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Unit Two: Dynamics
Section 1: Forces
Definitions
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What is the difference between dynamics and
kinematics?
What is a force? What can a force do? What
causes a force?
Key Terms:
Dynamics
Kinematics
Force
Gravitational Force
Electromagnetic Force
Strong Nuclear Force
Inertia
Net Force
Normal Force
Weight
What is dynamics???
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Kinematics: The study of how objects
move (velocity, acceleration)
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Galileo performed experiments that allowed him
to describe motion but not explain motion.
Dynamics: The study of why objects
move.
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The connection between acceleration and its
cause can be summarized by Newton’s 3 Laws of
Motion (published in 1687)
The cause of acceleration is FORCE.
Forces
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What is a force?
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Some forces occur when one object touches
another object.
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Example: Gravity pulls downward on anything you drop.
Some forces cause acceleration
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Example: If you put your coat on a hook, the hook pulls up
on your coat.
Some forces occur without any physical touch.
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A push or a pull
Example: gravity
Some forces cause stretching, bending, squeezing
4 Types of Forces
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1) Gravitational Force: an attractive force
that exists between all objects.
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It is the weakest force.
Ex: The moon is held in orbit by the Earth’s
gravity.
2) Electromagnetic Force: forces resulting
from electric charge.
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This force gives materials their strength, their
ability to bend, squeeze, stretch or shatter.
It is very large compared to gravity
4 Types of Forces
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3) Strong Nuclear Force: holds the
particles in the nucleus of an atom together.
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It is the strongest force but only acts over the
distance of a nucleus.
4) Weak Force: form of electromagnetic
force.
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Involved in the radioactive decay of nuclei
Newton’s First Law of Motion
- Newton’s Law of Inertia
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An object at rest or in uniform motion (ie,
constant velocity) will remain at rest or in
uniform motion unless acted on by an
external force.
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Reworded: An object at rest will remain at
rest until a force is applied. An object moving
at a constant velocity will continue to move at
a constant velocity if no force is applied (ie,
no acceleration).
Where did this come from?
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Galileo performed many experiments and
speculated that if a perfectly smooth object
were on a perfectly smooth horizontal surface
it would travel forever in a straight line.
Newton developed this idea.
Newton’s First Law Example
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If an apple is sitting on Mr, Nguyen’s desk, it
will remain there until the desk is removed
(so gravity acts on it) or someone lifts it up
(force).
If a car is driving along a straight road at
100km/h, it will continue to do so (given the
car still has gas!) until the brakes are applied
(force), there is a turn or the road surface
changes (more or less friction).
Net Force
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The sum of all vector forces acting on an
object.
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Example: What are the forces acting on a
stopped car? Draw a labeled diagram.
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Example: What are the forces acting on a
car moving at 100km/h [N]?
Normal Force
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A force that acts in a direction perpendicular
to the common contact surface between two
objects
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Example Diagram:
Inertia
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Tendency of an object to not change its
motion.
Quick Experiment
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Materials – cup, card, penny or coin
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What to do:
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Set up the card on top of the cup and the
penny on the card.
Flick the card. What happens to the card?
The penny? Why?
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Force - Units
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F=ma
Force units = mass units x acceleration units
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F = kg x m/s2
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Physicists decided to name 1.00 kg۰m/s2 = 1.00 N
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N = Newton = a force that causes a mass of one
kilogram to accelerate at a rate of one meter per
second squared
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Gravitational Forces
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Gravitational force decreases as we increase
how far we are from the centre of the Earth
It is a non-contact force
Gravitational Forces
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Example: Consider the following information
and then compare the gravitational force on
the SAME OBJECT in each case.
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A man standing near the equator (distance from
Earth’s centre = 6378 km)
A man standing near the North pole (distance
from Earth’s centre = 6357 km)
A man standing in the International Space Station
(distance = 6628 km)
A man in a space ship past Pluto
Weight Vs. Mass
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Weight and mass are NOT THE SAME.
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Weight = the force of gravity acting on a mass.
Weight can change. It is measured in Newtons
(force).
 Weight = mass x gravitational force
 Fg = mg
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Mass = the quantity of matter an object contains.
Mass for the same object is constant. It is
measured in kg.
Examples of Weight Problems
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Mr. Nguyen’s dog Pi has a mass of 17kg.
What would Pi’s weight be:
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A) On Earth?
B) On Jupiter (where g = 25.9 m/s2)
C) On the Moon (where g = 1.64 m/s2)
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Examples of Weight Problems
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A student standing on a scientific spring scale
on Earth finds that he weighs 825N. Find his
mass.
Friction
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A contact force
Electromagnetic Force (between surface
atoms of objects touching)
Friction
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There are 2 types of friction:
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Static Frictional Force
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When you start to move an object from rest
Larger than Kinetic Frictional Force due to Inertia
ųs
Kinetic Frictional Force
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Exists when the object is moving
ųK
Friction
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The strength of friction depends on…
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Surface materials
Magnitude of forces pressing surfaces together
The strength of friction DOES NOT depend
on…
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Surface area
Velocity of object moving
Coefficient of Friction
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“Stickiness value”
ų (symbol mu)
ų has no units
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Formula: Ff = ųFN
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Remember: FN = - Fg
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Friction Example
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During the winter, owners of pickup trucks
often place sandbags in the rear of their
vehicles. Calculate the increased static force
of friction between the rubber tires and wet
concrete resulting from the addition of 200.
kg of sandbags in the back of the truck.
Friction Example 2
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A horizontal force of 85N is required to pull a
child in a sled at constant speed over dry
snow to overcome the force of friction. The
child and sled have a combined mass of 52
kg. Calculate the coefficient of kinetic friction
between the sled and the snow.
Tug of War
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Sometimes we have more than 1 force acting
on an object (like in a tug of war).
What are the forces at work in a tug of war?
What direction are the forces?
If your team wins, what does that mean about
the forces?
If your team loses, what does that mean
about the forces?
What other forces are there on the players?
Free Body Diagrams
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We usually use a box or small
circle to represent the object.
The size of the arrow is
reflective of the magnitude of
the force.
The direction of the arrow
reveals the direction in which
the force acts.
Each force arrow in the diagram
is labelled to indicate the type
of force.
Use math symbols to show
equality if needed.
What can you tell about these
forces???
Free Body Diagram Examples
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1. A book is at rest on a table top. Diagram
the forces acting on the book.
Frames of Reference
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Imagine you are driving in a car. Does it feel
like you have moved?
If you are watching from the road, how does
your frame of reference change?
Newton’s Second Law
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Newton’s first law states that there an object does
not accelerate unless a force is applied to the object.
But how much will an object accelerate when there
is a net force?
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The larger the force the larger the acceleration.
Therefore acceleration is directly proportional to mass.
Acceleration also depends on mass.
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The larger the mass, the smaller the acceleration.
Therefore acceleration is inversely proportional to mass.
Newton’s Second Law
- Newton’s Law of Motion
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The acceleration of a body is directly proportional to
the net force and inversely proportional to its mass.
Force = mass x acceleration
F = ma
A net force acting on an object causes it to
accelerate. The larger the mass of an object, the
smaller the acceleration. We say that a massive
body has more INERTIA than a less massive body.
The acceleration is in the same direction as the
force.
Newton’s Second Law
Examples
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Ex. 1: What net force is required to
accelerate a 1500. kg race car at +3.00m/s2?
Newton’s Second Law
Examples
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Ex. 2: An artillery shell has a mass of 55 kg.
The shell is fired from a gun leaving the
barrel with a velocity of +770 m/s. The gun
barrel is 1.5m long. Assume that the force,
and the acceleration, of the shell is constant
while the shell is in the gun barrel. What is
the force on the shell while it is in the gun
barrel?
Newton’s Third Law
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When one object exerts a force on a second object,
the second object exerts a force on the first that is
equal in magnitude but opposite in direction.
These forces are called action-reaction forces.
Ex: If you push against a wall, you don’t go through
it as the wall “pushes back”.
Only the forces on an object determine its
acceleration.
Mass versus Weight
Friction
Inertial Frame of Reference
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A frame of reference that is at rest or moving
at a constant velocity.
Example: You moving in a car on cruise
control.
Example: You sitting at your desk.
Newton’s Laws of Motion are valid here!
Non-inertial Frame of
Reference
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An accelerating frame of reference
Example: When you suddenly stop in a car.
Newton’s Laws of Motion do not apply!