Transcript Forces

Unit Two: Dynamics
Section 1: Forces
What Do I Already Know?
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Take 10 minutes to discuss with the person
next to you. You can use your textbook
and/or phones as well if you don’t know!
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What is dynamics?
What is a force?
What does a force do?
What are some types of forces?
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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.
Summary Slide
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Dynamics: The study of WHY things move.
Forces (Summary Slide)
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What is a force?
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A push or a pull that causes a change in motion or shape.
It depends on the mass of an object and its acceleration.
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Force = mass x acceleration
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Symbol: F
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Formula: F=ma
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Units: kg x m/s2 = Newtons (N)
What does a force do???
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causes acceleration (like gravity)
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OR
causes stretching, bending, squeezing (like
spring forces)
The 2 Main Types of Forces
Contact Forces:
- Force when two objects are physically in contact with one
another
Example: push/pull (APPLIED force or Fapp), normal force
(FN), friction (Ff), spring force (Fe), tension (FT), air
resistance
Non-contact Forces:
- Force when two objects are not touching
Example: gravitational force (Fg), nuclear force, magnetic
force, electrostatic force (electric charge)
Summary Slide: Types of
Forces
Contact Forces:
- Force when two objects are physically in contact
with one another
Non-contact Forces:
- Force when two objects are not touching
Copy this down and add as we
go!
Name of
Force
Symbol
Contact or
NonContact
Description/Important
Ideas/Formula
Friction
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Symbol: Ff
A contact force that occurs because of
electromagnetic force (between surface
atoms of objects touching)
Acts in the opposite direction to motion
Spring Force
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Symbol: Fe
Also called elastic force
Contact force (spring is pushing something)
Occurs when a spring or elastic is stretched
and released or compressed and released
We will look at this in the Energy unit!
Tension
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Symbol: FT
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Contact force
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Tension is the magnitude of the pulling
force exerted by a string, cable, chain, or
similar object on another object.
Air Resistance
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Symbol: Fair
Contact force between the “air’s” molecules and the
object
Caused when things are moving through the air
In physics 11 we generally ignore air resistance
when completing force equations as it is very small
in comparison to most of the other forces (ie,
negligible)
HOWEVER, you may need to include air resistance
in free body diagrams (coming soon!)
Normal Force
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Symbol: FN
Contact force
A force that acts in a direction perpendicular
to the common contact surface between two
objects
Example Diagram:
Gravitational Forces
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Symbol: Fg
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We typically look at Fg as a a non-contact force
caused by the pull of the Earth’s centre on an object
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In reality, gravity acts as an attractive force between
any two objects. The force is proportional to the
objects so since Earth is so much larger than most
objects we look at, we only think about it as Earth’s
pull.
Can you answer most of the
original questions now?
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What is the difference between dynamics and
kinematics?
What is a force?
What can a force do?
What are the main forces we deal with
everyday?
Identify the forces and their
direction… for each situation
(Copy down and leave a few extra
lines for each)
A) A book is at rest on a table.
B) A book is being pushed by a person
horizontally to the right at a constant speed
C) The book in the previous part is let go, allowing it
to slow down and come to rest.
D) The book finally stops.
E)
A book is falling through the air, accelerating
downward.
Check Your Learning
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Identify the forces (with their direction) acting on the book in each
of the following situations.
A) A book is at rest on a table.
 Fg (acting downward)
 FN (acting up)
B) A book is being pushed by a person horizontally to the right at a
constant speed
 Fg (down)
 FN (acting up)
 Fapp (right)
 Ff (left)
C) The book in the previous part is let go, allowing
it to slow down and come to rest.
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Fg (down)
FN (acting up)
Ff (left)
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Since there is no longer any contact between the person
and the book, there is no longer an applied force. The
other forces remain until it stops.
D) The book finally stops.
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Fg (down)
FN (acting up)
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Ff is no longer present as there is no motion.
E) A book is falling through the air, accelerating
downward.
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Fg (down)
Fair (up)
What have you noticed about
Fg?
What have you noticed about
FN?
What have you noticed about
Ff and Fapp?