字幕列表 影片播放 列印英文字幕 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.