Oh, and I'm not removing the possibility that there is something other than just a gravitational force due to mass of the spaceship. Of course this spaceship could be even smaller if it had a higher density. Just for comparison, this is about the mass of many of the large asteroids with a radius of around 70 km. Putting in my values for the mass of the Earth ( m E), height of spacecraft ( h), and radius of the Earth ( R E) I get a spaceship mass of 3.7 x 10 18 kg. In this case, I can solve for the mass of spacecraft by setting the two gravitational forces on a building equal to each other. We don't really get a good view of this spacecraft, so I'm going to guess it is 5,000 meters above the surface of the Earth (it's probably much higher if it is really super big). Sure, the spaceship is closer but it's going to have to be massive to have a significant effect. In order for the building to be lifted, the gravitational attraction to the spacechip must be at least as large as that of the Earth. Now let's put a big spaceship (with a super-large mass) overhead. The only option is for the building to be at rest. If the building moves down, the ground force will increase (like a spring) and push harder on the building. Of course balanced forces also could mean the object is moving at a constant speed, but if the object moves up it will lose contact with the ground and there will no longer be a force pushing up. These forces are balanced and the building is at rest. Hopefully the distance is much greater than the size of the objects so you can just use the center-to-center distance. r is the distance between the two objects.m 1 and m 2 are the masses of the two interacting objects.This has a value of 6.67 x 10 -11 N*m 2/kg 2. The magnitude of this force can be written like this: The amount of twisting is related to the gravitational force between these masses. Place two large masses near them, and the gravitational force is strong enough to move the bar, twisting the wire. The idea is to place small masses on a bar suspended by wire. It's named for Henry Cavendish, who used it to determine the gravitational constant. This is a picture of a Cavendish torsion balance. However, there is an experiment that allows you to measure these forces. We don't normally notice these attractive forces because the magnitude is tiny. The gravitational force is an attractive interaction between any two objects with mass. But if a human exerts a gravitational force on the Earth, does a human also exert a force on another human? Yes.
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