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Intro to Forces

Force Diagrams and types of force interactions.
by

Kelli Warble

on 1 October 2015

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Transcript of Intro to Forces

Intro to Forces
By
The
Way,
Here in 20012 AD,
The World
IS ENDING
So the Council of Elders and Global Elder Harvi-San,
Decided to send out teams to the past,
to change the future
And thu
And thus was born our Mission!
Spring 2014
Physics Definition of Force
Simplest case: a
push
or
pull
on one object by another.
In general, there are
contact
or
long range
forces.
EVERY force is an
INTERACTION
...this is more profound than it seems.
We
deduce
the general state of forces by observing motion.
#1. Identify ALL forces acting:
Relating Forces and Motion
Draw a sketch of each situation.
A
B
C
D
Fill in the row "identifying the forces" acting on the object in each case.
Go back and draw a force diagram for each case:
In these cases, are the forces balanced or unbalanced? (And what is meant by "balanced"?) How do you know from the motion?
Introduction to Forces:
What is a force?
Naming Forces:
Physicists have specialized names for different forces:
e.g.upward force exerted by a surface the "normal" force
We are initially going to AVOID using specialized names because they typically obscure the idea!
INSTEAD use the noun associated with the object exerting the force.
"normal" force is just "force of table"
"gravitational" force is "force of ...
...gravitational force is exerted by the EARTH!
A
B
C
D
Newton's First Law:
Objects are
lazy.
This laziness is called
"inertia".
Objects tend to
keep
on doing what they are
already
doing... unless acted upon by a
net force
.
(Stay moving in a straight line at a constant speed.)
The MORE mass an object has, the MORE it resists changing velocity (accelerating)
MORE
MASS
= MORE
INERTIA
!
FORCE is measured in pounds or Newtons.
MASS is measure in kilograms or grams.
Free Body (Force) Diagrams:
Part 1
Forces on Stationary Objects
Part 2
Thought Question #2:
Will the earth ever stop spinning?
What does Newton's 1st Law have to say about this?
Why does the earth spin?
Thought Question #3: Minimum Force?
Suppose an object, such as a bowling ball, is at rest (motionless). Does ANY force start it going, even a REALLY small force? Or is there some minimum threshold force? Explain.
Can a
fruit fly
move a
bowling ball
?
What would
Newton's Laws
predict?
A baseball player slides into base.
Forces?
earth on player
(upward) ground on player
ground (friction) on player
velocity
What does Newton's 2nd Law tell us?
Person on an UPWARD elevator ride.
(Beginning of trip--speed up briefly.)
What type of motion will occur? What does this mean about the forces?
Forces?
Diagram:
floor on person
earth on person
F
F
EonP
FonP
What does N's 2nd Law say about this?
Newton's 2nd Law:
Since objects are
lazy,
it takes an
unbalanced force
to change their velocity (that is, to
accelerate
the object).
The size of the acceleration caused depends upon the size of the force, but also upon the object's ________.
mass
The direction of the
acceleration
will ALWAYS be the SAME as the direction of the
unbalanced force
!!!
AKA "INERTIA"
Bowling ball as it goes through "no touch" zone on VERY smooth floor.
v
Forces?
Diagram:
F
F
FonB
EonB
upward force of floor on ball
downward pull of earth on ball
Are the forces balanced or unbalanced? How do you know from the motion of the ball?
What does N's 1st Law tell us?
WAIT: Don't we need another force in the direction of the motion?
What about the book on a rigid table?
How does the table "know" how hard to push up on the book?
Does this
Microscopic Model of Matter
help?
force exerted by book
the more the book pushes down, the more the springs "bend", the harder the table pushes back up
THE TABLE ADJUSTS DEPENDING UPON THE WEIGHT OF THE OBJECT PLACED ON IT!!
What does N's 1st Law tell us about the forces acting on the book?
book balanced on (bending) meterstick
force of earth (gravity) down > force of meterstick up
force of earth (gravity) down = force of meterstick up
force of earth (gravity) down < force of meterstick up
How do we know they must be equal??
What about the book on a rigid table?
Full transcript