1:00:00 Great Lesson Ideas –
Graphing Two Dimensional Motion [music]
Rob (INTV)
Rob ROB:
My name is Rob LaPointe, I teach high school physics.
Rob ROB:
Want you to break this vector down into its components. A horizontal and a vertical component.
LESSON IDEA:
GRAPHING TWO-DIMENSIONAL MOTION ROB:
The name of my lesson today is “Graphing Two-dimensional Motion.”
Rob, Students ROB:
So tell me about dimensions. How many spatial dimensions are there and what makes them unique?
STUDENT:
Um, three?
ROB:
There’s three. Three space dimension, okay? So they’re forward and back, left and right, and then? Up and down.
GRAPHING TWO-DIMENSIONAL OBJECTS

Rob (VO) ROB:
The goal is to get them to take two-dimensional motion, to separate it into the two parts, to analyze those independently, and then to uh, marry them together at the end to get the solution.
Rob, Students ROB:
So, as long as the dimensions are 90 degrees from each other, they’re completely independent. So we can take them apart, and we can do all the mathematics for one dimension, and do all the mathematics for another dimension, and then marry them back together after we do our math and we’ll get a two- or three- or whatever-dimensional answer.
01:01:02 Rob (INTV) ROB:
So the first thing that uh, I would do, was to show them an experiment where two objects have identical conditions uh, vertically. But different conditions horizontally.
ROB (VO) ROB:
So shoving two quarters off the edge of a table uh, one will just fall straight down and the other one will go outward, but also fall. , and that, that does makes sense.
Rob (INTV) ROB:
And the question is, which one hits the ground first?
Rob, Students ROB:
So the front quarter should more or less fall straight down, the back quarter is gonna sail out some distance. Which quarter will hit the ground first
ROB (VO) ROB:
? Sort of the um, intuitive answer would probably be that the one that just falls straight down and hits the ground first. I mean, it just makes sense on a, on a simple level if something doesn’t have to go as far, it’s gonna take less time to go there.
01:01:50 Rob, Students ROB:
Yes?
STUDENT:
Same? They’ll fall at the same … ?
ROB:
Why?
STUDENT:
Because the vertical force of gravity never changes between the two.
ROB (VO) ROB:
To think about it more carefully, you realize that the horizontal motion that the one quarter has that goes farther really has no impact o-on the vertical motion which they both share. They both uh, are the same weight. They both fall at the same rate, they’re the same size. Um, and so they hit the ground at the same time.
Rob, Students ROB:
So you ready? R-You got your ears ready? Did you hear it?
STUDENTS:
Same time.
ROB:
Same time.
ROB (VO) ROB:
The next thing is that we have a demonstration, it’s classically called the monkey hunter, but um, I had a bear, so I call it the bear hunter. The demonstration is that you have a um, stuffed animal um, hung from a high place with an electromagnet. And I have a gun. It’s like a blow gun that shoots a marble. And when the marble leaves the gun, it trips a switch, which opens the circuit, turns the magnet off so the bear falls. The question then is, where do I aim? Do I aim at the bear? Do I aim below the bear? Do I aim above the bear?
01:02:57 Rob, Students ROB:
Should I as bear hunter shoot above the bear, shoot below the bear, or shoot right at the bear?
STUDENT:
No, you want it to be at it, ‘cause they’re gonna drop at the same time!
ROB:
Yeah?
ROB (VO) ROB:
That one’s a little harder to understand than the quarters. And it’s not that it’s inherently harder, it’s the same understanding. It’s that it has a component of initial upward motion. And the other one has an initial component of downward motion. The reality is that they are the same vertically.
Rob, Students ROB:
I’ve aimed it at the bear. So let’s try it. So the marble and the bear, regardless of their mass, accelerate at the same value. So we’re gonna get into our big job today. To shoot this gun right here. And we’re gonna be shooting these yellow plastic balls. This is the velocity that the projectile leaves the gun at.
01:03:48 Rob (INTV) ROB:
The third thing was, was kind of like a demonstration, but also, you know, a little bit of a lab activity.
Rob, Students ROB:
The bullet is gonna leave the gun and it’s gonna fly across the face of the white boards. And what I need you to do, to calculate its position in time and to plot where it will be as it goes across the white board.
ROB (VO) ROB:
They have to take that two-dimensional data, decompose it into its single dimensional component, its vertical part and its horizontal part.
Students STUDENT:
It always starts from zero when it’s vertical, and there’s always that acceleration.
ROB (VO) ROB:
And then come up with a theoretical function. Says where it will be when.
Rob, Students ROB:
Don’t worry at all about what’s happening vertically.
ROB (VO) ROB:
And so the groups work. I have two groups doing the horizontal and two groups doing the vertical.
01:04:31 Student STUDENT:
Cosine of 40, and then you get 4.72 [INAUDIBLE] 4.73.
ROB (VO) ROB:
They graph that data on the boards. We actually can use the white board as a drawing surface.
Rob, Students ROB:
Okay, so we’ve got .1 second intervals from .2 all the way to .8 seconds and you can see circle dots are where the ball should be. We need an iPad video of this. Okay. You ready?
STUDENTS:
Oh!
ROB:
Success? Yeah!
ROB (VO) ROB:
Lo and behold, if they do it right, it’s like wow! Like that’s incredible! That’s incre-. I could predict where this object would go and it actually does it. That’s a huge achievement. Even though it’s a simple thing, because you did it from scratch.
Rob, Students ROB:
Give yourselves a hand, that was awesome!
ROB (VO) ROB:
I think that’s one of the first real hard connections for them, between the theoretical structure and the physical reality. That’s what scientists do every day. You know, it may be a grand project, it may be a small project, but it’s the same job. It’s just trying to figure it out. And ultimately come to some knowledge on the issue.
01:05:40 Rob, Students [music]
01:05:45 With special thanks to Rob LaPointe and the staff & students at Orange Lutheran High School
CREDITS
Wingspan Pictures Logo [music]
01:05:55 Fade to black