Download Newton`s First and Second Law of Motion Video Script

Physics 401 – Newton’s First and Second Laws
VO
Can you explain what happens in this demonstration? Why does the tray fly across the table? Why
do the cardboard cylinders go with it? And why do the eggs drop straight down into the water below?
Watch as we set up the demonstration so that you can repeat it in your classroom. Notice that the tray
extends about ten centimeters over the side of the table. Make sure that the cardboard cylinders are
placed directly over the middle of the beakers of water. And it might be a good idea to use boiled
eggs in case anything goes wrong. Use a broom with stiff bristles, and stand on the brush part as you
pull the handle back. Then let it go.
Watch what happens again in slow motion.
Instructor
By the end of this lesson, you should be able to answer each of the questions you were asked about
the demonstration you just saw. In addition, you should be able to explain why seat belts are
necessary and why you feel pulled to the right when your car turns left.
Explanations of these and many other natural phenomena are based on this unit’s topic, Newton’s
Laws of Motion.
(Read objectives on screen.)
Instructor
Before we talk about Isaac Newton, we need to get some historical perspective on what great minds
thought about motion prior to his work. Let’s start with Aristotle. Now don’t take any notes on what
he taught because it all turned out to be wrong. And since so many people, including the Catholic
Church in Rome, believed whatever Aristotle said to be absolute truth, he retarded the growth of
science for hundreds of years.
Aristotle divided motion into two kinds, natural motion and violent motion. It was natural for
heavenly bodies, such as planets and stars, to move in perfect circles around the earth. After all, the
earth was considered to be the center of the universe.
For very light substances, like smoke, it was natural to move straight up, toward the heavens. But
most objects would seek their natural resting places on the ground, close to the center of the earth.
That’s why falling was considered to be a natural motion.
On the other hand, violent motion was motion forced on an object.
(sketch on screen)
VO
The early Greeks believed that vertical motion was natural, while horizontal movement was forced.
This is because a stone will fall vertically with no outside force appearing to be applied to it. But if
one wanted the stone to travel horizontally, a force such as the muscular force of the arm had to be
exerted on it.
Instructor
Since horizontal motion was not thought to be natural, there had to be some cause, a push or a pull. A
ball moves horizontally through the air because of the force exerted on it by the thrower’s arm. A
ship moves because the wind pushes it, just like the carriage moves because the horse pulls it. For
nearly two thousand years, it was accepted that if an object was moving “against nature,” it was
because some force was acting on it. Without a force, there could be no motion because the object
was in its natural place.
(earth on screen)
And since the earth’s natural place was at the center of the universe and no huge force was pushing or
pulling it, it was just common sense to believe that the earth did not move.
And then a Polish monk looked into the heavens and, based on his observations, decided that the earth
must move around the sun. You can imagine how popular this idea was at the time. In fact,
Copernicus did his writing in secret and only sent his ideas to a printer in the last days of his life. To
Aristotelians, the idea of the earth hurtling through space was unthinkable. Here’s why.
(tree on screen)
VO
Remember that the followers of Aristotle believed that heavy objects fell toward the center of the
earth because the earth was the center of the universe.
If the earth really was rotating and moving through space, as Copernicus said, then objects would fly
off, wouldn’t they?
And think about this. When an apple falls from a tree, why should it land directly beneath the limb of
the tree? If Copernicus was correct, wouldn’t the tree and earth leave the apple behind, making the
apple land miles away?
Instructor
It was Galileo who defended the ideas of Copernicus. He did it with his own observations of the
heavens using the telescope he invented and through scientific experimentation and creative thinking.
Watch this and be ready to tell what ideal conditions Galileo imagined in his inclined plane
experiment.
(inclined plane on screen)
VO
Unlike Aristotle, Galileo did experiments to support his ideas. He repeatedly observed and timed
balls rolling down one inclined plane and up another and found that the ball always gained its original
height.
If the second plane was shallower, the ball rolled farther until it reached its original height again.
The more shallow the second incline, the farther the ball rolled, trying to reach its original height.
Galileo concluded that on a perfectly horizontal plane, with a perfectly smooth surface, the ball would
keep on rolling forever.
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Instructor
For his study of falling objects, Galileo imagined ideal conditions without air. In his study of
horizontal motion, the ideal condition he imagined for the inclined plane was the absence of what?
Did you say friction? That’s right. Without friction to slow the ball down, Galileo imagined that it
would continue moving forever. He said that it was not the ball’s nature to come to rest, as Aristotle
had claimed, but that a property of matter, which he called inertia, would resist any change in the
objects motion. The word, inertia, comes from the Latin word for lazy or inert.
Galileo said that it was not the nature of the earth to come to rest, but that its inertia would keep it
moving. No force was necessary to keep it in motion. And objects on the earth would continue to
move with it, not fly off. We’ll learn more about that later. For now, we need to get back to the story
of Galileo. It seems that in addition to his experimenting, he did a great deal of talking and writing,
basically treating Aristotelians as fools. And this got him in trouble with the church officials in
Rome.
(picture of Galileo on screen)
VO
Galileo was put on trial in 1634, as part of the inquisition. Because he feared torture or execution, he
publicly recanted his Copernican views that the earth moved around the sun.
He was sentenced to house arrest and spent the last eight years of his life confined to his estate near
Florence.
But Galileo’s mind and pen would not be stopped. During his confinement, he wrote his most
important work. It was smuggled out of Italy to France, and published in Holland in 1638. This
work became cornerstone of the study of mechanics.
Galileo died in 1642. It was not until 1984 that the church admitted that it was wrong and Galileo was
right.
Instructor
Isaac Newton was born in England within a year of the death of Galileo. Just as I’m sure you do very
productive school work when you are stuck at home on snow days, Newton did some of his best
academic work during the plague years when he left Cambridge University. He spent that time at his
family’s farm that just happened to have – you guessed it, an apple orchard.
(picture of Newton on screen)
VO
At the age of only twenty three, Newton wrote his three laws of motion. His work goes beyond
Galileo’s description of motion to explain the reasons for that motion.
Instructor
Isaac Newton is known as the most influential scientist of modern times, but he gave credit to those
who went before him. His famous quote, “If I have seen farther than others, it is because I have stood
on the shoulders of giants,” surely referred to the work of Galileo. In fact, Newton’s First Law of
Motion simply restates Galileo’s original idea. He even uses the term, inertia, which Galileo first
used. In fact, Newton’s First Law is often called the Law of Inertia.
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(green chalkboard on screen)
VO
Newton’s First Law of Motion is also called the law of inertia.
It states that when no net force acts on an object, the object will either remain at rest or remain in
uniform motion.
In other words, objects at rest tend to stay at rest.
And objects in motion tend to stay in motion at constant speed and in a straight line.
Unless some outside force pushes or pulls the object, it will remain in its present state of rest or
uniform motion forever.
Instructor
The two most important terms involved in this law are inertia and force. Let’s talk about them one at
a time.
Inertia may be a term that is new to many of you. It is a property of matter, just like color,
temperature, and volume are properties that you can observe or measure.
(green chalkboard on screen)
VO
Inertia is a property of matter that resists changes in its motion.
How do we measure inertia? The best measure of an object’s inertia is mass. The more matter an
object has, the more mass it has, and the more sluggish or resistant to changes in its motion it will be.
Instructor
Don’t confuse mass with weight. Remember that mass does not depend on gravity. It would be
easier to lift this barbell on the moon because lifting involves opposing the force of gravity, which is
less on the moon. But it would be just as difficult to shake this barbell on the moon or in space as it
is on earth.
(green chalkboard on screen)
VO
A force is a push or pull. Only net forces actually result in changing motion. Remember that forces
are vectors, so direction is important.
Look at this book. There are two forces acting on it. The weight of the book is the force of gravity
pulling down on the book. But the table is supporting the book. That means that it is pushing upward
on the book. Since the two forces are equal, the net force on the book is zero and the book will
remain at rest.
Instructor
Examples of the Law of Inertia are all around you. What happens to your head when a car suddenly
moves forward? It feels like your head snaps backwards, but an observer outside the car would see
the car accelerate forward and your head remain at rest. The inertia of your head resists any change in
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motion. Now watch what happens when a car stops suddenly.
Newton’s first law of motion explains why the use of automobile seat belts is critical to passenger
safety. When a car stops suddenly, unrestrained passengers will continue to move forward until they
make contact with the windshield, steering wheel, or dashboard. Most injuries in a car accident are
caused by the passengers remaining in motion and smashing into the dashboard or steering wheel.
When passengers are restrained by seat belts or air bags, their motion becomes roughly equivalent to
the car’s motion. And the vehicle absorbs the majority of the collision’s force”
Instructor
Imagine this. You are in the passenger seat of a convertible that turns sharply to the left. What do
you experience? It feels like you are being pulled toward the right. With no seat belt, you could
actually slam into the door. But what would an observer on a platform above your car see?
He would see the car turning and your body continuing to move in a straight line. Inertia resists any
form of acceleration, including changing direction.
Now it’s your turn to come up with some examples of Newton’s first law in your everyday life. Your
teacher will turn off the tape and give you some time to think of some situations and share them with
your class. Then we’ll be back.
(Pause Tape Now graphic)
(text on screen)
VO
Here are some examples of Newton’s First Law of Motion.
(paper towel roll on screen)
VO
When you use one hand to rip off a paper towel from a full roll, the inertia of the full roll resists
moving and it will stay put. But try the same thing with a half-full roll and the whole thing will come
with you. It has less inertia.
(hammer on screen)
VO
To tighten the head of a hammer on its handle, we bring the hammer down quickly and then stop it
suddenly on a hard surface. The inertia of the massive head keeps it in motion after the handle is
stopped.
(toy boat on screen)
VO
Float a toy boat in a bowl of water. When you turn the bowl, the water and boat don’t turn. Inertia
resists the change in direction.
(plate on screen)
VO
Here’s a good one. If you use heavy plates and glasses, with lots of inertia, you can pull a tablecloth
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out from under them. Make sure that there’s no hem on the cloth and pull quickly in a horizontal
direction only. Here goes.
Instructor
Now you should be able to answer the questions about the egg-drop demonstration we showed you at
the beginning of this program. Take another look and then your teacher will pause the tape and give
you a chance to answer the questions. In your answers, try to use the terms we’ve learned during this
lesson.
(egg demo on screen)
VO
Can you explain what happens in this demonstration? Why does the tray fly across the table? Why
do the cardboard cylinders go with it? And why do the eggs drop straight down into the water below?
Instructor
First, why does the tray fly across the table? You should have said that a force was applied to the
tray, causing it to start moving. We used the broom handle so that this force would be horizontal.
The cardboard rolls went with the tray because they are very light and don’t have much mass or
inertia. So they were not able to resist the force exerted on the tray, and because of friction, on them,
too.
Now the eggs have lots of inertia and were able to resist moving with the tray and cardboard rolls.
They remained at rest until the net force of gravity pulled them down.
How did you do? I hope this program made it easier to answer the questions.
(text on screen)
VO
Here’s a physics challenge for you.
Your friend says that he doesn’t believe that objects in motion tend to stay in motion because, even on
a smooth, flat road, he has to keep giving his car gas or it will slow down and stop. How do you
explain to him that the Law of Inertia still applies?
Tell your teacher.
Instructor
Remember that Newton’s 1st Law applies when there is no net force acting on the object.
It would be easier to explain this to your friend if you had this apparatus. Watch what happens when I
give this plastic puck a push across the table. It slows down and stops.
Now watch what happens when I let this balloon furnish air for the puck to slide on. It just keeps on
going. What’s the difference? Did you think of friction? With the first puck, the force of friction
acted on the puck to slow it down. With the air from the balloon, we eliminated most of the friction,
so the puck’s inertia could keep it in uniform motion. Did you get it?
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(rocket sled on screen)
VO
Here’s a drastic example of the Law of Inertia. In December of 1954, Colonel John Stapp was
strapped into a rocket-powered sled called the Sonic Wind. In just five seconds he was accelerated to
a speed of 632 miles per hour, making Colonel Stapp the fastest man on earth. The rocket sled was
then stopped in one point two five seconds.
The force required to decelerate the sled at this rate was equal to the force you would experience by
hitting a brick wall at 120 miles per hour. Although Colonel Stapp’s body was securely tied to the
sled, his eyes tried to obey Newton’s first law of motion. Colonel Stapp temporarily lost his sight,
and many broken blood vessels resulted in a frightening pair of black eyes. To see pictures of Colonel
Stapp after the test, look in the August, 1955 issue of National Geographic magazine. The
experiments with the Sonic Wind were halted because the risk to sight was too great. Objects in
motion, even eyes, do tend to stay in motion.
Instructor
Imagine this situation. You are sitting on a plane or train or even in a car that is moving uniformly, in
other words at a constant speed in a straight line. What will happen if you toss a coin straight up?
Will you and the vehicle move out from under it so that the coin slams into the back of the car or
plane or train? No. That’s because objects in motion stay in motion. The coin is already moving
horizontally with the vehicle. So, as it goes up and down, it also continues to move forward. It will
come straight back down into your hand.
Now this kind of smooth ride was not common in Galileo’s days, so it was hard for people to imagine
such a situation. So Galileo asked them to think about this.
(sketch of boat on screen)
VO
Galileo asked people to picture a boat moving smoothly through the water.
If a sailor dropped a ball from the top of a mast, the sailor would see the ball fall straight down and
land on the deck at the foot of the mast. To the sailor, whose frame of reference is the ship, the mast
is standing still, and it would be logical for the ball to fall straight down.
But to people of the shore, the ship and its mast are moving. They would see the ball moving forward
as it falls. Galileo explained that since the ball was already in horizontal motion with the boat, it
would continue to move forward as it falls.
Instructor
With the grid behind me to use as your frame of reference, I’ll hold a ball over my head like this and
move uniformly from left to right. When I reach this point, I’ll drop the ball. Watch what happens.
VO
You can see that the ball continues to move forward as it falls. It was already in horizontal motion,
and objects in motion stay in motion.
Instructor
Now we’ll do something a little different. This time the ball will be at rest at this position. When I
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reach it, I’ll simply knock it off its support. What do you expect to happen? Tell your teacher.
VO
Because the ball was not initially moving with me, it had no horizontal motion. So when it was
released, it fell straight down. Is that what you predicted? I hope so.
Instructor
So far, we’ve talked about objects at rest staying at rest and objects in motion staying in uniform
motion. Now let’s talk about changing an object’s motion. What is the one word we use for change
in motion? Tell your teacher.
We’ve been talking about changes in motion. What is the one word we use for change in motion?
If you said acceleration, you learned something in the last unit! Let’s go over some examples of
accelerated motion.
(text on screen)
• The most obvious example of accelerated motion is an object speeding up.
• Then there’s slowing down or decelerating.
• Starting from rest is accelerating.
• And so is stopping.
• Changing direction is accelerating.
• And that includes motion in a circle, like revolving.
Instructor
So it all boils down to this. The inertia of the object in each example resists the change in motion, and
a net force must be exerted on each object to produce the change. Inertia resists acceleration and a net
force causes it.
Newton’s first law describes an object’s motion when no net force is acting on it. The logical next
step is to describe an object’s motion when there is a net force acting on it. That brings us to
Newton’s Second Law of Motion.
(green chalkboard on screen)
VO
Newton’s Second Law of Motion is often called the Law of Acceleration.
When a net force is applied to an object, the object will accelerate in the direction of that force.
(Read fact or fiction statement on screen)
(text on screen)
VO
No, an object does not always move in the direction of the net force acting on it. That’s not what
Newton’s second law says. The object’s motion changes in the direction of the net force acting on it.
(cartoon of boy on screen)
Look at this example. The ball is moving upward, so we’ll call that the positive direction.
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But the net force acting on the ball is the force of gravity, or the ball’s weight. And that’s in a
downward direction.
Newton’s second law says that the object will accelerate, or change its motion in the direction of the
net force. So the ball will accelerate downward. How does it move up and accelerate down?
The answer is that the ball slows down or decelerates at 9.80 (nine point eight zero) meters per second
squared.
Slowing down is accelerating in the opposite direction of the motion.
There’s more to Newton’s Second Law of Motion, but we’ll need to do an experiment to discover it.
Let’s save that for the next program.
(text on screen)
VO
But now, it’s time to …SHOW WHAT YOU KNOW!!
Jot down your answers only. Your local teacher will go over the answers with you.
(Read Show What You Know questions on screen)
Instructor
That’s it for your introduction to Newton’s Laws of Motion. But we’re not through with the old guy
yet. Next time, we’ll find out why Newton knew that a feather and a hammer fall at the same rate on
the moon, and this was almost 250 years before humans actually walked on the moon. Glad it was an
apple rather than this thing that conked him on the head. Until next time, may the net force be with
you.
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