Monday, December 3, 2012

Conceptual: More Kinematics Word Problems Practice

After bellwork we had time to finish Kinematics Word Problems, which is due today.

Posted by L. Volkening
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2 comments:

  1. SerinaM45December 4, 2012 at 8:16 PM

    Ms., I tried to explain to my dad what you told us today about the gun and the bullet, but my dad is adamant that it is wrong. Can you please explain it to me here so that he can read it... Because it is seriously bugging me. -.-

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    1. L. VolkeningDecember 4, 2012 at 8:55 PM

      I'll try, but I'm not very good without pictures and a lot of gesturing :) If it helps, I finally got today's entry posted and it has the links to the videos we watched. http://dvhsphysics.blogspot.com/2012/12/conceptual-independence-of-x-and-y.html

      So...No matter what else it is doing, gravity makes an object fall. Gravity makes all objects fall with the same acceleration (assuming negligible air resistance, which we have to if we don't want to solve a differential equation. And we really, really don't want that). So both bullets will fall with the same acceleration and therefore their velocities will change at the same rate in the vertical direction.

      This means if a bullet is released from rest, after one second it will be moving 10 m/s down and have fallen 5 meters (using the second and first of our kinematics equations, respectively). If a bullet was already moving in the horizontal direction, after one second it will STILL be moving 10 m/s down and have fallen 5 meters; it just has ALSO moved quite a ways horizontally in that one second.

      So let's say we drop and fire two bullets from a height of 5 meters. After one second, the dropped bullet has fallen 5 meters and so hits the ground. The fired bullet has also dropped 5 meters and so hits the ground, albeit very far away. But they do hit the ground at the same time - after falling for one second.

      Sorry, that's the best I can do at bedtime. I'm afraid my brainpower diminishes as the day goes on. Like a battery.

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