How does a skater start, stop, or turn? The answer is that a force acts on the

skater and causes the skater to accelerate.

Once again, it's an answer that involves a new physical quantity.

This time, acceleration. And it's time for me to introduce that

term. As I said earlier, a skater's velocity

consists of the speed in which the skater is traveling and the direction in which

he's heading. If he's inertial, that is if he's

experiencing no overall force, then his velocity is constant.

He has a constant speed and a constant direction of travel.

But if he's not inertial, if he's experiencing an overall force, then his

velocity changes with time. He's accelerating.

Acceleration measures the rate at which an object's velocity is changing with

time. It's a subtle quantity.

It's not so easy to observe. You have to look carefully.

Moreover, it's another vector quantity. It has an amount and a direction.

Let me show you. I'm going to, I'll be the object, and I'm

going to accelerate. But first, let me get myself some room.

I'll head over here to start with. And I'm about to accelerate toward your

right. Now initially, I'm not going to

accelerate. Here I am at velocity zero.

I'm still at velocity zero, still at velocity zero. So, my velocity

is not changed with time, I'm not accelerating.

But I'm about to accelerate to the right at a slow rate and my velocity will

develop. Here we go,

toward the right. And it will become faster and faster and

faster and faster until I disappear from view.

Alright. So, what you should be looking for is not

mine, my movement, per se, that I simply have a velocity.

You should look for my velocity changing. It goes from slowly to the right, to

faster and faster and faster and faster to the right.

Now, let me show you acceleration towards your left.

And I'm going to do a rather fast acceleration to the left.

Ready, get set. Whoa,

off I go. Alright.

In those cases, I was accelerating in the direction of my velocity.

And my velocity was increasing as a result.

I was going faster and faster. My speed was steadily increasing and

that's how a skater starts. The skater starts from rest with a

velocity of zero, chooses a direction to accelerate,

and begins to pick up speed in that direction.

And continues to accelerate in that direction,

thereby, going faster and faster and faster.

The SI unit of acceleration is the meter per second per second or meter per second

squared. Now, acceleration isn't all that familiar

to anyone. So, the familiar unit for acceleration in

the United States is not so familiar. But it is the foot per second squared.

Well, we now have velocity and acceleration. My velocity is the speed of

my motion in the direction which I'm heading.

My acceleration is how that velocity is changing with time.

And that acceleration has both an amount and a direction.

Well, let me show you a couple of examples of this.

If I accelerate in the direction of my velocity as we've seen, I speed up.

On the other hand, if I accelerate opposite the direction of my velocity,

I am going to slow down. Let me show you.

Don't watch yet. Let me get started because I,

I, I can't do everything at once. So, I'm going to get started.

Now, I'm moving to the right but I'm accelerating to the left, and watch what

happens to my velocity. I come momentarily to a stop and if I

keep accelerating backwards here towards your left, I pick up speed and disappear

off out of view on the, on the left. Well,

accelerating forward, that is in the direction of my velocity, I pick up speed

we, we're comfortable calling that acceleration.

if I however, I accelerate opposite my velocity, so let me do this again.

So, I'm going to, let me get started. So, here we go, ready, start.

And now, I'm going to accelerate opposite my velocity, I come to a stop, and then I

pick up speed in the opposite direction. During the stopping part of that story,

so as I was heading to the right but accelerated to the left and slowing down,

we have a special name that we often apply to that type of acceleration,

acceleration opposite your velocity. We call it deceleration.

It's, it's not necessarily a completely separate concept.

It's a, just a special case of acceleration.

Acceleration opposite your velocity causes you to decelerate.

But there's at least one other interesting example of acceleration.

What happens if instead of accelerating in the direction of my velocity or

opposite the direction of velocity, I accelerate to the side.

So, suppose I get started, I'll get started heading to your right, my

velocity will be at the right. Here it is.

Okay, I'm now moving right. And I'm going to accelerate away from

you. Oh, I turned and now I'm going to keep accelerating

to my left, look at what happens to me.

I go in a circle. The point of this is, that if you

accelerate not in the direction of your velocity, and not opposite but rather to

the side, you turn. So, we now can really answer the question

of how a skater, starts, stops and turns. To start, the skater, say, the skater is

at rest, the skater has to pick a direction and

begin to accelerating that direction. So, so here, I'll,

I'll be the skater and I will start from rest and I want us,

I want to hit to the right. We'll talk about what causes this

acceleration surely but I am going to develop a velocity that gets larger and

larger toward the right. Here we go.

Okay. So, to start, I pick a direction and then

I continue to accelerate in that same direction,

thereby picking up speed in the direction that I had chosen.

Alright. To stop, what does the skater do?

Well, the skater then starts the situation already moving and accelerates

opposite the skater's velocity. So, I'll do it again.

Here we go. So, let me get started.

Okay, I'm a skater moving along and I want to stop.

I accelerate backwards opposite my velocity and I slow down.

Come to a stop. And if the skater wants to turn and the

skater accelerates in a direction neither forward or backward but rather, to the

side. So, the skater might have been heading,

let me do it differently. Might have been heading again to the

right. And now, decides to, to, to turn more and

more towards you all. And so, the skater is heading to the

right and accelerates towards you. And the skater's velocity changes and the

skater begins to turn. So, let's see whether you got all of

this. I'm going to move.

And then, I'm going to ask you about my velocity and my acceleration.