You Might Also Like
Physics: Physical Pendulums
Physics: Gravity on Earth
Physics: Physical Quantities & Measurement Units
Physics: Collisions in Two Dimensions
Physics: Frictional Force Between Two Surfaces
College Algebra: Solving for x in Log Equations
College Algebra: Finding Log Function Values
College Algebra: Exponential to Log Functions
College Algebra: Using Exponent Properties
College Algebra: Finding the Inverse of a Function
College Algebra: Graphing Polynomial Functions
College Algebra: Polynomial Zeros & Multiplicities
College Algebra: Piecewise-Defined Functions
College Algebra: Decoding the Circle Formula
College Algebra: Rationalizing Denominators
About this Lesson
- Type: Video Tutorial
- Length: 11:51
- Media: Video/mp4
- Use: Watch Online & Download
- Access Period: Unrestricted
- Download: MP4 (iPod compatible)
- Size: 126 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”.
About this Author
- 2174 lessons
Founded in 1997, Thinkwell has succeeded in creating "next-generation" textbooks that help students learn and teachers teach. Capitalizing on the power of new technology, Thinkwell products prepare students more effectively for their coursework than any printed textbook can. Thinkwell has assembled a group of talented industry professionals who have shaped the company into the leading provider of technology-based textbooks. For more information about Thinkwell, please visit www.thinkwell.com or visit Thinkwell's Video Lesson Store at http://thinkwell.mindbites.com/.
Thinkwell lessons feature a star-studded cast of outstanding university professors: Edward Burger (Pre-Algebra through...More..
This lesson has not been reviewed.
Please purchase the lesson to review.
This lesson has not been reviewed.
Please purchase the lesson to review.
I'd like you to visualize two different situations. Situation number one: Voyager spacecraft - it's a spacecraft we launched back in the `70's, and by now it's past the outer planets and it's leaving the solar system. It's cruising along at really high speeds. It's going many tens of thousands of miles per hour, heading away from our solar system. That's one object. Second object to visualize. There's a golf ball sitting on the ground and Tiger walks up and whacks the golf ball and off it goes, launches into a beautiful parabolic arc. Very different motions of these two different things, the spacecraft and the golf ball. What we can do with kinematics is to describe the motions. I just look at them, I say, "position as a function of time." I can draw a graph, and once I know that I can calculate velocity as a function of time or acceleration as a function of time.
The description of motion of both of those objects is relatively straightforward. But the much deeper question which we want to address is why. Why does the spacecraft travel the way it does, at high speeds, in a straight line. Its engines are long dead and it's cruising along at these incredible speeds. Why does it do that? The golf ball was sitting still. It's perfectly happy sitting there on the ground. All of a sudden it's launched into a parabola. Can we understand why things move they way they do rather than just describing how they move? That's a big goal of physics, and the word for the study of why things move the way they do is called "dynamics," as compared to kinematics, which is merely descriptive.
The central idea of dynamics was really articulated by Isaac Newton. Galileo started the process in a very clear way, and Isaac Newton really, finally, for the first time in history, I think, understood the prime ideas of dynamics. So we're going to be talking about Newton's ideas and Newton's first idea, which is now called Newton's first law of dynamics, is the following. Very simple, very intuitive. At least part of it is very intuitive.
An object a rest remains at rest. That's the natural state of affairs of things. And an object in motion continues in motion. So let me try to write that down in sort of a concise way. Newton's first law is an isolated body - isolated means lets think about a body that's not being pushed or pulled, and it will maintain uniform motion, and the word "uniform" is just a word for steady motion. When I talk about an isolated body, I have to explain it in terms of force. Force is a central ideas to Isaac Newton, and a force you can think of as nothing more than a push or a pull. When you apply a force to an object you are pushing it or pulling it, and we're going to quantify forces later. But for now, an isolated body is one which isn't feeling forces. Let me write down Newton's first law in a slightly more long-winded way, just a little bit more careful statement of Newton's first law.
In the absence of a net force - that means that you could be pushing on one side, but if you are also pushing on the other side, equal and opposite, those two forces would cancel out, and there would be no net force. So what happens when there's no net force on an object? A body at rest stays at rest. A moving body continues moving in a straight line with constant speed. That's what uniform motion means. The first one I think is very reasonable. It's called "the law of inertia," and everybody, I think, can accept that fact.
The second part of Newton's first law is rather counterintuitive. Let me convince you that you may not believe this at first. You're driving your car down a straight highway. You're in uniform motion and your foot's on the gas. You are clearly applying a force to the car. What happens if you stop applying a force? In the absence of a force, well, what you think, what's correct, is your car slows down and comes to a halt. That's not in disagreement with Newton's first law, because you're not thinking about the net force. What you're thinking about is one of the forces, the force, say, of the engine pushing the wheels, the subtlety of exactly what's applying that force is something we'll get to. You can just think of the force as coming from your foot on the accelerator pedal. When you ease your foot off the pedal there is still a force on the car. It's friction. All cars, everything in the ordinary world always experiences friction, and it's the frictional force that just another force of nature which is causing the car to slow down.
For many, many years, people didn't really think about friction as a distinct and separate force. It's so engrained, kind of hidden. It's always present. If you don't really think explicitly about friction, it does seem like the natural state of objects is at rest, and moving objects will come to a halt. But that's because of friction. If you could just eliminate friction and ask, what is the deep, underlying motion of objects, you would discover experimentally that Newton's First Law is absolutely correct. Galileo was the one who really first did these experiments and saw that when you drop a marble down a track and then watch it moving with very low friction it will continue in motion.
The law of inertia is very intuitive. It's always fun to do some quick demos. Here's a simple demo of the law of inertia. Here's a jar and a little object and I'm going to lay the object on top of a card. The object is at rest, and I'm arguing that in the absence of force an object at rest will stay at rest. So what happens if I flick the card out of the way? Well, it's not going to float there because there's a force of gravity pulling it down. But on the other hand, if I flick the card fast, as long as there's not much friction, and there's not, there's not going to be any important force in this direction. No force according to Newton's means, there will be no change in motion in this direction. So it started off at rest horizontally. It will still be at rest. It should plunk down into this jar. Sure enough.
Let's do another demo. These are just so much fun, I can't resist. Nice tablecloth sitting here, little dishes, some flowers on the table. Here's a classic trick. Try this at home at your own risk. Objects at rest remain at rest as long as there's no net force on them. There's a tiny force here of friction between the tablecloth and the object, but fundamentally demonstration of inertia. Inertia is the word that describes the fact that objects like to continue doing what they were doing. When a person has inertia, it's hard to change them. That's what Newton's First Law is telling us, that if you don't push on something, then it's not going to change its motion.
This is correct and sometimes people just can't believe it. You think, "No, objects in motion want to come to a halt." Here's another example where people sometimes have a hard time believing it. Just got a little metal ring here and a marble. I'm going to put the marble along the edge and roll it, so it's going to roll all by itself along this circular hoop. And the question is what will it do when it reaches the edge here? A lot of people have a sense -you know, Newton's First Law, when we wrote it in our inarticulate way, and we said, "it maintains uniform motion," so some people might argue, "Well, gee, uniform motion, it's running around in a circle. Won't it keep running around in a circle?" So it will kind of loop around and maybe it'll just keep going in a circle and maybe it'll kind of arc away. That's an intuition that a lot of people have, but it's just not right. Newton's Law says it will maintain straight-line motion with constant speed. So that's what we should see. Try it again - it moves fast. As soon as it leaves it travels in a straight line with constant speed. Everything does that as long as there's no net force, so as long as there's no friction on the table to worry about.
I need to make one little caveat here. What I've been saying is true in inertial reference frames. An inertial reference frame is a reference frame that's sitting still or moving with a uniform velocity. If you're in a rocket ship that's accelerating upwards, or even if you're in an airplane that's experiencing lots of turbulence, jerking up and down, Newton's First Law will appear not to hold. When you're in an airplane and you're sitting eating your lunch and there's something on the table, all of a sudden it jumps and flies out of the table. That's a clear violation of Newton's First Law - an object at rest spontaneously comes into motion. That's because you're not in an inertial reference frame if your reference frame is accelerating and jerking around.
The idea that objects want to come to a rest is an old one. It's deeply engrained in a lot of people's intuition. It was first articulated clearly thousands of years ago by Aristotle, a Greek philosopher, and he said objects want to head towards rest, and he was wrong. Galileo and Newton really made perhaps the most significant discovery of modern physics that really lead us to our ability to describe and understand all motions, anything. It can be any system, simple, complicated, moving fast, moving slow. This idea, the prime idea is that if there is a force, then the motion will change, and Newton's First Law says, it's a corollary of that, it's a starting point. if there's no force, the motion won't change.
You know, if you insist that Aristotle's right and objects tend to want to slow down, I call that - you're a closet Aristotelian. I know I even I have a little bit of that in me, because we've got lots of world experience where friction is just so deeply engrained that we can't forget about it. If you want to become a master at some sport, if you want to drive a sports car, if you want to understand, or for that matter, design a rocket that's going to travel through interplanetary space, you must understand Newton's Law.
So, Voyager, the example that we started with - it's an object out in deep space. It's experiencing no forces of any significance whatsoever. Why does it continue moving at huge speeds - 30,000 miles - something like that, 30,000 miles an hour. Why doesn't it slow down? Newton's First Law - an object in motion remains in motion.
How about the golf ball? Well, that was an object in motion remaining in motion until it got whacked by the golf club. So what we're going to talk about next is what happens when you do apply a force, and what we're going to see is it changes the motion.
Newton's Three Laws
Newton's First Law Page [1 of 3]
Get it Now and Start Learning
Embed this video on your site
Copy and paste the following snippet: