Day 72: Bullet Drop vs. Shot

MythBusters Bullet Fired Dropped screenshot

 

Today (ok, you got me, it was yesterday…a bit behind here) We white boarded a horizontal launch projectile motion problem for the first time. The students rocked it, I was surprised at how easily they seemed to be ok with separating the X and Y directions despite that I didn’t really talk about it. Another score with Modeling Instruction, plus the fact that MI does projectile motion after forces. Anyway, at some point in each class a discussion came up about using the initial velocity in the y direction; with the launch velocity of 10 m/s horizontally, should the initial velocity in the y position equation be zero or 10? They were able to resolve to zero well. So after solving for time of fall, I raised my hand. “So you’re telling me, if I drop a bullet and shoot a bullet horizontally at the same time, they’ll land at the same time on horizontal ground?” I tried arguing against it, but they wouldn’t have it. Yep, they said, they land at the same time. “I sure wish the Mythbusters had explored this…oh, wait.” So we watched this 3 minute clip of the end result of their demonstration. It bothers me a little that the difference is 36 ms, that seems large to me considering all the pains they took in making sure the bullets drop at the same time…but it is convincing enough to students that they should land at the same time. We also have a quick conversation about sources of uncertainty that cause that difference; they always think it’s that they aren’t dropped at the same time despite the high speed video showing that the drop time difference is 1/3 of a ms. I suspect a combination of a not perfectly horizontal shot and a not perfectly level ground, but that’s just my suspicions.

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