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Intro to Forces
Transcript of Intro to Forces
Here in 20012 AD,
So the Council of Elders and Global Elder Harvi-San,
Decided to send out teams to the past,
to change the future
And thus was born our Mission!
Physics Definition of Force
Simplest case: a
on one object by another.
In general, there are
EVERY force is an
...this is more profound than it seems.
the general state of forces by observing motion.
#1. Identify ALL forces acting:
Relating Forces and Motion
Draw a sketch of each situation.
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?
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!
Newton's First Law:
This laziness is called
Objects tend to
on doing what they are
doing... unless acted upon by a
(Stay moving in a straight line at a constant speed.)
The MORE mass an object has, the MORE it resists changing velocity (accelerating)
FORCE is measured in pounds or Newtons.
MASS is measure in kilograms or grams.
Free Body (Force) Diagrams:
Forces on Stationary Objects
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.
A baseball player slides into base.
earth on player
(upward) ground on player
ground (friction) on player
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?
floor on person
earth on person
What does N's 2nd Law say about this?
Newton's 2nd Law:
Since objects are
it takes an
to change their velocity (that is, to
The size of the acceleration caused depends upon the size of the force, but also upon the object's ________.
The direction of the
will ALWAYS be the SAME as the direction of the
Bowling ball as it goes through "no touch" zone on VERY smooth floor.
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?
Microscopic Model of Matter
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?