Escape velocity how can u count?

[ramanan]

When we talk of escape velocity of an universal object we mean to say of the minimum velocity needed by an object to escape the gravitational attraction of that universal object. right?

say for ex. an object from earth must need a minimum velocity of 11km/s.

so is this velocity applicable only for objects trying to get away from the earth or is it applicable for objects coming towards the earth.

to make it simpler let's imagine of a meteor coming towards the earth at speeds more than E.V .can this meteor able to maintain the same speed till it reaches the earth.

 

(VELOCITY how scientists expain it in this case. i mean

is it unidirectional?)

imagine before anwering

[swansont]

The object will tend to speed up as it approaches the earth. But unless it loses energy along the way, it won't be captured.

[YT2095]

11km/s is applicable for objects leaving Earth.

for an object "dropped" an allowed to fall naturaly, it`s velocity will increase at 32 feet per second per second, untill it reaches a maximum know as "Terminal Velocicy" (I forgot what exact speed that was, but it`s roughly 200 to 300 MPH).

an asteroid traveling if space at several 10`s of km/s will only be slowed down a tiny bit by the friction in our atmosphere and usualy burm up. a larger body could quite easily maintain it`s original velocity though and not be significantly hindered by the air friction, there is no realistic upper limit at what speed it could aproach at (barring light speed of course)

[iglak]

why can't we escape the earth's gravitational pull at a velocity less than 11km/s? what causes escape velocity?

 

 

 

P.S. on "Myth Busters" (a TV show on Tech TV), they found out that terminal velocity for a penny is approximately 74 mph (because of all of the air resistance), which is not enough to pucture skin, break bones, or tear muscles... just an interesting little tidbit....

[Janus]

iglak said in post #4 :

why can't we escape the earth's gravitational pull at a velocity less than 11km/s? what causes escape velocity?

 

 

 

 

Imagine you are throwing a ball up in the air. As it leaves your hand it starts with an upward velocity, but immediately starts to lose it due to gravity as it climbs. Eventually it loses all its velocity, comes to a stop and then begins to fall back down.

 

The greater the velocity it has when it leaves your hand, the higher it will travel before falling back.

 

If Earth's gravity reached to infinity and maintained the same strength the whole way, there would be no speed at which you could throw the ball that it wouldn't fall back. But Earth's gravity doesn't do both. It does reach to infinity, but it gets weaker the further from the center of the Earth you get. (By the square of the distance; at twice the distance of the Earth's surface, it has fallen to 1/4 strength)

 

Thus, when you throw the ball upwards, as it climbs, the force of gravity pulling it back weakens as it gets further from the Earth. As long as you throw it at less than 11km/sec, the Earth will win out and eventually slow the ball to a stop and pull it back. But at 11km/sec, something happens, the ball gains height and the pull of gravity weakens faster than the ball loses speed.

 

As the ball climbs, it still loses speed, but at a smaller and smaller rate. No matter how high it has climbed it still has some speed left and it will never fall back to the Earth.

 

Another way of looking at Escape velocity is that it is the speed that an object dropped from an infinite distance would be traveling when it hits the surface of the Earth. (neglecting resistance)

[iglak]

ah, okay. so spaceships lose those big fuel tanks once they are empty, and at that point they've passed that speed by a bit.

 

so then, you don't need to actually be going that fast, just at that point you can start cooasting, and you will keep going farther away from earth. i get it.

 

so, is that speed the escape velocity from ground level, or somewhere high above it (where the spaceships generally run out of fuel)?

[Janus]

iglak said in post #6 :

ah, okay. so spaceships lose those big fuel tanks once they are empty, and at that point they've passed that speed by a bit.

 

so then, you don't need to actually be going that fast, just at that point you can start cooasting, and you will keep going farther away from earth. i get it.

 

so, is that speed the escape velocity from ground level, or somewhere high above it (where the spaceships generally run out of fuel)?

 

Escape velocity is usually figured as from the point of departure, in this case, the surface of the Earth, even if the rocket never quite reaches that velocity.

 

For instance, assuming a rocket fires its engines until it reaches an altitude of 300 km (About Leo) . At the Surface of the Earth, Escape velocity is 11.205km/s. At 300km its is 10.950km/s. So the rocket would only have to attain 10.950km/s to escape. But the total amount of energy the rocket had to expend to do this is the same as it would have been to give the rocket a velocity of 11.205km/s at ground level.

 

It is actually more advantagous to reach escape velocity as soon and as low as possible. When you burn most of your fuel close to the ground, you use less fuel to get the same effect because you are using less fuel to lift fuel. (If you burn your engines until you get to 300km, you have to lift at least some of the fuel to 300km, and that takes extra fuel as opposed to burning all your fuel before you reach 10km, where you only have to lift fuel a maximum of 10km.)

[Radical Edward]

escape velocity is a bit of a useless term really. really what you should try to think of is escape energy; the kinetic energy of the escaping object has to be greater than the potential energy of the object it is escaping at infinity.