Top 5 Student Mistakes in Projectile Problems – Part 1 of 2

This is part 1 of a 2-part video series about the most common mistakes students make when setting up and solving projectile motion problems. Don’t make these mistakes! Watch this video to learn how to do projectile problems the right way.

You can watch part 2 here: https://youtu.be/W40DilVai7Q

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Transcript:

This is Scott from http://ift.tt/1ETlQg4 and in this 2-part video series I will list the top 5 mistakes students make with projectile problems and show you how to avoid them.

Mistake #1 is forgetting that the acceleration is NOT zero at the projectile’s highest point! Here’s an example in which we plan to shoot a raging chicken onto the edge of a nearby rooftop with a nice parabolic trajectory like this.

If I were to film this misadventure with a video camera, I would get these three snapshots centered on the highest point. At the highest point, which I’ll call “now”, the velocity is purely horizontal; there is no vertical component. In the “before” snapshot, on the left, the chicken was going to the right and up with velocity v_before. In the after snapshot, on the right, the chicken is going to the right and down with velocity v_after.

Average acceleration is delta v over delta t, and in projectile motion the average acceleration is the same as the constant acceleration. To compare accelerations we need to consider the changes in velocity, delta v, and the changes in time, delta t. Delta v between before and now is v_now minus v_before, which I can sketch like this. Delta v prime – between now and after – is v_after minus v_now, which I can sketch like this. Notice that delta v and delta v prime are the same; each change in velocity has the same magnitude and the same direction: Down. Now what about delta t? The camera records video at a constant frame rate so the amount of time between any two successive snapshots is constant (for example, at 30 frames per second, there would be 1/30th of a second between two successive snapshots). Therefore delta t between before and now is the same as delta t between now and after. Since acceleration is delta v over delta t, and these terms are the same in both intervals, the accelerations are the same.

Some of you may be asking, but what about the exact _instant_ that the projectile is at the highest point? What if acceleration _was_ zero at the highest point? Well, in that case our chicken would miss the building. Zero acceleration would mean constant velocity, and the poor little raging chicken would just keep on going without ever falling back down to the ground. If this happened during a soccer or a baseball game, it would make history – but it would also be extremely boring to watch the ball fly away every time it got high in the air!

Remember this: in projectile motion, the projectile is _always_ accelerating because of gravity, and that brings us to the second mistake.

#2 is forgetting that the horizontal velocity is constant. In projectile motion, we only consider the force of gravity – with very rare exceptions – and gravity is, well, down. Vertically down. If you forget, it’s easy to check: Just take a small and preferably soft object, hold it over the floor, and let go – and observe which direction it falls. Silly? Yes. But it’s important! Too many students forget that there is no horizontal acceleration and that the horizontal component of a projectile’s velocity is therefore constant.

One quick note: This assumes a flat surface. If you’re considering planetary effects, like when you’re throwing a baseball to your friend on the other side of the world, (Superman?) remember that gravity points to the center of mass.

That’s only 2 mistakes, but I’m out of time for this week. The other 3 will be posted next week; or, if you’re watching after September 2015, you can click on the link in the description to go straight to part 2 of the video.

I’m Scott Redmond and I help students pass physics. If there’s a topic or homework problem that you would like me to explain in a video like this one, please contact me at http://ift.tt/1ETlQg4!

via YouTube http://youtu.be/-WgcN1Il6so