LET'S MOVE IT: NEWTON'S LAWS OF MOTION

Transcription

1 LET'S MOVE IT: NEWTON'S LAWS OF MOTION 1 videocassette minutes Copyright MCMLXXXVH Rainbow Educational Media 4540 Preslyn Drive Raleigh, NC Distributed by: United Learning 1560 Sherman Ave., Suite 100 Evanston, IL

2 CREDITS Author: Michael Hardy William McNeil Producer: Michael Hardy William McNeil Editor/Computer Graphics: Roger Meyer Camera: Hal Landon Tom Buchbinder Post Production: McNeil, Murphy and Rector Special Thanks to: Palisades Interstate Park Commission NASA U.S. Department of Defense Vermont Travel Division, Killington Monroe Woodbury High School Hockey Team Produced for Rainbow Educational Video by Michael Hardy Productions

3 TABLE OF CONTENTS Introduction...! Program Description...2 Discussion Questions...5 Suggested Activities...? Bibliography...8 Script...9

4 INTRODUCTION "It turns out that the number of laws of nature needed to describe the world is very small. There are three laws of mechanics, four of electricity and magnetism, three for thermodynamics, one for relativity, and a few more (depending on how you count) for quantum mechanics. This means that all of the billions of natural phenomena we encounter every day are comprehensible using only a few natural laws." James Trefil Science is a dynamic discipline, a nonstatic process. Theory and hypothesis are subject to revision and even abolition as new observations made possible by more technically precise methods reveal to the scientist heretofore unknown phenomena that alter, subtly or radically, existing paradigms. But, as Trefil has noted, the number of laws governing the universe is exceedingly small. So while the upheaval of theory and the overturning of paradigm occur historically, the laws governing any newly observed phenomena remain few. That Einstein changed, in the most revolutionary sense, the way we perceive the universe, is undeniable. The mysteries and paradoxes of the subatomic world of quantum mechanics has challenged our very concept of reality itself, prompting men and women of science to begin to speak in a language strikingly akin to the utterances of monks and mystics. Nonetheless, Sir Isaac Newton and the natural laws he discovered and formulated remain indispensable. For example, scientists have recently uncovered evidence that in nature there are more forces at work than the four previously recognized: electromagnetic force, strong and weak nuclear forces, and gravity. It is now believed that a fifth and sixth force exist, and that these fifth and sixth forces may, even if only slightly, counteract the force of gravity under certain conditions. It is therefore possible that our notion of gravity as postulated by Newton may be altered. Does this then render Newton's discoveries obsolete? Not at all, for not only does this new discovery in no way change the fundamental laws of nature, it was itself made possible by computations based on Newton's second law: F=MA. 1 MCMLXXXVIII Rainbow Educational Video, Inc.

5 This program is a presentation of Sir Isaac Newton's three laws of motion. Students will be given each of the three laws accompanied by animation and live action footage of motion as it is experienced in everyday life, from the gliding movement of birds in flight to the explosive movement of a hockey player on ice. And throughout, a clear and precise narration unites the action with the scientific facts that made it possible. PROGRAM DESCRIPTION A live action shot of a hockey player accelerating in a fastpaced hockey match leads into our topic, Newton's three laws of motion. The team that best controls motion, the motion of the skates, the motion of the puck, is the team that wins. It is by taking a close look at the game of hockey, as well as some other activities, that we will begin to arrive at a knowledge of motion itself. To begin, we note that all motion, whether subtle or explosive, obeys a set of three physical laws, laws formulated three hundred years ago by Sir Isaac Newton. According to Newton's first law, an object at rest remains at rest unless a force acts upon it, and an object in motion stays in motion unless a force acts upon it, as is illustrated by a hockey player's stopping a speeding puck with his stick. Newton's second law of motion states that the acceleration of an object depends on two things; (1) trie mass of the object and (2) the amount of force applied to the object. Here, animation illustrates this law at work. Next, animation of a recoiling cannon serves as an example of Newton's third law of motion: for every action, there is an equal and opposite reaction. Having thus previewed the three laws of motion, we now zero in on the first law. Nothing will move unless a force starts it in motion. A hockey puck at rest on the ice will stay there forever unless a force, such as a hockey player's striking it with a stick, makes it move. And once the puck is moving, it will keep moving in a straight line at the same rate of speed until a force makes it change. This phenomenon is called inertia. Whenever we observe an object slow down, speed up, or start or stop, we know that there is a

6 force acting upon it. Sometimes the force operating on objects are visible to the naked eye, but just as often they are not. This brings us to the subject of friction. A hockey puck in motion would race into infinity were it not for the friction caused by the rubbing contact of the puck with the ice, which slows the puck down. So, with friction, we have an example of a force's affecting the motion of an object. When we ride a bike, friction slows it down. That's why we have to peddle to keep it moving. And when we stop the bike, it's the friction of the brakes that stops the wheel. Friction is always present on Earth. Friction with the water slows down a boat; friction with the air slows down birds, and also makes it possible for them to fly. This program next notes another force acting on all earthly motion. It is gravity, which pulls everything down to the center of the Earth. From a boy's sliding down a pole, to the downward crash that occurs when a building is demolished, gravity is at work. So we've now seen that a force is required to change the motion of an object. How much will the motion change? According to Newton's second law of motion, it depends on the mass of the object in question and on the amount of force applied to it. Here the program specifically defines mass as the amount of matter making up an object. A heavier object has more mass than a lighter object. For example, a bowling ball has more mass than a softball. Muhammad Ali has more mass than Pee Wee Herman. Next, we define acceleration as a change in motion. Acceleration doesn't mean speed, it means a change in speed. When a motionless hockey puck gets struck by a hockey stick, it's acceleration. Acceleration is also at work when an object moving at a steady or constant rate speeds up or slows down. It is change in speed. The program now illustrates how mass, force and acceleration are interrelated. Anytime equal force is applied to two objects of different mass, the object with less mass will accelerate more. If a large force and a small force are applied to two objects of the same mass, the larger force makes the object accelerate more. If the same amount of force is applied to two objects of the same mass, they will accelerate at the

7 same rate. Here, animation and action footage provide illustration. We are now taken into the world of Newton's third law of motion: forces come in pairs, every action has an equal and opposite reaction. This law is immediately demonstrated by live action footage of a man hurling a shot-put out of a kayak. As he propels the shot-put forward, the kayak moves backward. This is the third law. We see it in operation again when a hockey stick slaps a puck. The stick sets the puck in forward motion and, as it does so, the puck exerts an equal force backward against the stick. The hockey player can feel it in his hands. The program offers further illustrations of the third law. As a rocket lifts off, the hot gasses blasting downward propel the rocket upward. Action, reaction. We also see a cat pushing a basket backwards, and the basket pushing the cat forward. It was while observing his own cat that Newton formulated the third law. The program now blasts us off into outer space, because space is an excellent place to observe Newton's laws. Why? Because in space, as a shot of an astronaut floating freely in orbit shows us, gravity doesn't affect things as it does on Earth. With graphic illustrations of objects and astronauts in space, we see how the conquest of space would not have been possible without prior mastery of Newton's laws. Motion, as the program demonstrates, is experienced in a multitude of ways: skating on ice, riding a bike, floating through space, watching the moving hands of a clock, running, or bouncing a ball. Motion is everywhere. This propels us to our conclusion: a review of the three laws, highlighted by animation and graphic scenes from the far reaches of space. Whether we remain on terra firma or reach for the stars, we experience motion, the primary characteristic of animate being. And where we have motion, we have Newton's three laws at work.

8 DISCUSSION QUESTIONS 1. Why would it be easier to stop a bicycle on concrete than on ice? [Because concrete is a more uneven surface than ice and therefore creates more friction with the bicycle. The velocity of the bicycle decreases more quickly on concrete than on ice.] 2. Why is it necessary to bank the turns on roadways traveled at high rates of speed? [An object in motion keeps moving in a straight line unless a force makes it change. When a highway turns, a force is required to make the car change directions. On most roads and at normal speeds, the frictional force of the tires on the road surface is enough to keep an automobile from continuing in a straight line and skidding off the road. But at higher rates of speed, a greater frictional force is required. Banking the turns provides this force.] 3. How much force is required to change the motion of an object? [According to Newton's second law, it depends on the mass of the object and the rate of acceleration.] 4. What is the difference between speed and acceleration? [Speed is the rate at which a moving object travels; acceleration is any change in speed.] 5. A Cadillac and a Honda both accelerate from zero to 55 m.p.h. Why will the Cadillac have to consume more fuel than the Honda if it is to keep pace with the smaller car? [The Cadillac has more mass than the Honda. Therefore if it is to accelerate at the same rate as the Honda it has to have more force applied to it. So for this reason it must burn more fuel than the Honda. This is in accordance with Newton's second law.]

9 6. Why is it impossible to construct a perpetual motion machine? [Newton's first law tells us that an object in motion will stay in motion unless a force acts upon it. So, many hopeful inventors have tried to build perpetual motion machines, devices that can stay in motion at the same rate of speed forever. None have succeeded and none ever will, for it is impossible to create a situation in which no forces are active. Friction and gravity affect all moving objects. Thus perpetual motion is an unrealizable concept.] 7. Why does a car's engine have to run all the time just to keep the car moving at the same speed? [According to Newton's laws, the car, once it's in motion, will keep moving in a straight line at the same speed forever, unless a force makes it change. If no other forces were acting on the car, it wouldn't need an engine to maintain a constant speed. But, of course, many forces do act on the car all the time. Friction, with the air, with the road, between moving parts within the car, tends to slow it down. Gravity pulls it toward the center of the Earth, deflecting it from its straight path. Sometimes this makes the car slow down; sometimes it makes it speed up. In order to keep the car moving forward at the same speed, the engine (or the brake) must exert a force exactly equal to the sum of all these other forces.] 8. Why is space a particularly good place to observe Newton's laws? [Newton's third law is sometimes hard to understand on Earth. When we throw a baseball, we aren't aware of the "equal and opposite" reaction. That's because, without even thinking about it, we use friction and gravity to counteract the forces that would push us backwards. In an orbiting space station, however, the affect of gravity virtually disappears. And, if you're "weightless" you will be affected only slightly by friction. If you throw a baseball in a space station, your body will move in the opposite direction and won't stop. Newton's other laws are also easy to observe in this environment. For instance, some of the

10 NASA footage in this videotape shows an astronaut letting go of a ball in mid-air. Since it is motionless when he releases it, it stays motionless: Newton's first law. (Note that an orbiting satellite is not in a "gravity-free" environment but rather in a state of continuous "free-fall": its forward momentum perpendicular to the Earth exactly balances the pull of gravity toward the Earth. Astronauts inside the satellite experience weightlessness, or a "lack of gravity," because they are "falling" at exactly the same rate as the satellite. It's just as though they were in an elevator with a broken cable but which never hits bottom.)] 9. There are many examples in the videotape of ways that friction slows things down. How does friction enable things to move? [Without friction we couldn't walk because we couldn't push our feet backward against the ground. Friction of the paddle in the water makes the boat move. Friction of the bird's wings against the air enables the bird to fly. Friction of a bicycle against the pavement enables the bike to move forward. What are some more examples?] SUGGESTED ACTIVITIES 1. Have students simultaneously drop a tennis ball and a baseball onto a hard, even floor. Ask them to explain why the tennis ball bounces higher than the baseball. Can they observe the second law at work here? 2. Cover half of a long, flat table with a sheet of aluminum foil, and the other half with a sheet of sandpaper. The only other equipment needed are two round marbles. Have the students first roll one marble down the sheet of sandpaper and then roll the other marble down the sheet of aluminum foil. This is an excellent way of demonstrating friction. 3. Obtain one 5 lb. dumbbell and one 10 lb. dumbbell. Have the students hold the 5 lb. dumbbell in their left hands

11 and the 10 IB. dumbbell in their right hands. On the count of three, ask them to push both arms straight out. Then ask them why they had to apply more force with the right hand carrying the 10 IB. weight than they did with the left hand carrying the 5 IB. weight. This experiment enables students to feel Newton's second law.] BIBLIOGRAPHY Allman, William F. and Schner, Eric W., Newton at the Bat: The Science in Sports. Mew York: Scribner, Asimov, Isaac, Understanding Physics: Motion, Sound, and Heat, hew York: New American Library, Besancon, Robert Martin, The Encyclopedia of Physics. New York: Reinhold, Bixby, William, The Universe of Galileo and Newton. New York: Harper and Row, Gardner, Robert, Ideas for Science Projects. New York: Franklin Watts, Greenleaf, Peter, Experiments in Space Science. New York: Arco Press, Gerholm, Tor Ragner, Physics and Man: An Invitation to Modern Physics. Totowa, New Jersey: Bedminster Press, Pals, Abraham, Inward Bound: Of Matter and Forces in the Physical World. Oxford: Oxford University Press,

12 SCRIPT Narrator Hockey is a game of speed and motion. The team that wins is the one with players who control motion the best: The motion of their skates; the motion of the puck; and the motion of their bodies. We're going to take a close look at this game and other activities to learn some things about motion itself. TITLE: Let's Move It: Newton's Laws of Motion Narrator All motion obeys a set of physical laws, whether its fast and bold or small and delicate. These laws were observed three hundred years ago by the great scientist, Sir Isaac Newton. They can be described in three simple statements. Newton's First Law says that an object at rest, stays at rest unless a force acts on it. And an object in motion, stays in motion unless a force acts on it. Newton's Second Law of motion tells us that an object's acceleration depends on the mass of the object and the amount of force applied to it.

13 Newton's Third Law tells us that forces come in pairs. Every action has an equal and opposite reaction. Now let's take a closer look at the first law. It takes a force to make something start moving. If an object, such as a puck, is not moving, it will stay at rest until a force makes it move. And once it's moving it will keep moving in a straight line at the same speed until a force makes it change. This characteristic, or an object in motion stays in motion, is called inertia. Because of inertia, anytime you see something slow down, speed up, start, or stop, you know there must be a force acting on it. Often, you can see the force - - a push or a pull. Some forces that operate on most objects are not as easy to see. One example is friction..which results from the rubbing together of two objects. Without friction, this player..or this puck, would go on forever. This puck is slowed down by rubbing against other things..such as the ice..or even the air. This is friction,one example of a force affecting an object. 10

14 Friction makes your bike slow down. That's why you have to pedal to keep it moving. And when you want to stop, the friction of your brakes, slows the wheel. Everything that moves on earth is affected by friction. Friction with the water makes boats slow down. Friction with the air makes birds slow down. It also enables them to fly. And friction between moving parts would make machines slow down if more force didn't keep them moving. This is friction... And so is this... There's another force that acts on all motion on earth. It's called gravity. Gravity pulls everything toward the center of the earth. It's one of the forces that acts on all objects. Here is an example of gravity. This also is an example (skiing). And here's another (child bouncing ball). We've seen that it takes a force to change the motion of an object. But how much will the motion change? That depends on a couple of things: According to the second law, it depends on the mass of the object and the amount of force. Scientists have some special words to describe this law: Instead of 11

15 talking about an object's size or weight, they use the word "mass." Mass is the amount of matter in an object. A bowling ball and a softball have different masses. And mass is related to the weight of the object. If one thing weighs more than another, we say it has more mass. This is true, no matter how large the objects are. Acceleration is the word for a change of motion. Acceleration doesn't mean speed..it means a change in speed. When something that is standing still starts moving, that's acceleration. It's also when something that's moving at a steady speed goes faster. Or when something that's moving slows down. Mass, force and acceleration are all related. When the same force is applied to two objects of different mass, the one with less mass will accelerate faster. If a large force and a small force are applied to objects of the same mass, the larger force makes the object accelerate faster. 12

16 If the same amount of force is applied to two objects of the same mass, they will have the same acceleration. You can experience this easily. A small car can be moved with a small force. Something big enough to ride on requires more force. And it takes a lot of force to move a real car. Newton's third law tells us that forces come in pairs. Every action has an equal and opposite reaction. Here's one way to get a feel for that: If you were sitting in something that could move very freely such as this kayak and you threw something very heavy..like a shot-put..you would go backwards. And that shows Newton's third law. Here's that law again - - can you see the reaction? The stick sends the puck flying. Is there an equal reaction back against the stick? Yes, when the stick hits the puck it exerts a force in one direction. The puck exerts an equal force back against the stick. You know the force is there because you can feel it in your hand. Newton discovered this law by watching his cat. Can you see the equal and opposite reactions? The cat 13

17 pushes the ball backwards and the ball pushes the cat forward. A rocket is a good example of the third law. As hot gases are pushed out the back, the rocket is propelled upward: action and reaction. Space is a good place to observe Newton's laws because as you can sec..gravity doesn't affect things as it does on earth. Here's a good example of the first law. Things at rest, stay at rest. And things in motion, stay in motion until the force makes them change. For humans to conquer space, they had to master Newton's laws. They had to know just how much force was required to move objects of different mass. And they had to understand how an equal and opposite reaction could send an astronaut floating away from his spacecraft. Motion. Think of some of the ways you might experience it: While skating; on a bicycle; in space; watching a clock; skiing; running or bouncing a ball. Let's look once more at the three laws: 14

18 First: an object at rest, stays at rest unless a force acts on it. And an object in motion stays in motion unless a force acts on it. Here's a good example of the first law: (boy with baseball bat hits ball). Second: an object's acceleration depends on the mass of that object and the amount of force applied to the object. Here's that law again: (go-cart being pushed/ car being pushed) Third: Forces come in pairs; every action has an equal and opposite reaction. Here's one way to get a feel for that - (spacecraft taking off) These three laws govern all motion you can see. Whether it's on earth., on the moon..or anywhere in the Universe. END 15