Transcript document

Force &
Motion
Once upon a time in a land far, far away…
…Aristotle (384 – 322 BC) proposed
the idea that the “natural” state of
any earthly object was to be at rest.
Trees didn’t move on their own, boulders rolled down
hills and then stopped on their own, spears fell to the
ground after traveling for a while:
Everything tended to come to a state of rest.
Experiment #1:
Elevate one end of the plastic ramp 1 – 2 feet, place the
block near the top of it, and let it go…now (before you
click).
Did it slow down and stop once it got to the table?
If it didn’t, run away as fast as you can!!!
Aristotle would say that the block wanted to
be at rest, so it stopped…all by itself.
He reasoned that all earthly objects were made of the
earth to some degree; and so, they all wanted to be
part of the earth again.
Therefore, their natural state was to be at rest
– one with the earth.
At the time this idea made a lot of sense. In fact,
this idea dominated thought for almost 1500
years.
I told you it was a quality idea!
Experiment #2:
This time, place the wooden sphere at the top of the
ramp and let it go…before you click the mouse (just
like last time).
Did it stop like the block did? Aristotle said it would.
Since it is made of the same material as the block, it
would have the same tendency to come to rest.
So, I’d say that Aristotle has a little problem…
his theory no work so good, eh?
This is exactly what Galileo Galilei (1564 – 1642)
noticed about nature, and he went about overturning
Aristotle’s work.
I don’t think so, Scooter.
Galileo didn’t buy the “from the earth, to the earth”
theory of motion that Aristotle championed because
it didn’t explain why the block would stop but the
sphere would keep rolling.
Galileo set up an experiment in which he rolled spheres
down a track that was also inclined at the opposite end,
as shown below.
He noticed that the spheres came to rest on the
opposite slope at approximately the same height they
started on the first slope.
The smoother he made the tracks and spheres the closer they got to that height.
As he made the second half of the track a gentler
slope, the spheres still rolled to the same height,
WHICH MEANT THEY ROLLED FURTHER BEFORE
COMING TO REST.
He extrapolated these results to consider a track that
didn’t end in a slope, but rather remained flat to infinity.
To infinity and beyond!
This sphere should never stop! (since it would never get back to its
original height)
He concluded from all of this that a force must be
responsible for slowing objects down as they moved
across the ground.
THEREFORE, THE NATURAL STATE OF MOTION OF
AN OBJECT IS TO DO WHATEVER IT IS DOING AT
THE MOMENT!
Specifically, an object at rest would remain at rest
while an object placed into motion would remain in
motion. (You’ve heard this before, I know it.)
Galileo explained this predisposition of an object to
maintain its current state of motion by instilling in all
objects a certain physical property called…
INERTIA
Basically, this INERTIA enabled the object to
resist any change in its current state of motion.
The amount of inertia an object contained was
related to its mass – the greater the mass, the
more inertia the object had.
So, if you want to move a box out of your way you have
to push it. If the box is heavier, you have to push it
harder to make it move because it has more inertia –
more resistance to moving.
But there are a couple problems with that inertia thing:
Number 1:
Inanimate objects have been given the ability to
assess their current situation and to decide what they
are or are not going to do. (In other words, we have
personified them – which you can only do in
literature.)
Number 2:
An object’s inertia is not a consistent property. It exists
in certain situations but not in others. It’s as if a banana
is only yellow when it is sitting on the table but not when
you push it across the table.
To illustrate this problem, take the textbook that is
sitting on the table and make it move slowly across the
table.
Did you do it? (Git-R-Done!)
You had to apply a force to it, didn’t you? Galileo would
have said that you had to overcome the book’s inertia in
order to get it to move. Its inertia was trying to keep it at
rest.
Now align the dowel rods parallel to one another, place
the book on them and try to make it move.
You didn’t have to push as hard, did you?
So, where did the book’s inertia go? It was there when
the book was on the table, but now it’s gone (or
seriously diminished).
But how could that be? You didn’t change its mass or
shape or orientation; you just placed it on the dowel rods.
Do you see the problem with the
concept of inertia now?
It’s kind of hard to be consistent with
it. Sometimes it exists, and
sometimes it doesn’t.
So what is INERTIA exactly?
I’ll tell you this much: it is NOT a force.
The object you are pushing against is not fighting you or
pushing on itself to stay in its current state of motion.
INERTIA is just the idea that an object will not change its
motion for no reason. Something must make its motion
change.
In fact, if you truly comprehend the ideas presented next,
you will see that INERTIA is an unnecessary concept.
This man’s name is Isaac Newton. He was born
on Christmas Day, 1642, and died in 1727.
By the way, your physics teacher was also born on Christmas
Day. Coincidence?
...or DESTINY?
Isaac Newton took hold of Galileo’s idea of motion and
adjusted it in a very important way.
Newton realized that when you tried to slide a heavy
crate across a floor, the crate wasn’t trying to keep
itself from moving, SOME OTHER OBJECT (in this
case, the floor itself) WAS ALSO PUSHING ON THE
CRATE, and the two of you were canceling each other
out.
Newton concluded that when all of the pulls and pushes
acting on an object were cancelled out, the object
would remain in its “natural” state: moving in a
STRAIGHT LINE at a CONSTANT SPEED
or remaining at rest (which is a constant
speed of zero).
In other words, an object would maintain a
CONSTANT VELOCITY
unless a push or a pull acted on it to
change that velocity.
And instead of the property of inertia causing this
“tendency to remain as it was”, Isaac utilized the idea of
BALANCED OR UNBALANCED FORCES.
BALANCED FORCES would be those that cancel each
other out.
The forces acting on the box will be BALANCED and
If
you
push
a box in
with
force of
10 pounds
the
crate
willonremain
its acurrent
state
(at rest).
directed to the EAST…
10 lb.
10 lb.
While I push on it with a force of 10 pounds directed to
the
WEST…
total
F
= (+ 10 lb.) + (- 10 lb.) = 0 lb.
The forces will add together to produce a total
force of zero pounds.
So if you push the textbook so it slides away from you a
little bit…
…go ahead, do it…
{I said, “Push the textbook!”}
…the book slides a bit and comes to rest, but not because
it “wants” to stop.
It stops because a frictional force
is acting on it, pushing on it
opposite its direction of motion.
If we could get rid of the friction, then once you put it
into motion by pushing it…
…it should continue at constant
velocity forever!
(which is a very long time)
We can do that – kind of – if we use the
balloon puck that is sitting on your desk.
Experiment #3:
Detach the balloon-stopper from the puck and inflate
the balloon; reattach it to the puck.
Air escaping through a tiny hole in the bottom of the
puck will lift the puck off the table – reducing friction
to a negligible level.
Before all the air runs out, give the puck a small push.
It didn’t slow down and stop, did it?
It continued across the table at a (fairly)
constant velocity, didn’t it?
In order to stop the puck, you have to apply an
unbalanced force to it, don’t you?
This first principle of motion:
“An object will maintain a constant velocity
unless acted upon by an unbalanced force.”
is known as
You may now go back and take
notes on any “main ideas” within
the presentation, if you wish.
(Although, it’s mostly just background
information for the upcoming chapter.)
When finished, please log off the
computer, return to your seat, and
read sections 1 & 2 of chapter 4 in
the text.