Escape velocity from the surface of the Earth is about 11.2 kilometers per second (or just over forty thousand km/h, or twenty five thousand mph), meaning that if you want an object to leave Earth and never fall back, you have to throw it at least that fast. How does that work? Well as you’ll remember from high school physics, Newton’s law of universal gravitation tells us that an object’s weight (The downward force applied by gravity) depends inversely on the square of the distance from the Earth’s centre. If you double this distance, the downward force is quartered. Increase it by three times, and the force is reduced to a ninth and if it’s increased by four times, the force is reduced to a sixteenth. This law also tells us that the gravitational influence extends outwards to infinity – you’re NEVER free, so that any object moving away in freefall will be constantly slowing down until it eventually stops and starts falling back. But if your starting speed is high enough, then the downward force will weaken fast enough to never quite slow the object all the way down. It will never come to a stop, even though it continues to decelerate at a slower and slower pace. The exact speed required for this to happen is the escape velocity. Incidentally, the actual speed required depends on your starting point – if you start from a very high altitude, your escape velocity is lower because the gravitational force is already quite weakened. But on Earth, the few kilometers you can gain by moving from sea level to the top of the tallest mountains make only a negligible difference.
Written by Allen Versfeld
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