWhen Net Force Equals Zero.
Equilibrium a situation where equal and
opposite forces act on an object. If an object is
in Equilibrium, then no net forces act on it and
neither its speed or direction will
change. Equilibrium does NOT mean No forces
it means that forces are balanced.
Example A wooden block weighing 5 N is
hanging motionless from a spring which pulls
upward with a force of 5 N. Net force
5 N 5 N 0 N
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Balanced and Unbalanced forces.
Balanced Forces forces that are equal and
opposite Balanced forces are in equilibrium and
motion can be either no motion, or constant speed
motion Unbalanced forces are not in equilibrium
and there is a net force acting on the object.
An object subject to unbalanced forces will
accelerate in the direction of the net force.
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Newtons Second Law The law of constant
acceleration
Net force causes acceleration
CONCLUSION acceleration is directly
proportional to the applied force a ? F
and always acts in the same direction as the
net force.
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Mass resists Acceleration
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Newtons Second Law
Newtons second law The acceleration produced
by a net force is directly proportional to the
size of the net force and inversely proportional
to the objects mass.
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Video Newtons 2nd Law or Motion
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Weight and The Force of Gravity
Gravity A force of attraction between two massive
objects that is directly related to the product
of the two masses and indirectly proportional to
the square of the distance between the
objects. Ø     Gravity gets stronger if the
distance between two masses decreases. Ø    
Gravity gets stronger if one of the masses is
very large (like a planet) Ø Gravity gets
weaker the further away the masses get.
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Video Gravitation
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Free Fall A body that is continually falling
toward the Earth, pulled by only gravity.
The only difference between the apple and the
moon is horizontal velocity. The apple has none
so it falls straight down.
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Newtons universal law of gravity
The force of gravity (weight) is ? Directly
proportional to the product of the Earths mass
mE and a falling objects mass mO. ?
Inversely proportional to the square of the
distance between the centers of the Earth and
the falling object.
Where G is a coefficient 6.67 x 10-11 m3/(kg s2)
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Acceleration of Gravity is the acceleration an
object experiences when freely falling toward the
ground. Since Fg mog then Near the
surface of the Earth, objects have an
acceleration of 9.8 m/s2 in falling. This
value is called called g and has the same
units as any other type of acceleration.
Weight is a measure of the force that gravity
pulls down on an object. Specifically, if you
have 1 kilogram mass of matter the Earths
gravity pulls down on it with a force of (9.8
m/s2)(1 kg) 9.8 Newtons.
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Your weight is different on different planets,
since every planet has a different amount of mass.
Ø    The moon is about 1/6th the Earths mass
so, you weigh about 1/6th what you do on
Earth. Ø     Jupiter has 2 ½ times the Earths
gravity Ø     The sun has 27 times more gravity
than the Earth Ø     Mass does not
change wherever you are, your weight changes
depending on the strength of gravity which
depends on the mass of the planet you are on.
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Video mass and weight
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Following Newtons second law F ma Replace
the F with a W for weight and the acceleration
a with a g to represent the acceleration of
gravity
To calculate an objects weight. Multiply mass
in kilograms by g W mg To calculate an
objects mass Divide the weight in Newtons by
g
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Newtons Second Law and Acceleration Region A
Acceleration
The positive slope of the line indicates that the
box is accelerating (velocity increases with
time) as the pulling force is greater than the
friction force between the box and the table. FP
gt Ff The Net Horizontal Force FN FP Ff
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Region B Motion at constant speed
In this region, the Pulling force is equal to but
in the opposite direction from the force of
friction FP Ff and FN FP Ff
0 The box is in equilibrium moving at constant
speed as shown by the horizontal graph The Net
Force 0 Therefore acceleration is zero and
velocity is constant and to the right.
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Region C Deceleration
In this region, the slope is negative since the
pulling force is either reduced or zero so FP lt
Ff . This makes the net force negative, and
acting in the opposite direction from the
velocity. If the Net Force is negative then the
acceleration will also be negative but the
velocity remains positive, to the right
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      If the net force is in the same
direction as the velocity, the object will
increase speed in that direction.
     If the net force is zero, the object is in
equilibrium, either motionless or its inertia
will keep it moving at constant speed in the
same direction.      If the net force
is in the opposite direction from the velocity,
the object will decelerate or slow down while it
continues moving in the same direction.
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Friction a force that resists motion
FRICTION a force that resists motion of two
objects that are in physical contact. Friction
always acts opposite to the direction of motion.
Pull Force (P) and motion
Friction (F)
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Causes of Friction
Metal surfaces may look smooth to the eye but are
actually composed of microscopic hills and
valleys. As the hills of two objects interact,
they momentarily make and break electrical
forces, attraction between positive and negative
charges in atoms which results in the force we
know of as friction
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The strength of Friction depends on

• The type and roughness of the surfaces in
  contact with each other

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Ways of reducing friction
? Reducing the weight pressing the surfaces
together
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Coefficient of friction ? is the ratio of the
force of friction and the weight pressing the
surfaces together.
F is the force of friction in Newtons W is the
weight of the upper object, in Newtons.
Use F ? W to find the frictional force
between two surfaces. Look up values of ? in
a table.
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Fluid Resistance a force that resists the
motion of a solid object moving through a liquid
like water, or a gas like air.
caused by particles of air or water hitting a
moving object. The faster the object is moving,
the more force the particles hit it with.
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Streamlining is one way to reduce resistance by
smoothing out the shape of an object
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Turbulent flow high velocity flow where eddys
and whirlpools form in the flow causing the fluid
resistance Laminar Flow low velocity flow where
fluid resistance is uniformly low. Fluid flows
smoothly over the object.
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Air resistance an upward force, caused by
friction between air molecules and a falling
object. Air resistance force increases as the
downward velocity increases.
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Video Newtons 3rd Law of motion
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Newtons Third Law Action and Reaction
Force occurs as part of an INTERACTION between
two or more objects.
Since no motion occurs when I push on the wall,
the force I apply and the opposite force the wall
applies must be equal and opposite
Whenever one object exerts a force on a second
object, the second object exerts an equal and
opposite force on the first object. For
every action (force), there is an equal and
opposite reaction (force)
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Action and Reaction forces
Action reaction pairs never act on the same
object they always act on different objects.
BUT the action and reaction forces are equal and
opposite BUT the action and reaction forces are
equal and opposite
The bullets mass is smaller than the rifles but
it exits the rifle at a very large acceleration a
while the rifle moves backward with a smaller
acceleration. So the while the action and
reaction forces are equal and opposite, their
effects (accelerations) are not. for the bullet
F m ? a small mass times large
acceleration equals Action force For the Rifle
F m ? a large mass times small
acceleration) equals reaction force