An introduction to physics in the context of everyday objects.

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From the course by University of Virginia

How Things Work: An Introduction to Physics

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An introduction to physics in the context of everyday objects.

From the lesson

Falling Balls

Professor Bloomfield examines the physics concepts of gravity, weight, constant acceleration, and projectile motion working with falling balls.

- Louis A. BloomfieldProfessor of Physics

How would a ball fall on the moon? The answer to this question is simple.

Â The ball would fall much more slowly than on Earth.

Â Every aspect of the ball's fall would be slower.

Â It would pick up speed in the downward direction more slowly.

Â It would cover distance toward the ground more slowly, and it would take longer to

Â hit. [SOUND] To understand why that's the

Â case, however, we need to revisit the issue of gravity and take a look again at

Â the relationship between weight and mass. Up until now, I've talked about falling

Â near the Earth's surface. And I've told you that every kilogram of

Â mass, near the Earth's surface, requires a weight of about 9.8 Newtons.

Â And if you drop an object near the Earth's surface, it accelerates downward

Â at about 9.8 meters per second squared. You'll notice that I keep saying, near

Â the Earth's surface, and that's because those two statements are dependent on the

Â Earth's gravity here, the local strength of gravity.

Â Now, as we'll see in the episode on rockets, does local strength of gravity

Â is local? It varies from place to place.

Â And the details, we'll save for that episode.

Â But, for the moment, it's important to know that the strength of gravity, that

Â is how much weight is produced for each kilogram of mass, depends on two things.

Â It depends on the mass of the object producing that gravitational force, and

Â it depends on your distance from that object.

Â In the present case, here near the Earth's surface, the object that's

Â producing the gravity is the Earth. And so, we care about the, the mass of

Â the Earth and the distance that's involved.

Â That is the distance between the object in question,

Â namely the Earth and us, is about the distance between us and the

Â center of the Earth. Quite a distance away.

Â So, the Earth is very massive and in fact, quite distant from us. And

Â together, that leads to a strength of gravity that gives every kilogram of mass

Â a weight of 9.8 Newtons, and causes falling objects to accelerate downward at

Â about 9.8 meters per second squared. If we go somewhere else, that relation,

Â those relationships may change. For example, if we go to the Moon where

Â the strength of gravity locally on the surface of the Moon, is about 1/6th that

Â on the Earth. Well, every kilogram of mass will acquire

Â a weight of only about 1.6 Newtons. And if you drop a ball or any other object

Â there near the surface of the Moon, it will accelerate downward at about 1.6

Â meters per second squared. You might think this is all very

Â hypothetical and unimportant to everyday life. But actually, where you are in the

Â Earth surface matters. When I say that a kilogram of mass weighs

Â about 9.8 Newtons, here in the Earth's surface,

Â it's really in about. But, there are places you can go on Earth

Â that have stronger, local gravity, and places you can go that have weaker local

Â gravity. Every time you go upward, for example,

Â into the mountains or into a plane and get farther from the center of the Earth,

Â the strength of gravity, the Earth's gravity weaken slightly, and you weigh a

Â little less. You don't actually have to go up or down,

Â you can move to different locations on Earth.

Â The Earth, it turns out, is not perfectly spherical. Because it's spinning, it is

Â in effect flung outward, around its equator.

Â It's, it's got, the diameter of the Earth is larger around the equator than it is

Â across, across the poles. So, you can get closer to the center of

Â the Earth by going to the North or South Pole.

Â And you can go, get farther from the center of the Earth by going to the

Â equator and that will affect your weight by about half a percent.

Â So, you will actually weight, about half a percent more on one of the poles, than

Â you do on the equator. Half a percent is not trivial,

Â and so, that 9.8 meters per second squared acceleration of a falling ball.

Â Don't, don't trust the next digit all that much.

Â You have to, to be careful about it. So, the relationships between mass and

Â weight depend on where you are, and the acceleration of a falling ball also

Â depends on where you are. If you visit the grocery store, you'll

Â find items being sold by weight, and items being sold by mass, and some items

Â that are sold by both. This chocolate bar, for example, is

Â labelled according to both weight and mass.

Â It says here, net weight 3.5 ounces. That's a weight listing.

Â The ounce is a unit of force which is equal to 1/16th of a pound force.

Â So, that's the weight of this chocolate bar as, as promised by, by the

Â manufacturer. And a second label here says this

Â chocolate bar has a mass of 100 grams. A gram is a unit of mass equal to

Â 1/1000th of a kilogram. So, we have this bar labelled according

Â to its weight and according to its mass. The same is true of this bag of cookies.

Â We have a weight listing. It says net weight 16 ounces or 1 pound,

Â those are both units of those are both force amounts, so we have a weight. and

Â we also have 453 grams. That means that this bag of cookies has a

Â mass of 453 grams. And again, a gram is a unit of mass.

Â So, these two items are labeled and sold by both weight and by mass.

Â Which brings us to a question. If I take these items to the Moon, are

Â they still labeled correctly? And if they're not labeled correctly, what has

Â gone wrong with the labeling? Their labels still accurately specify their

Â masses. But those labels are way off when it

Â comes to weight. Mass, after all, is the measure of an

Â object's inertia. It has nothing to do with gravity.

Â So, the mass of say, this chocolate bar is the same on the Moon as it is on

Â Earth, as it is in deep space. It's, it simply reflects how difficult it

Â is to make this chocolate bar accelerate. So, the mass is 100 grams here, it's a

Â mass, mass of 100 grams on the Moon, mass of 100 grams anywhere you like.

Â On the other hand, weight depends on the local strength of gravity.

Â So, this bag of cookies [SOUND] weighs 1 pound here on Earth, where the strength

Â of gravity is a certain amount. But if we go to the Moon where the

Â strength of gravity is only 1/6th that on Earth, this bag of cookies is no longer

Â one pound. It's about 1/6th of a pound.

Â And so, it's no longer accurately labeled.

Â Long and short of it is, if you're going to be selling items on an intergalactic

Â basis, you do best to label them according to mass because they'll always

Â be properly and accurately labeled, regardless of where they're shipped to.

Â If you label them according to weight, you're likely to run into trouble with

Â the authorities for selling under, or possibly over weight items.

Â The bottom line is that a ball's weight and its acceleration due to gravity both

Â depend on the local strength of that gravity.

Â In most cases, however, we don't notice that dependence. And that's because the

Â Earth's gravity is so nearly the same anywhere we can go That the variations in

Â weight and accelleration of gravity are very subtle.

Â Whether you're playing baseball at sea level or in the mountains, or on the

Â North Pole or on the Equator, the game is essentially the same.

Â It's very hard to notice any changes in the ball's weight or its acceleration due

Â to gravity as it falls. But if you go and play baseball on the

Â Moon where the local strength of gravity is only about 1/6th that on Earth, the

Â game is going to change significantly. The ball will weigh only 1/6th its Earth

Â weight. And as it falls, its acceleration due to

Â gravity will be only about 1/6th its value here on Earth.

Â The game would be a very different game.

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