## Thursday, November 4, 2010

### WCYDWT: Projectiles

The camera angle messes with the perspective a bit, but I still find these images interesting. Does it mess with the perspective so much that it ruins the problem or does it just lead to another discussion on how things aren't always as they seem. I'm thinking that I'll show the video, but make digital and hard copies of the photos available to students.

Question #1

Projectile Question (YG) from David Cox on Vimeo.

Question #2

Projectile Question (PG) from David Cox on Vimeo.

Projectile Answer (YG) from David Cox on Vimeo.

Projectile Answer (PG) from David Cox on Vimeo.

Next up:
Create something that helps kids see vertical and horizontal motion independence.

Anonymous said...

I feel cheated.

So I'm not sure if students would pick up on if the ball really slows down that much from hitting the wall. It's tough that the wall is struck so late, but I think if you provide another frame it becomes an obvious part of the question.

I leave this with a takeaway that balls slow down a LOT when they ricochet off walls. Is that your goal here?

I'm thinking that you can guide the question more towards "will it hit the other building") by providing a few more frames of the green ball, enough that the green ball would clearly win. Then before you close up the initial discussion, you can point out that it was really hard to edit the video, so the green ball is just there for reference. That way you hit your goal and the ball slowing down is just a side effect.

Even if you don't do that, I might.

David Cox said...

I keep looking at this problem from an Algebra 1 perspective. I want my students to notice that the ball accelerates as it drops. 8th graders can pick up on how each point (strobe) has a rate of change from one to the next. We can talk about how the rate of change has a rate of change. From there they should be able to predict which may hit the ground first. When the answer is different, then the discussion can be about how the perspective of the camera may distort and/or the ball hitting the wall slows the fall down.

We may not be able to calculate a nice, neat answer, but it'll make for good conversation.

Unknown said...

My physics students are currently conducting their own investigations into projectile motion, and I'd love to be able to use that strobe effect to help them analyze their videos. Would you be willing to explain how you did it? Our attempts to reproduce it have not been fruitful.

For what it's worth, we have access to Adobe After Effects, and our footage was shot with a Flip Ultra HD camera.

untilnextstop said...

Hi David, I was playing with Logger Pro this morning. It's a proprietary software for physics, that our school has a site-wide license for. I imported your Answer 1 video and was able to retrieve using Logger Pro a list of points (times and heights, or times and horizontal position), and when plotted together (Logger Pro can do that easily for you) you can generate an excellent discussion of why the height is quadratic and the horizontal positions are changing linearly, and in fact, why the horizontal position of the yellow ball over time looks like an upside down absolute value graph. I recommend buying a single copy of Logger Pro and playing around with it! Maybe your school would be willing to invest in the license in the future...

untilnextstop said...

FYI, the screenshot from Logger Pro looked like this after importing the data: https://dl.dropboxusercontent.com/u/39075205/screenshot.png