If two identical balls are connected to the same very long rope, and one of the balls are floating in space and the other one is hanging above earth’s surface. What would happen?

Question

So my 7-year old asked me this and as much as I would love to be able to give him a proper answer, I couldn’t.

Does ball A (in space) float away, taking ball B (above earth’s surface) with it? Will the balls stay somewhat fixed or will ball B force ball A down?

In: Physics

Answer 1

It depends on the rope and just where in space the ball is floating.

The big problem is that if you are just as high up as the International space station, you would need a 400 km long rope to reach the ground.

Any normal rope that long would rip apart under its own weight, but you could probably make one from Kevlar or something that might work.

But there is another even bigger problem the ISS and the space ships flying to it may look like they are just floating in space, but they are actually moving very quickly compared to the ground beneath them.

Objects that high fly sideways at speeds that would be 22 times the speed of sound here on earth.

So what would happen that the drag of the weight of the rope and the lower ball and the air resistance of the lower ball and the part of the rope in atmosphere would either slow down the upper ball until it fell down from orbit or it would simply snap the rope under the strain.

Even if you could keep the rope from snapping you would still have to deal with the upper ball being slowed down by the air resistance of the lower ball and the lower ball being speed up by being dragged by the one above.

It is something that would be very destructive to anything around it.

One way to escape that problem of one part going very fast is to simply take the upper ball and take it much much higher into an obit where it always was above the same patch of ground.

This way you would have no speed difference between the ball near the ground and the one in orbit.

This sort of construct is more or less what we call a space elevator and it would work the way you would want it to. You have a rope that you could climb up to get to the the ball floating in space.

The problem is that instead of a few hundred kilometer long the rope would now need to be 36 thousand kilometer long.

We don’t really have any material that could be used to make a rope that long. Some think that some sort of fancy carbon nanotube fibers could work for this and some tricks that involve tapering one end and some other stuff.

We can’t build anything like that yet, but it is a serious idea that could work in theory.