Scene #

Description

Narration

1

A green chalkboard background is showing on screen. The narrator draws a car in pink chalk, he adds an arrow going to the right and rights 60 miles per hour on the top of the arrow. He then draws a stick figure outside of the car. Followed by drawing a stick figure inside the car.

OK, so today's lesson is going to be about relative motion. And relative motion is really just about how you view things as they move. So let's say that a car is moving along the highway at 60 miles per hour. And you're standing off to the side observing this. Well, to you from your viewpoint right here, this car is moving 60 miles per hour. However, if you were to be inside of that car, the car itself would not be moving 60 miles per hour. The things around you would be moving 60 miles per hour or the things outside of the car would be moving 60 miles per hour. Not things inside the car because, of course, the umbrella is moving with you and the coffee cup is moving with you.

So you actually don't view that moving 60 miles per hour.

2

The narrator draws a second car below the first one and adds an arrow going to the right. He writes 75mph on top of that arrow. He writes 15 mph in the top right corner of the board. Below the two cars he drew he adds a divider. On the bottom of the divider he adds another car with an arrow going left and writes 60 mph on the arrow. To the right of the car he rights 120 miles per hour. Below the 120 miles per hour he writes 60 minus negative 60 equals 120mph.

So with that being said, let's say that another car moves by you at 75 miles per hour. They decided to past you the fast lane there now. Well, to you, this car from inside your car appears as though this car is moving 15 miles per hour. And the reason being is because it's only moving 15 miles per hour faster than you. Well, let's say that we had a divider, and we look across the highway, and another car is moving this way at 60 miles per hour following the speed limit.

Well, how fast would this car look to you is 120 miles per hour. And this comes from the idea that you're moving 60 miles per hour this way, and the car is moving 60 miles per hour this way. So if I was to say that I'm moving 60 miles per hour minus the negative 60 of this car moving in the opposite direction, you're going to get 120 miles per hour. And that's just the appearance of how fast this moves. Because if you're still off to the side of the road, this car is moving 60 miles per hour. This car to an off road observer is 75 miles per hour, and then this car right here is 60 miles per hour in the opposite direction.

3

The narrator draws an arrow pointing south, labels it S for south and writes 120 miles per hour. Above 120 mph he writes 30 mph with an arrow pointing south. He draws a stick figure on the ground. To the left of the stick figure below the arrows he writes 150 mph. He draws a stick figure on the arrow that was originally drawn that represents the plane.

So let's take another look at this. Let's say that an airplane is moving this way, it's moving south at 120 miles per hour. And there is a tailwind of 30 miles per hour behind it. Well, if you were on the ground watching the plane, you would notice that this plane is actually going 150 miles per hour to you who is watching on the ground. However, if you were to be on the plane, you would not notice a difference of more than 120 miles per hour. You would only feel as though you're moving 120 miles per hour. So once again it really only depends on where you are at.

4

The narrator draws another arrow pointing south to represent a plane. He writes 120 to the left of the arrow. To the right of the arrow he draws a smaller arrow pointing north to represent a headwind and writes 30 mph by this arrow. At the bottom of the arrows he writes 90 mph. He draws a stick figure on the 120 mph arrow. He draws a tree to the left of the arrows.

Let's take another example. You have a plane that's moving like this, 120 miles per hour south. And it has a headwind of 30 miles per hour. Well, to an on ground observer, you would notice that this plane is going 90 miles per hour south. However, if you were to be on the plane, you would still feel as if the plane is moving 120 miles per hour, or you would think that the objects around you outside, maybe a tree of some sort, is moving at 120 miles per hour past you.

5

The writing on the chalkboard is cleared and the narrator writes an arrow pointing to the right and writes 120 m/s below it. To the right of the arrows he draws a current in the water. He then adds an arrow moving north from the tip of the front of the first arrow and labels it 30 m/s. He draws a stick figure at the end of the 120 m/s arrow and draws a stick figure to the right of the corner. He connects the arrows to create a triangle. He puts a question mark at the diagonal of the triangle. The narrator proceeds to write out the steps to solving the problem as he says it out loud.

Let's just talk a little bit about this in terms of angles here and do one more example. Let's say that you were on a boat moving 120 meters per second across the river to the other shore over here. And there was a current in the water that was moving your boat, let's just say, 30 meters per second north. Well, to you on the boat, you feel as though you're going 120 meters per second across the river. However, to an onshore observer, somebody who is not on the boat, they're going to actually see the boat go like this at an angle.

So what is that actual apparent motion? Well, we're going to use Pythagorean theorem, and we're going to say 120 squared plus 30 squared is actually equal to c squared, which would be our hypotenuse here. And then c actually comes out to an answer of 124 meters per second. And then we also need an angle, so we're going to take tan of theta equal to opposite over adjacent, which is going to be 30/120. And then we also have to take the inverse tan of 30/20 to get the angle itself-- or 30/120, I apologize. And then the angle actually comes out to an angle of 14 degrees. So to an onshore observer, you're actually moving 124 meters per second at 14 degrees.