Transcript to Ms. D`s Power Point Presentation on Chap 6-1

Chapter 6-1
Force & Motion
What is force?
A push
A pull
Contact vs Long Range Forces
A Contact force acts only on an object by touching it
Example: Push a crate across the floor
Contact vs Long Range Forces
Long Range force:
force is exerted without contact
Examples:
Gravity
Magnetism
So,
Push your physics book
across the desk:
contact force of hand on book
Push your physics book
off the desk:
long range force of gravity takes
over
Forces have agents!
The agent is the specific,
identifiable, immediate cause of
the force
Agents for example contact force:
• Friction of the crate against the floor
• Push of your hands on the crate
The agent is an object: a real, tangible thing!
Agents for Examples of Long
Range Force:
For gravity:
The mass of the Earth attracting
(gravitationally) the mass of the crate
The agent is an object: a real, tangible thing!
Agents for Examples of Long
Range Force:
For magnetism:
The north pole of the magnet in your hand
repelling the north pole of the hanging
magnet.
The agent is an object: a real, tangible thing!
If you can’t name an
AGENT
then a force doesn’t exist!
Some pictorial models with vectors
drawn for forces:
Be able to draw these kinds of pictorial models and force vectors:
Notice that force vectors are different from velocity or acceleration vectors
because they are drawn from the same spot.
How does acceleration depend on Force?
The more Force, the more acceleration.
A direct relationship
How does acceleration depend on the
mass of the object?
The more mass, the less acceleration.
An inverse relationship
An unbalanced force (this is, a Net Force) is the cause of a change in velocity!
BIG TOPIC
Drawing Free Body Diagrams (FBD)
Steps to follow in drawing a FBD:
Draw the object as a dot with a box around it
Drawing FBD Step 2
Draw all forces starting on the dot:
These are force vectors, so forces are
Represented as arrows:
Length is proportional to magnitude of force
Direction is drawn in the direction of the
force
Remember our hard working crate-pusher?
The scenario is that the crate is accelerating
to the right (it’s important that it is accelerating).
Let’s see what a FBD of the forces on the crate
looks like---
Drawing FBD of crate being
accelerated to the right:
Draw the force of the push as an arrow to
the right
Draw the friction force of the floor on the
crate as an arrow to the left
It is important the friction force is less than
the pushing force for this scenario—Why?
Drawing FBD of crate being
accelerated to the right:
There must be a net force, Fnet
to the right
Draw FBD of these scenarios:
No. 1
Draw FBD Scenario 2:
Draw FBD Scenario 3:
Let’s use the parallelogram method
to solve these vector diagrams:
Solve these problems: draw the
FBD and solve for the net Force
What is a Newton, anyway?
I know what a pound feels like but
what does the force of a Newton
feel like?
The typical apple weighs 1N
Other typical weights/Forces
measured in Newtons:
Remember Galileo?
Well, he really came up with this concept,
Newton just codified it as his first law of motion and,
thus, got credit for it.
“An object at rest will remain at rest or
An object that is moving
will continue to move
•in a straight line
•at a constant speed
Unless acted upon
by an unbalanced (net) force.”
Often called the “Law of Inertia”
What is inertia?
It is the tendency of an object to resist change.
Things want to keep doing what they’re
already doing.
Another BIG CONCEPT:
EQUILIBRIUM
• Occurs when net force = zero
– The object is at rest or
– The object is moving at constant velocity
(Remember: no net force, no acceleration–
the object is in uniform motion or
the object has constant velocity)
Different Types of Forces and their
symbols:
Note the use of “system” and “environment” above:
System is a defined collection of objects—it is the subject under consideration.
Environment is the rest of the world outside the System. Note: the environment
can exert forces on the system.
Common misconceptions about
Forces:
• When a ball has been thrown, the force of
the hand that threw it remains on it.
• A force is needed to keep an object
moving
• Air does not exert a force
• The quantity ma is a force
Fin 6-1