SPACETIME AND GRAVITY
GRAVITY
Gravity is the force of attraction between objects that have mass. The strength of gravity depends on two main things: how much mass an object has and how far you are from its centre.
A planet with more mass generally has a stronger gravitational pull. However, distance matters too. If two planets had the same mass but one were much larger, you would stand farther from its centre on its surface, so you would experience weaker gravity there.
This is also why gravity becomes weaker as you move farther away from a planet. A rocket leaving Earth is still affected by Earth's gravity, but that pull gradually decreases as its distance from Earth increases.
The same idea works between planets, moons, stars, and other objects: the farther apart two objects are, the weaker the gravitational attraction between them.
So remember:
More mass = stronger gravity.
More distance = weaker gravity.
But where does this pull come from?
A planet with more mass generally has a stronger gravitational pull. However, distance matters too. If two planets had the same mass but one were much larger, you would stand farther from its centre on its surface, so you would experience weaker gravity there.
This is also why gravity becomes weaker as you move farther away from a planet. A rocket leaving Earth is still affected by Earth's gravity, but that pull gradually decreases as its distance from Earth increases.
The same idea works between planets, moons, stars, and other objects: the farther apart two objects are, the weaker the gravitational attraction between them.
So remember:
More mass = stronger gravity.
More distance = weaker gravity.
But where does this pull come from?
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Newton’s explanation of gravity is based on what he observed in our planet; that is, gravity is a force that pulls objects with mass toward each other, and since the Earth was so big, it pulled everything else down towards it.
This works beautifully for most of the things we do in physics: measuring how fast a rocket will fall, knowing where a basketball will fall after released, etc.
But then Einstein came along, looked past the Earth, and found out something deeper:
Gravity can also be understood as the effect of mass changing spacetime.
The tricky word is spacetime. Space tells us where something is, while time tells us when something is there.
Einstein realized that these are connected, so physicists often describe the universe using both together: spacetime.
Now imagine an object floating far from planets or stars. If nothing pushes or pulls on it, it naturally follows the straightest possible path through spacetime.
Earth changes the shape of spacetime around it because Earth has a lot of mass. So an object near Earth still tries to follow the straightest path it can, but the spacetime itself is curved, and so the straightest possible path brings the object toward Earth. An airplane, for instance, is not trying to curve down as it flies from continent to continent; the Earth is pulling it in a way that it seems to stay at a certain altitude.
And this is where life gets weird: Imagine you drop your phone.
While it is falling, ignoring air resistance, it is in free fall. According to Einstein, during the fall, the phone is actually following its natural path through spacetime, which means it is not being pushed by the ground or by your hand.
But when your phone is sitting on the table, even if it looks like it is not moving, the table is prevents it from following that natural falling path. The table is pushing the phone up, and so an accelerometer would be able to detect acceleration on your phone. The accelerometer detects the "push" of the table, the force stopping the phone from continuing its path toward the centre of the Earth.
This works beautifully for most of the things we do in physics: measuring how fast a rocket will fall, knowing where a basketball will fall after released, etc.
But then Einstein came along, looked past the Earth, and found out something deeper:
Gravity can also be understood as the effect of mass changing spacetime.
The tricky word is spacetime. Space tells us where something is, while time tells us when something is there.
Einstein realized that these are connected, so physicists often describe the universe using both together: spacetime.
Now imagine an object floating far from planets or stars. If nothing pushes or pulls on it, it naturally follows the straightest possible path through spacetime.
Earth changes the shape of spacetime around it because Earth has a lot of mass. So an object near Earth still tries to follow the straightest path it can, but the spacetime itself is curved, and so the straightest possible path brings the object toward Earth. An airplane, for instance, is not trying to curve down as it flies from continent to continent; the Earth is pulling it in a way that it seems to stay at a certain altitude.
And this is where life gets weird: Imagine you drop your phone.
While it is falling, ignoring air resistance, it is in free fall. According to Einstein, during the fall, the phone is actually following its natural path through spacetime, which means it is not being pushed by the ground or by your hand.
But when your phone is sitting on the table, even if it looks like it is not moving, the table is prevents it from following that natural falling path. The table is pushing the phone up, and so an accelerometer would be able to detect acceleration on your phone. The accelerometer detects the "push" of the table, the force stopping the phone from continuing its path toward the centre of the Earth.
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These things can be checked by using an app called "Physics Toolbox Suite". It's an awesome app filled with physics measurements using your phone to collect and record information! If you look to the right, you will see the data recorded. The acceleration of my phone was at 1 while I was holding it; I then proceeded to drop it, and all three axis dropped down close to zero. If I had let my phone drop off from a bigger height, it would become more stable and form a line! But I like my phone. It is new!
So: Falling: no support force pushing on you Standing: the ground pushes upward on you But what about when you jump on a trampoline, and as you get to the very top, you feel weightless? Are you now not affected by gravity at all? |
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ZERO-GRAVITY WHO?
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We all have heard of zero gravity. It's a term used when talking about astronauts and NASA training, about wind tunnels and other contraptions. But That explains something you already experience. When you jump off something, for a tiny moment you feel almost weightless.
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You have not escaped Earth's gravity; you are simply in free fall, and for a little time, have no other forces acting on your body, ONLY gravity.
Astronauts on the International Space Station are also trapped by Earth's gravity; in fact, gravity up there is still about 90% of Earth's gravity. They are, however, experiencing constant freefall. |
As the Space Station orbits the Earth in a forward motion, it is premanently falling into it, simultaneously suffering a centripetal force .
That is orbit!
That is orbit!
The spacecraft and everything inside it are all falling toward Earth together, so there is no floor pushing up on them. That lack of support force is what creates the sensation of weightlessness.
So the better terms are microgravity or apparent weightlessness, not literally zero gravity.
So the better terms are microgravity or apparent weightlessness, not literally zero gravity.
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So, summarizing the two theories of gravity:
Newton:
Earth pulls the apple downward.
Einstein:
Earth changes spacetime, and the apple follows its natural path through that changed spacetime.
Both descriptions predict ordinary falling pretty well. Einstein's description becomes especially important around things with enormous gravity, such as stars, neutron stars, and black holes; while Newton's observations are still useful for general Earth physics.
Newton:
Earth pulls the apple downward.
Einstein:
Earth changes spacetime, and the apple follows its natural path through that changed spacetime.
Both descriptions predict ordinary falling pretty well. Einstein's description becomes especially important around things with enormous gravity, such as stars, neutron stars, and black holes; while Newton's observations are still useful for general Earth physics.
The Equivalence Principle
Imagine you are inside a windowless spaceship.
Case A: The spaceship sits on Earth.
You feel your feet pressed against the floor.
Case B: The spaceship is far away from every planet, but its engines accelerate upward at 9.8 m/s².
Would you believe me if I told you that you would feel your feet pressed against the floor in exactly the same way? Without looking outside, you might not be able to tell which situation you were in. If you accelerate your body at exactly the rate of gravity on Earth, your body will feel the same weight.
Einstein called this the equivalence principle. That idea became one of the foundations of general relativity.
Five Big Ideas:
- Newton described gravity as a force.
- Einstein described gravity as curved spacetime.
- Massive objects such as Earth change spacetime around them.
- Falling objects are following natural paths through that spacetime.
- When you stand on Earth, the ground pushes upward on you, which is why you feel weight.
Here is a free headache for you:
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