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• Hi. It's Mr. Andersen and this AP Physics essentials video 24. It is on the gravitational

• force which is the force an object with mass experiences when it is placed inside a gravitational

• field. And on our planet we call that its weight. So imagine I say take this orange

• and go weigh it for me. Well you might not know what device am I going to use to figure

• out its weight. We have only talked about gravitational and inertial mass so far. Would

• you use a balance? No. That measures the mass. You would use a scale. That is going to measure

• the weight. Now you might say that is crazy. They both have the same answer. They are both

• going to be 0.200 kilograms. But what I would say is that balance measures the mass and

• it is never going to change, the mass of an object. So if we were to take both of these

• devices to the moon, that balance is still going to measure 0.200 kilograms. There is

• less gravitational field on the moon, less force, and that is going to affect both the

• orange the weights on the other side. But you can see that the scale is going to give

• us a lower value. And if we were to take that scale and the orange and just drop it and

• let it free fall to the planet it would measure 0.0. And so a scale measures weight because

• it is measuring that gravitational force. And I would also say this is not correct units.

• We have to measure that in newtons. And we will go through that in just a second. And

• so if you take any object that has mass and place it inside a gravitational field it is

• going to experience a gravitational force. And it is really easy to calculate that gravitational

• force. Remember it is a vector in the direction of the gravitational field. But all you do

• is take the m or the mass and you multiple it times g, which is also a vector. And it

• measures the strength of the gravitational field or the gravitational field's strength.

• And so on our planet that gravitational force is going to be equal to its weight. It is

• the weight of the object. Now let's say we take something like an orange and we just

• drop it and it goes in free fall, then we would note that the acceleration of that orange,

• as it falls towards the earth is going to be numerically equal to the gravitational

• field strength. The units are going to be different, but on our planet it is always

• going to be 9.8. And so remember a gravitational field is going to be created by any object

• that has mass. And since we are sitting on the earth all of that is going to go towards

• the center of the earth. But since the earth is so large we can treat these field vectors

• as essentially being parallel to each other. And so if we take an object and place it inside

• that gravitational field it is really easy to figure out its gravitational force. It

• is simply m times g. Or the mass of the object times the gravitational field strength. Now

• you know that the mass is always measured in kilograms. And the gravitational field

• strength will be measured in newtons per kilogram. And so let's go to that orange. It is has

• got a mass of 0.20 kilograms. What is the gravitational field strength? It is going

• to be 9.8 newtons per kilogram. And so all we do is multiply those two values together

• and I get a gravitational force or a force vector of 2.0 newtons. So that is going to

• be the force acting on that orange. And so if we were to just let it go in a gravitational

• field, what is it going to do? Well Newton's Second Law, it is going to accelerate in the

• direction of the field. But on our planet it does not do that. On our planet it still

• has the same force, it will accelerate, but eventually the earth is going to stop it from

• going to the center of the earth. It is going to exert a normal force in the opposite direction.

• And so that force on the scale is what we would measure as its weight. There is a spring

• in there that is measuring that force. And so it is giving us a value. And we would measure

• that in newtons if we were in a physics lab. And so objects that are in free fall are different

• from objects that are just sitting on the planet's surface. And so imagine we take a

• large object, 250 kilograms. So could you figure out the weight of that object? You

• simply multiply it times 9.8 newtons per kilogram. And you would get a force of 2500 newtons.

• So let's say we let it go from the top of a building, this massive object. They are

• going to accelerate towards the planet. And so in physics the gravitational field strength

• which is in newtons per kilogram is going to be equal to, numerically equal to the acceleration

• due to gravity, which we call that little g. And so it is not a coincidence that these

• two are exactly the same. But the units are different. We are measuring acceleration remember

• as a change in velocity over time. And gravitational field strength is the force of that field

• based on the amount of material in that object. And so did you learn to calculate the gravitational

• force? It is simple. You simply multiple m times g. And I hope that was helpful.

Hi. It's Mr. Andersen and this AP Physics essentials video 24. It is on the gravitational

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B1 US gravitational field gravitational force object strength weight

# Gravitational Force

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林裕庭 posted on 2014/10/04
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