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Lecture Comments (5)

1 answer

Last reply by: Professor Selhorst-Jones
Mon Feb 17, 2014 11:21 PM

Post by Christopher Bunce on February 15, 2014

is there any college level courses offered on this website?

1 answer

Last reply by: Professor Selhorst-Jones
Mon Sep 16, 2013 7:38 PM

Post by Neeki Ahmadi on September 16, 2013

Great at explaining the concepts!

0 answers

Post by Norman Cervantes on February 18, 2013

picturing a car speeding towards me at the speed of a bullet made me chuckle.

Newton's 1st Law

  • An object at rest stays at rest unless a force acts upon it.
  • An object in motion stays in motion unless a force acts upon it.
  • Without a force acting on an object, its velocity will not change. In other words: no force, no acceleration.
  • Mass is a measure of inertia. The more mass something has, the more force it takes to accelerate it. The less mass, the less force.
  • While mass is associated with volume it is not the same thing. There are large objects with low masses and small objects with high masses.
  • While mass is associated with weight it is not the same thing. The weight of an object is connected to gravity. You can change the weight of an object by putting it on a different planet, but you do not change its mass.

Newton's 1st Law

Lecture Slides are screen-captured images of important points in the lecture. Students can download and print out these lecture slide images to do practice problems as well as take notes while watching the lecture.

  • Intro 0:00
  • Newton's First Law/ Law of Inertia 2:45
    • A Body's Velocity Remains Constant Unless Acted Upon by a Force
  • Mass & Inertia 4:07
    • Mass & Inertia
  • Mass & Volume 5:49
    • Mass & Volume
  • Mass & Weight 7:08
    • Mass & Weight
  • Example 1: The Speed of a Rocket 8:47
  • Example 2: Which of the Following Has More Inertia? 10:06
  • Example 3: Change in Inertia 11:51

Transcription: Newton's 1st Law

Hi, welcome back to, today we are going to be talking about Newton's first law.0000

First off, if we were to take a book and just put it down on a table, and watch the book, would it eventually start to move?0005

We just stayed there, and stared at it long enough.0014

You might think that this is a trick question, but it is not.0017

Your intuition is perfectly right, the book is just going to sit there, for basically a very long time, assuming the Earth does not manage to hurdle into the Sun because we have waited there for billions of years, we will be fine, the book is just going to stay in place.0020

What if we were to slide a hockey puck on a asphalt?0034

If we were to take a hockey puck and slide along a road, it would slide along for a little while and then frictional forces would cause it to grind to a halt and it would stop, stop after a few metres.0039

What if we were to take a hockey puck and slide it on a piece of well-polished wood, the same hockey puck would slide along for a while, and then it would probably slide considerably longer than the asphalt, because the wood is going to have less friction.0050

But it is going to eventually come to a stop, probably twice as long as it takes on the asphalt.0062

What if we had an arbitrarily large ice ring and slide a hockey puck across it?0067

In a normal sized ice ring (like a hockey ring), it would manage to slide probably the entire way across the hockey ring.0074

If it were arbitrarily large, it would eventually stop, it would take a really really long time.0081

Why?, because we got even lower friction.0086

The friction between these things just keeps going down each time we do it.0088

Asphalt has more friction than wood, which has more friction than ice.0091

What if we had some hypothetical frictionless surface?0098

The puck would never stop, because there would be nothing to stop it.0100

The puck on the asphalt slides a little while, the puck on the wood slides a little while longer, the puck on the ice, it slides really long, but eventually it would come to a stop as well, because the ice will have an effect.0106

But on the hypothetical frictionless surface, it is just going to keep sliding and sliding.0118

There is nothing to stop it.0131

What this means is that, it is not that the things want to come to rest, it is that things want to stay the same, giving them a certain human personality, which they do not necessarily have, probably not at all.0133

But, we do have that a hypothetical frictionless surface would give us the ability to slide something forever, because there is nothing stopping it.0145

If something is moving, it continues to move until something stops it.0157

If there is something sitting still, it continues to sit still until something comes along and forces it to move.0161

This brings us to Newton's first law.0166

Newton's first law's idea is that a body's velocity remains constant unless acted upon by a force.0169

If you are still, you stay still until something comes along and moves you, something puts a forces on you, which makes you move.0178

Or, if no force is acting on an object, that will have no acceleration.0184

It would continue to stay in the same state, whether it is moving in a given velocity, or it is sitting still, which is just another kind of velocity, it is just going to stay the same.0194

Isaac Newton is the person who first came up with these formulations of these laws, and we are going to be talking about these laws for the next few sections.0205

You should possibly look into them, it is going to be in the book of any course you are taking, or the internet, Newton is an interesting character and created a lot of Physics as we know of it today.0215

He is one of the founding fathers of, basically the modern movement of Physics that we have gotten used to.0229

He lived in the 1600's and involved in the creation of calculus as we know of it today, and he did a lot of things for Physics, and Math.0235

Mass is the same thing as inertia.0248

The other phrase for what we are talking about is the law of inertia, Newton's first law or the law of inertia.0253

Mass and inertia are actually the same thing.0260

If we came along and kicked an empty bucket, it would just bounce away, it would clatter along.0263

But if we came along, and kicked a bucket that is completely full of concrete, it would break your foot, you would hurt yourself really badly, because t is not going to go anywhere, you have to give it a much stronger force to actually get it to act.0267

Instead, it acts on your foot, and it impedes on your foot, which causes your foot to experience a lot of pain.0277

What is the difference?0283

Mass!, one of these objects is a massive object, it has a lot of mass, the empty bucket: low mass, it just moves on it, very small force is required to move it anywhere.0284

But a heavy mass means that it has high inertia.0296

Mass is inertia, inertia is mass, they are the same thing.0299

If something is hard to move, then it has a lot of inertia; if something is easy to move, then it has little inertia.0302

That is not necessarily true, you can take a light object, and stick it to something using a high strength adhesive.0310

But then you are getting into different forces.0318

But if you had something on a frictionless surface, if you want to shake something back and forth, all these things will be measurements of inertia.0320

If something is able to move freely, then how hard it is to get moving is a measure of its inertia, its mass.0327

If something has more mass, it is harder to move, if something has less mass, it is easier to move.0332

If something has more mass, it has greater inertia, if something has less mass, it has less inertia; if something has more inertia, it is harder to move, if something has less inertia, it is easier to move.0336

Mass is inertia, inertia is mass.0346

What about volume, what about how big something is?0349

If something is this big, does not it imply that it is heavier, more massive than something that is this big?0353

Not necessarily.0360

Volume is just a measure of what is the size of your thing.0362

Something can be very large and have very little mass, say, a pillow full of feathers, or a balloon.0363

But something can also have very low volume and very high mass, like a brick of lead, or a plate of Uranium.0370

These things are very small, but they are going to be very massive objects.0377

So, just because something is very big does not mean it is massive.0382

That said, if we have one brick of lead, and take a brick a lead exact same size and density under all the same specifications that this brick of lead was made, we are going to double the mass because we have doubled the volume.0384

But that is because you have got a consistent object.0396

Comparing two object is like comparing apples and oranges, you are not necessarily going to have the same mass to volume ratio.0398

Even within one kind of object, you can have different mass to volume ratios.0404

You could have a loaf of bread, and you could have a hollow loaf of bread.0408

One of them will have more of mass, but they will have the same volume.0412

You got to keep in mind that there are a lot of things that is going to affect mass, and mass is a really important characteristic.0416

Volume can have a lot effects, but mass is a really important characteristic, and you want to remember that it is not always going to be the same thing as volume, and in many cases, it is not at all.0420

What about mass compared to weight?0428

What does weight mean, we have not really talked about this before.0431

Weight is the measure of the force of gravity, not your mass.0434

They are related, and we will talk about that later, when we talk about Newton's second law, you will see very clearly what the connection is between mass and weight.0439

But, you do not necessarily have the same weight throughout.0448

You have all heard about that, if we were to go from here on Earth, to being on the moon, your weight would change.0450

That is because there would be less gravity taking effect.0456

But your mass would stay the same.0458

None of the particles that make you up, none of the matter that makes you up, (matter is what gives the things mass), would be gone, it is just you in a different place.0460

So, the weight changes, the mass does not change.0468

If we got a stone on Earth, with some weight, and we took that stone to the moon, its mass would remain the same, but it will have one sixth the weight that it has on Earth.0471

If we took that stone to deep space, where there is no gravitational effect on it at all, it would just float there, it would have nothing pulling it anywhere, it would have no weight.0487

Weight is the pull of gravity, it is the force given to something by gravity.0497

So it would have no weight in space, but it would still have the same mass, the same inertia.0502

That stone, for you to shake it back and forth on Earth, it would take a little amount of difficulty.0507

But if we were an astronaut in space shaking that same stone, it would have the same difficulty, because its mass is the same, so its inertia is the same.0512

So shaking it back and forth is still difficult, because that is a measure of mass, not the weight.0521

First example: (these are all idea examples, there is no Math here, these are all ways to think here.)0527

We have got a rocket that we fire in deep space.0534

It has got a velocity of 104 m/s .0537

So, we are out here in deep space, and if we nothing acts on it, there is no gravity, it does not hit anything, nothing is pulling it in different direction, and it is now going at a constant velocity, its burners are done firing, its fuel off, it is just moving at a speed, it is not pushing itself along.0549

What will its speed be in a 150 years?0570

It seems like a really long time, and it is a really long time, but nothing acts on it.0572

It is moving along, but there is nothing to change it moving along, so since it is moving along, it continues to move along.0578

No forces mean same speed.0584

So, in 150 years, in 1500 years, in a billion years, it is still going to be the same as long as no forces act on it.0587

If there is no gravity, if there is no additional fuel, if nothing comes along and runs into it, it is ust going to be 104 m/s for no matter how long we choose to look at it.0595

Second example:0607

Which of these are going to have more inertia?0608

A falling leaf, a thrown rock, a stopped car, or a bullet that has just been fired.0611

The bullet is clearly the thing moving fastest.0623

The thrown rock is also moving at a reasonable speed, the falling leaf pretty slow, the stopped car completely not moving at all.0626

It might seem clear which one is going to be the thing with more inertia.0631

It is going to be the thing moving really quickly.0635

But, that does not matter.0637

If you were standing there, and the bullet hit you, that would be really bad, because the bullet would be moving really fast, so it would hurt you.0639

But if you were standing next to the stopped car, the stopped car would not hit you.0645

That is because your relative velocities are different.0648

So, the reason why you are thinking that inertia is about the speed is because of how much damage it can do to you, but that is not what inertia is, it is about change in that.0653

If you were running along with the same speed of the bullet, it would not be able to hurt you, because it would just be sitting there next to you, and if a car would hit you at a speed the bullet would hit you, you would no longer be.0665

There would be nothing.0676

So, the idea if inertia is, how massive is the object.0678

Which one of these is the most massive?0682

The car, the car is the hardest thing by far, to be able to get up to the speed of something else, is going to be the car.0684

The bullet can be stopped very easily.0690

We can just put a small piece of wood in front of the bullet, and it is going to come to a stop.0692

But if we had a car moving at even a 100 m/s, which is a considerable fraction of what a bullet would be moving at, and we put a piece of wood in front of that car, that car is going to keep moving, that wood is going to have no effect.0696

So the inertia here, is really just a measure of mass.0707

Finally, what is the change in inertia of a 2 kg skateboard that goes from 5 m/s to 10 m/s?0711

We have got a change here, change in speed = 5 m/s .0720

But does that matter?0728

No, it has no effect.0729

Once again, the speed has no effect on your inertia, it is just the mass.0730

Has the mass of the skateboard changed?0734

No, the mass has remained the same, 2 kg, so there is no change in inertia, it is just the same mass.0735

Nothing has changed, nothing has come along to change the mass.0741

The only thing that will affect the inertia of an object its mass.0744

If we know its mass, we know what its inertia is; and that is really the important thing.0747

Speed does not matter.0751

Hope you enjoyed that, see you again for the next lesson at