More on Satellites using Newton's Laws
Escape Velocity: A Thought Experiment
When an object is
thrown upwards, we know by experience that it falls back to earth.
How is it that we can launch rockets into space? We will investigate
this question.
Let's start with a mental experiment. When you throw a stone upwards it eventually falls back to earth. Does it make a difference how hard you throw the stone? Will it go higher if you throw it harder? Throwing the stone harder means a larger `initial velocity'. How hard would you have to throw to prevent the stone from falling back to earth?
The concept of excape velocity is simply an extension in understanding orbits. As we increased the speed of the steel ball shot from the cannon, we put it into orbit, and we can calculate EXACTLY what speed the cannon ball must travel to stay perfectly in orbit. We can also calculate the exact speed needed so that the cannon ball NEVER RETURNS. If a rocket leaves the Earths surface going faster than 11.5 km/sec, thats 25,039 mph (Earth's escape velocity), it will leave the Earth's gravity forever. The space shuttle and all Low Earth Orbiting (LEO) satellites move at a speed of approximately 8 km/sec (5 miles per second). Obviously, they do NOT have sufficient velocity to leave Earth's orbit.
So, how do we accelerate Rockets to such high speeds?
All
satellites today get into orbit by riding on a rocket or by riding in
the cargo bay of the Space Shuttle. Both work on the physical concept
of thrust.
Thrust is a measure of how much force an engine can push with. Thrust
is equal to the exhaust velocity times the amount of mass that exits
the rocket every second (mass x velocity per time). using Newton's
third law of motion, the Rocket will move in the opposite
direction of the thrust with the same force (mass times acceleratoin).
Obviously, the rocket weighs more, so the velocity of
exhaust from the rocket far exceeds that of the rocket. Rockets need a
lot of thrust in order to escape Earth's gravity and atmosphere.
Once a spacecraft is in interplanetary space, having a lot of
thrust is not as important. Several countries and businesses have
rocket launch capabilities. Satellites as large as several tons
regularly make it safely into orbit.
For most satellite launches, the rocket is aimed straight up at first. This gets the rocket through the thickest part of the atmosphere most quickly and best minimizes fuel consumption. In most cases, the flight plan calls for the rocket to head east because Earth rotates to the east, giving the launch vehicle a free boost. The strength of this boost depends on the rotational velocity of Earth at the launch location. The boost is greatest at the equator, where the distance around Earth is greatest and so rotation is fastest.
Can you imagine why the government or private agencies might want a launch going in the North-South direction?
How far above the earth's surface is the Space Shutte flown? How about the Global Position System? How about Geostationary orbits? See here