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Lesson 4: Fields

# Fields

Forces that act across a distance, like gravity, electric forces, and magnetic forces can be explained using the idea of fields. Fields extend through space and can be mapped around objects using a test charge that interacts with the field. Learn about different fields and the types of objects that interact with them. Created by Khan Academy.

## Want to join the conversation?

• from what i learned i thought that the moon was going to float away but from what she said it will eventually crash into earth because of gravity? which one is it?
(13 votes)
• The gravity bit is true, but the magnetic field surround the earth keeps the moon at a distance. As a result, it is being pushed away, but the gravity is keeping it on its orbit.
(9 votes)
• If you have two magnets and move them closers until they touch, will the fields combine and become stronger? Thanks!
(6 votes)
• No they will keep thier respective fields.
(5 votes)
• Are there any other types of fields?
(3 votes)
• there are gravitational electric electromagnetic magnetic and more
(1 vote)
• Where can you go to escape all forces?
Just wanted to know, so I can go there and see what it's like.
(6 votes)
• Space like out of earth
(6 votes)
• At the video said that all objects with mass have a gravitational field. But I thought that, according to general relativity, there were no gravitational fields, and that different objects with mass are attracted to each other because of spacetime curving different ways.
(6 votes)
• Massive objects warp spacetime, or curve it, as you said. Warped spacetime generates gravitational attraction. I hope that I answered your question.
(5 votes)
• What's the difference between field force and non-contact force?

I assume field force is any force that has field and non-contact force is any force that can be exerted without contact.
Then they are the same thing right? Or is there something other than field that can help exert force without contact?
(5 votes)
• I don't really know as to why, but I'm willing to guess that a non-contact force is literally a force that doesn't require physical contact, and that fields are just another more specific subject related to non-contact forces; fields remain constantly around an object (up until a certain point, kind of like a boundary) and can include non-contact forces, soooo. maybe? probably just fields are just related to them/sub-sections

(please someone verify this)
(3 votes)
• If this is so then how does a rocket stay in orbit around the earth?
(1 vote)
• tl;dr: it's gonna have to be FAST! Sideways fast. VERY very fast.

If the rocket is in orbit, then it has enough sideways motion that it doesn't fall to the earth, if you could stop its sideways motion it WILL fall down.

That's also how the moon doesn't crash on the earth. It IS falling towards the earth, but it's constantly moving fast enough to the side so it misses it all the time.

The International Space Station stays in space in the same way, but it's so close to earth that it actually feels some drag from the upper atmosphere, which slows them down and makes them lose some of that sideways motion and start falling down to earth, so every now and then they have to fire up their engines to give them more sideways motion and boost them up again.

Btw the space station is moving SO FAST that if it were to whizz by right in front of your eyes, you would NOT see it! It's going faster than a bullet! That's how fast it needs to go to stay that close to earth in orbit. If it orbited much farther away, it wouldn't need to move this fast, the moon for example is slower than the ISS cuz it's much farther away.

Numbers for the curious:
ISS orbits at 8km/SECOND! That's 28,800km/h!!
The moon orbits at 1km/s
A bullet can have a speed of about 0.8km/s
(12 votes)
• vector from despicable me!
(3 votes)
• I think you mean .
(6 votes)
• Upvote if you homeschool.
(4 votes)
• so magnets are connected and used in the earth's force? because to pull the moon (which is enormous) you need something substantial. And well... if the earth was made with a magnetic force, the magnet is around us?
(3 votes)
• I cannot answer well but... I will do my best. Earth has a magnetic field AND a gravitational field. the field that is exerting force on the moon is gravitational, not magnetic, and while being inferior to the pull of an object, such as the sun, is superior to the other forces on the moon, therefore keeping the moon in orbit. the magnetic field of earth does not have any relation to the gravitational field. the magnetic field is generated by the movement of molten metal inside the earth's crust. Yes, we are inside of earth's magnetic field.
i hope this helps even though its four months late :)
(2 votes)

## Video transcript

- If you hold a ball up in the air and let it go, you know it's going to fall, but why? Nothing is touching it once you let it go. How can there be a force on it? Well, this is because Earth's gravitational force is pulling the ball and gravity is a non-contact force. Non-contact forces don't have to touch an object to exert a force on it. Instead, these forces act over a region. So if an object is in that region, it will be affected by the force. In this case, the ball is in Earth's gravitational field, and so it feels an attractive force towards the Earth and the ball falls to the ground. Field forces include non-contact forces, such as electric, magnetic, and of course, gravitational forces. So since these forces are non-contact, they can exert a force on objects they aren't touching, but how do these objects know if there's a force between them? To explain these non-contact forces, scientists eventually developed the idea that these objects were surrounded by something called a field. So what is a field? A field extends through space from an object with certain physical properties. What are those? Well, for gravitational forces, these affect objects with mass. So any object with mass has a gravitational field surrounding it that points towards the object's center. The further you move away from the object, the less dense the field and weaker the field becomes. Electric forces effect charged objects. So an electric field surrounds any object with a net charge, and the direction of this field will depend on the charge. Magnetic fields will affect magnets and any other material with magnetic properties. Each spot on a field has two things associated with it: magnitude and direction. And these help us predict what forces objects will experience when they're in the field. So let's look at an example to help understand this. Say we have a planet. Now, the planet has a lot of mass so we know it's going to be surrounded by a gravitational field that points towards the center of the planet. I can draw these little field lines that show the direction of the field and its strength. As we move away from the planet, the field will start to weaken, and I'm going to represent that by a less dense field with these arrows. Now, let's say there's an asteroid moving near the planet in this direction. I know that the asteroid, as it's shown here, is in the outskirts of this planet's gravitational field. So it is going to feel some gravitational attraction towards the planet, which we can draw with this vector, Fg, which is force of gravity. Now, because it's attracted to the planet, the astroid will continue to move towards the planet. And the closer the asteroid gets to the planet, the stronger the field and the stronger the force of attraction it will feel. And so in this way, scientists can use fields to help predict behavior of objects experiencing non-contact forces. And all of this may sound kind of odd, but you probably already think about forces this way. For example, if we go back to the ball that you know is going to fall, you knew this because the force of gravity from Earth was going to pull the ball towards the Earth. But now you also know that that's because Earth's gravity is a field force. And so the ball is in the field of gravity for Earth and experiences an attractive gravitational force. So while fields may sound mysterious, they really just mean that a force is felt over a distance. Gravitational, electric, and magnetic forces are all field forces, which means they act over distance and can affect a lot of objects.