All of us ( I can safely assume it's most of us) love looking at the moon. It looks stationary isn't it ? We've heard that it goes around the earth. Ever wondered why? Well, you ask any adult , and pat comes the reply - "it's earth's gravity man, elementary my dear Watson " . Well, if u look a little deeper, earth exerts gravity pull on the moon. Ok... But like how we stay grounded to the earth, why doesn't the moon crash into the earth because of gravity? How does it orbit the earth ? Now that's something we could think about .
To understand this, we need a fair understanding of gravity or rather the gravitational force. To be more precise earth's gravitational force . What is gravity? A pull exerted by the earth on objects. And for an understanding of force, we need to know newtons laws of motion ( they really go a long way in helping us understand. Thank you Newton ! )
Let's dig into gravity a little later. For now let's say, it's a "force exerted by the earth pulling objects closer to it or towards it " ( crude explanation for now ) let's try and analyse force. And so, let's look at newtons law of moving objects or the first law of Newton .
"An object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force"
That having said, imagine I have a ball with me and I hold it in my right hand and just drop it. It goes down . That's not new. We've seen it many times. Now I do something little different. Instead of just dropping it onto the ground, I throw it forward. Point to note here is that I 'throw' it, that is, exert a force on it. Now what happens? The ball moves a little forward and then takes a curve and hits the ground. Easy peasy. Again, nothing we haven't seen before .
A good point to think about here would be, in both the cases, gravity exists. Why did one ball fall right down and the other move forward and fall ? The only difference was the force we exerted on the ball in the second case when I 'threw' it forward. Gravity , tried pulling it towards the earth. But the ball already had some speed/velocity here because of the throw. The ball moved forward because of it. And because of air friction ( that's another discussion totally, let's do it later. Let's assume friction hurts the motion or opposes the motion of the ball ) the ball lost its velocity and gravity got better of it, thereby eventually pulling it to the ground. So, gravity force tried altering the path of the moving object.
Let's extend the experiment a little further.( I better give Newton the credit. It was his experiment or theory rather ! I just read it in the internet ) so what he did, he got on to a mount ( or so says the Internet ) with a cannonball in hand and first like we did, dropped the ball. It went down. Then threw the ball. It took a forward motion and fell down in a curve. Now , he threw the ball with a little more force. The ball went further and fell down. Little more force, the ball would fall further away. The earth is not a rectangle but a sphere. Imagine if u threw it away with little more force this time . It would not fall off the earth because of gravity, but will fall near the equator ( say ) . More push, the ball might fall near the South Pole. More push, the other side of the equator this time. A little more push and it might finish one whole circle and hit u at the back of your head !!! Isn't that something ?
[ Note : please read 'newton's cannonball' from wikipedia for exact details and an animation of the cannon ball and its outcomes. The above para is a simplified version ]
What is happening all this while is that, the gravity is altering the path of the ball which wanted to move in a straight line. And how far the ball went depended on the speed it had with it earlier. Let's try and apply this to the moon. Moon wants to move in a straight line, but because of the gravity, it is constantly falling towards the earth. And goes one full circle. I know I know, you have a million more and new questions popping into your head now. Keep it mum just for a little while longer. I would like to take a small detour now.
Imagine I have a ball tied to a string and I fling it into a circle. The centre of the circle being the start of the string or my hand. The ball moves in circles. If at any point I cut the string, will the ball fall down? No? It will fly away tangential to where it is while I cut the string , and then eventually fall down. Now the string is gravity. The ball is the moon. If the gravity one day disappears then the moon would go flying away. So far so good ?
In the ball example, the ball keeps on moving coz of my exerting a force. In the example before that the ball completed one big circle and hit the back of my head and fell down. But the moon keeps going round and round. What's different ? Air friction. The air particles bombard against the moving object and slow it down . In the absence of air or otherwise in vacuum, no opposing force and we could keep the ball moving on and on.
In the example above , I gave the ball its initial speed by giving it a fling. Who gave the moon its speed? Our explanation forces us to assume that the moon has a velocity of its own. Right ? That maybe, the moon was already moving in a straight line after it was born or because of some complicated spatial parameters. And it happened to come near the earth and got drawn into an orbit because of earth's gravity! Seems like an explanation isn't it ?
The velocity with which the moon goes round the earth is called orbital velocity. That's how we send man made satellites into space. Like how I could make the ball move one complete cycle can I make the ball fall off the earth? Or escape the gravity pull? That's possible too. Give it so much force or velocity that ( in the absence of air friction to slow it down ) it could be so much greater than the pull of the earth that it could go flying away. That's the escape velocity.
To sum up, the points to take back home from this post will be, the combined effect of the moon's own speed and the gravity pull of the earth together makes it go into an orbit around the earth. The speed the object(here the moon) possesses when it orbits another object(here the earth) is the orbital velocity. The speed required to escape the orbit is escape velocity.
We've given ourselves a new reason to feel happy. The moon is going to be with us for while ( touch wood ) and better so, we know why :)
To understand this, we need a fair understanding of gravity or rather the gravitational force. To be more precise earth's gravitational force . What is gravity? A pull exerted by the earth on objects. And for an understanding of force, we need to know newtons laws of motion ( they really go a long way in helping us understand. Thank you Newton ! )
Let's dig into gravity a little later. For now let's say, it's a "force exerted by the earth pulling objects closer to it or towards it " ( crude explanation for now ) let's try and analyse force. And so, let's look at newtons law of moving objects or the first law of Newton .
"An object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force"
That having said, imagine I have a ball with me and I hold it in my right hand and just drop it. It goes down . That's not new. We've seen it many times. Now I do something little different. Instead of just dropping it onto the ground, I throw it forward. Point to note here is that I 'throw' it, that is, exert a force on it. Now what happens? The ball moves a little forward and then takes a curve and hits the ground. Easy peasy. Again, nothing we haven't seen before .
A good point to think about here would be, in both the cases, gravity exists. Why did one ball fall right down and the other move forward and fall ? The only difference was the force we exerted on the ball in the second case when I 'threw' it forward. Gravity , tried pulling it towards the earth. But the ball already had some speed/velocity here because of the throw. The ball moved forward because of it. And because of air friction ( that's another discussion totally, let's do it later. Let's assume friction hurts the motion or opposes the motion of the ball ) the ball lost its velocity and gravity got better of it, thereby eventually pulling it to the ground. So, gravity force tried altering the path of the moving object.
Let's extend the experiment a little further.( I better give Newton the credit. It was his experiment or theory rather ! I just read it in the internet ) so what he did, he got on to a mount ( or so says the Internet ) with a cannonball in hand and first like we did, dropped the ball. It went down. Then threw the ball. It took a forward motion and fell down in a curve. Now , he threw the ball with a little more force. The ball went further and fell down. Little more force, the ball would fall further away. The earth is not a rectangle but a sphere. Imagine if u threw it away with little more force this time . It would not fall off the earth because of gravity, but will fall near the equator ( say ) . More push, the ball might fall near the South Pole. More push, the other side of the equator this time. A little more push and it might finish one whole circle and hit u at the back of your head !!! Isn't that something ?
[ Note : please read 'newton's cannonball' from wikipedia for exact details and an animation of the cannon ball and its outcomes. The above para is a simplified version ]
What is happening all this while is that, the gravity is altering the path of the ball which wanted to move in a straight line. And how far the ball went depended on the speed it had with it earlier. Let's try and apply this to the moon. Moon wants to move in a straight line, but because of the gravity, it is constantly falling towards the earth. And goes one full circle. I know I know, you have a million more and new questions popping into your head now. Keep it mum just for a little while longer. I would like to take a small detour now.
Imagine I have a ball tied to a string and I fling it into a circle. The centre of the circle being the start of the string or my hand. The ball moves in circles. If at any point I cut the string, will the ball fall down? No? It will fly away tangential to where it is while I cut the string , and then eventually fall down. Now the string is gravity. The ball is the moon. If the gravity one day disappears then the moon would go flying away. So far so good ?
In the ball example, the ball keeps on moving coz of my exerting a force. In the example before that the ball completed one big circle and hit the back of my head and fell down. But the moon keeps going round and round. What's different ? Air friction. The air particles bombard against the moving object and slow it down . In the absence of air or otherwise in vacuum, no opposing force and we could keep the ball moving on and on.
In the example above , I gave the ball its initial speed by giving it a fling. Who gave the moon its speed? Our explanation forces us to assume that the moon has a velocity of its own. Right ? That maybe, the moon was already moving in a straight line after it was born or because of some complicated spatial parameters. And it happened to come near the earth and got drawn into an orbit because of earth's gravity! Seems like an explanation isn't it ?
The velocity with which the moon goes round the earth is called orbital velocity. That's how we send man made satellites into space. Like how I could make the ball move one complete cycle can I make the ball fall off the earth? Or escape the gravity pull? That's possible too. Give it so much force or velocity that ( in the absence of air friction to slow it down ) it could be so much greater than the pull of the earth that it could go flying away. That's the escape velocity.
To sum up, the points to take back home from this post will be, the combined effect of the moon's own speed and the gravity pull of the earth together makes it go into an orbit around the earth. The speed the object(here the moon) possesses when it orbits another object(here the earth) is the orbital velocity. The speed required to escape the orbit is escape velocity.
We've given ourselves a new reason to feel happy. The moon is going to be with us for while ( touch wood ) and better so, we know why :)
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