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About this Lesson
- Type: Video Tutorial
- Length: 4:53
- Media: Video/mp4
- Use: Watch Online & Download
- Access Period: Unrestricted
- Download: MP4 (iPod compatible)
- Size: 52 MB
- Posted: 07/01/2009
This lesson is part of the following series:
This lesson was selected from a broader, comprehensive course, Physics I. This course and others are available from Thinkwell, Inc. The full course can be found at http://www.thinkwell.com/student/product/physics. The full course covers kinematics, dynamics, energy, momentum, the physics of extended objects, gravity, fluids, relativity, oscillatory motion, waves, and more. The course features two renowned professors: Steven Pollock, an associate professor of Physics at he University of Colorado at Boulder and Ephraim Fischbach, a professor of physics at Purdue University.
Steven Pollock earned a Bachelor of Science in physics from the Massachusetts Institute of Technology and a Ph.D. from Stanford University. Prof. Pollock wears two research hats: he studies theoretical nuclear physics, and does physics education research. Currently, his research activities focus on questions of replication and sustainability of reformed teaching techniques in (very) large introductory courses. He received an Alfred P. Sloan Research Fellowship in 1994 and a Boulder Faculty Assembly (CU campus-wide) Teaching Excellence Award in 1998. He is the author of two Teaching Company video courses: “Particle Physics for Non-Physicists: a Tour of the Microcosmos” and “The Great Ideas of Classical Physics”. Prof. Pollock regularly gives public presentations in which he brings physics alive at conferences, seminars, colloquia, and for community audiences.
Ephraim Fischbach earned a B.A. in physics from Columbia University and a Ph.D. from the University of Pennsylvania. In Thinkwell Physics I, he delivers the "Physics in Action" video lectures and demonstrates numerous laboratory techniques and real-world applications. As part of his mission to encourage an interest in physics wherever he goes, Prof. Fischbach coordinates Physics on the Road, an Outreach/Funfest program. He is the author or coauthor of more than 180 publications including a recent book, “The Search for Non-Newtonian Gravity”, and was made a Fellow of the American Physical Society in 2001. He also serves as a referee for a number of journals including “Physical Review” and “Physical Review Letters”.
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We're here in a jungle located in the Physics Department of Purdue University. We're witnessing a hunter trying to hunt down his dinner by trying to shoot a monkey out of a tree. This monkey, whose name is Newton, is very clever. And is going to try to avoid being hit by trying to be clever, but maybe not clever enough. Let's watch the hunter and watch the monkey, and with a diagram show you what each of them is trying to do.
Here is our vicious hunter aiming his rifle toward the monkey located at coordinates (A, B). That's where the tree is. Now suppose there was no gravity at all. All the hunter has to do is aim his rifle to the monkey. And if he points right on, he's going to hit the monkey. That's the end of it. But there is gravity. We are living in the Earth's gravitational field, and that's where Newton is going to try to be smart. At the instant that he sees the flash from the rifle, Newton is going to say, "Let me just fall out of the tree." Hopefully then the projectile will fly over his head, and he'll be safe.
So he tries to drop out of the tree, and let's see what happens. In the time t that it takes the projectile to move from the gun to the line along which the monkey is falling, the monkey will fall at distance , where g is the acceleration of gravity. And he will end up, let's say, over here. That's what Newton thinks is going to help him out. However, what he's not taking into account is that in the same time interval t, the projectile will also fall a distance . Instead of ending up over here, he'll end up at exactly the same vertical position. The end result, they hit. Projectile hits the monkey.
Notice that in this discussion I've made no reference to the velocity of the projectile. However fast or however slow it is, the same result will ensue. Now notice what happens. If the projectile were faster, it would take less time for it to reach the fall line over here. The monkey would fall a shorter distance, but so would the projectile. In any case, they would meet. Now let's see what this looks like in a real live demonstration.
Here's our friendly monkey, Newton, banana and all. And here's our vicious hunter, Randy. And what we've done is arranged to have a rifle pointed toward the spot where Newton's going to be in a few seconds. Let's bring him over here. Good luck Newton.
Now the way we we've set this up is as follows. This is a rifle arranged to fire a projectile like this, which is just a big ball bearing. And it's going to be aimed directly toward Newton, who will be up in the tree. Well Newton is climbing up in the tree momentarily. He's getting up there. And it's arranged in such a way that at the instant the projectile leaves the rifle, a switch will trip and will release the electromagnet which is keeping Newton in the tree. While Newton is climbing up in the tree, the gun is being loaded. Now we're going to make sure to start with the rifle aimed toward Newton. So we have a laser on this rifle. And as soon as Newton is in place, Newton is resisting, now we're going to put a spot. You can see the red spot that tells us that the rifle is aimed directly toward Newton. Now again, we're simulating the flash of the rifle with this switch. At the instant the projectile leaves this rifle, Newton is going to fall. We want to see where the projectile and Newton make contact.
I'll go ready, aim, fire. You look very carefully at what's going on, and see whether, in fact, the projectile hits Newton. Ready, aim, fire. Bingo, yes, it really did hit. Now what's the bottom line of all of this? Newton tried to be smart and tried to avoid being hit by hoping that if he fell out of the tree, the projectile would go over his head. In fact, what he forgot to take into account was that, during the same time interval when he falls, the bullet also falls. So if the projectile were aimed toward him, the projectile is going to hit him no matter what.
What generally we see that projectile motion and the Earth's gravitational field always has this property. Whatever you shoot in the Earth's gravitational field, any object falls at the same acceleration g. And so at a given time interval t will fall a distance . That applies to the monkey. It applies to projectiles. It applies to anything we drop in the Earth's gravitational field.
Investigating Motion in Two Dimensions
Physics in Action: The Hunter and the Monkey Page [1 of 1]
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