You Can Play With Escape Velocity—Without Leaving the Planet

WIRED | 3/22/2019 | Rhett Allain
malik778 (Posted by) Level 3
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If you like cool-sounding science terms, escape velocity should fit the bill. But what the heck does it even mean? Allow me to explain.

Surely, you have heard this before:

What goes up, must come down.

How about an experiment to see if this is even true? I'm going to take a ball and toss it up. Let's see what happens. You can try this at home.

That seems to work. What if I throw it even higher?

It still works. It goes up. It comes down. But why? Here is a force diagram for the ball right after I throw it up.

Ball - Arrow - Velocity - Velocity - Force

Since the ball is moving upward, I put a dashed arrow to represent the velocity. I just want to point out that velocity is not a force. The real force is the downward "mg" arrow. This is the force due to the gravitational interaction between the Earth and the ball. In this case, the force depends on the mass of the ball (m) and the local gravitational field (g) with a value of around 9.8 Newtons per kilogram.

But what do forces have to do with motion? In general, forces change the motion of an object. Yes, the "change" part is super important. When a force acts in the opposite direction as the velocity, the object slows down. This means that the upward-moving ball slows down. With a constant gravitational force, the object slows down until it stops. After it stops, the downward force makes the object increase in speed since the force and velocity are in the same direction.

Ball - Force - Gravity - Stop - Fact

It doesn't matter how high you throw the ball, the downward force of gravity will eventually slow it down to a stop. Except this is wrong. In fact, the gravitational force is not constant.

We like to write the gravitational force on the surface of the...
(Excerpt) Read more at: WIRED
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