How do you describe the fluid, effortless motion of a coasting skater?
A simple answer to that question is that a coasting skater moves at a constant
velocity. The term velocity is new, and I'm about
to define it. In fact, in a short time, I'll restate
Newton's first law, using better, more sophisticated language, but in order to
do that, I need to introduce three physical quantities associated with
motion. Those physical quantities are position,
velocity, and force. Position is the measure of an object's
location in space. For example, my position is, is about 6.5
feet, or two meters in front of this jar. You'll notice I needed three things.
First, here's the reference point from which to work with, this jar, second,
there's my distance from that jar, about six and one-half feet or two meters, and
finally, there's the direction, from that reference point, to me, namely in front
of. If you leave out any one of those three
aspects of position, the reference point, the distance and the
direction from that reference point to the lo, location you have in mind,
you don't know the objects position. To show you that, let me change my
position. Well first off, if, we lose that bottle.
And I say I'm six feet in front of eh, we don't know, you don't know where I am
anymore. Okay, we need the bottle, the reference
point, okay. So now, suppose you know that I'm in
front of the reference point but you don't know how far, you lose the distance
part. I could be here, I could be here.
It's a problem. And finally suppose you forget to mention
the direction from the reference point to me, well I could be six one-half feet
that is two meters for the reference point here,
also here, also here, so the direction matters as
well, you need all three. Position is a distance and a direction
from a reference point. Physical quantities like position that
have both an amount and a direction are known as vector quantities.
You're probably familiar with physical quantities that have just an amount, like
the temperature of this room or the duration of a concert or the size of your
computer screen. But there are also many important
physical quantities that have both an amount and a direction.
They're vector quantities, and if you leave out the direction part of a vector
quantity, you haven't specified the whole thing.
We measure position in units of distance. Physicists normally use the SI, or system
international of units. And the SI unit of distance, is the
meter. That's about a meter right there.
Well, because most people in the United States are relatively unfamiliar with the
SI units, the meter in particular, and because this class is all about familiar
things, I'll also use more familiar units.
In this case, more familiar units of distance include the foot and the mile.
When an object is moving, it has a velocity that's not zero.
Velocity measures the rate at which an object's position is changing with time.
Velocity is another vector quantity. It has an amount, and a direction.
The amount of your velocity is the speed at which you are traveling.
And the direction of your velocity is the direction in which you're heading.
Let me show you my velocity. Right now, my velocity is zero.
But I'm about to develop a velocity to your left.
Here we go. I'm now moving toward the left.
And my velocity is, oh, maybe half a foot per second to the left.
That's different from this velocity. Now, I'm moving about half a foot per
second toward your right. That's a different velocity, even though
my speed is the same. As you can see, velocity has an amount,
my speed, in this case, and a direction.
The direction in which I'm heading. And if you don't specify both of them in
specifying my velocity, for example, you've left out some important
information. You haven't fully specified my velocity.
We measure velocity in units of distance per time.
The SI unit of velocity is the meter per second.
In the United States, more familiar units of, of velocity are, feet per second,
and miles per hour. I have one more physical quantity to
introduce. Force.
Force is the physics term for a push or a pull.
An it's another vector quantity. For example, if I push on this book.
. I'm exerting a force on it, and that
force has both an amount which is how hard I'm pushing, and a direction, which
in this case, is away from you. The SI unit of force, is the Newton, one
Newton, is about the force that a small, apple exorts on your hand when you hold
it, this, is actually not a small apple, this about a three Newton apple.
Louis A. BloomfieldProfessor of Physics
