So, the gravitational force is super tiny. The only way we ever notice this force is if one of the interacting objects has a super huge mass—something like the mass of the Earth (5.97 x 1024 kg). If you replace your friend with the Earth and put the distance between you and your friend-Earth as the radius of the Earth, then you get a gravitational force of something like 680 Newtons—and that is a force you can feel (and you do).

Is There Gravity in Space?

Now for the real question. Why do astronauts float around in space unless there is no gravity? It sure seems like there is no gravity in space—it's even referred to as "zero gravity." OK, I've answered this before, but it's important enough to revisit the question.

The short answer is "yes"—there is gravity in space. Look back at the gravitational equation above. What changes in that equation as you move from the surface of the Earth into space? The only difference is the distance between you and the center of the Earth (the r). So as the distance increases, the gravitational force decreases—but by how much does the gravitational force change? How about a quick estimation?

Let's use an Earth radius of 6.371 x 106 meters. With this value, a person with a mass of 70 kg would have a gravitational force of 686.7 Newtons. Now moving up to the orbital height of the International Space Station, you would be an extra 400 km farther from the center. Recalculating with this greater distance, I get a weight of 608 Newtons. This is about 88 percent the value on the surface of the Earth (you can check all my calculations here). But you can see there is clearly gravity in space.

Oh, here is some extra evidence. Why does the moon orbit the Earth? The answer: gravity. Why does the Earth orbit the Sun? Yup, it's gravity. In both of these cases, there is a significant distance between the two interacting objects—but gravity still "works," even in space.

But why do astronauts float around in space? Well, they float around when in orbit—if there was a super tall tower reaching into space, they wouldn't float around. The "weightless" environment is caused by the orbital motion of the people inside a spacecraft or space station. Here is the real deal. If the only force acting on a human is the gravitational force, that human feels weightless. Standing on a tall tower would result in two forces (gravity pulling down and the tower pushing up). In orbit, there is only the gravitational force—leading to that feeling of weightlessness.

Actually, you don't even need to be in orbit to feel weightless. You can be weightless by having the gravitational force as the only thing acting on you. Here is a situation for you to consider. Suppose you are standing in a stationary elevator at the top of a building. Since you are at rest, the total force must be zero—that means the downward gravitational force pulling down is balanced by the upward pushing force from the floor. Now remove the force from the floor. Yes, this is difficult but it can be accomplished. Just have the elevator accelerate down with the same acceleration as a free falling object. Now you will be falling inside an elevator. The only force is gravity and you will be weightless.

Some people think this falling elevator is fun. That's why many amusement parks have a ride like The Tower of Terror. Basically, you get in a car that drops off a tower. During the fall, you feel weightless—but you don't crash at the bottom. Instead, the car is on a track that somehow slows down more gradually than if it smashed into the ground. They have one of these types of rides at the NASA center in Huntsville. went on this with my kids—it was actually scarier than I had imagined.