# Speed of gravity (water balloons)

I watched this video: popped water balloons

And as you see when the needle pops the balloons the water stays in the same shape as the balloon for a few fragments of a second.

The force impacting the water after the balloon has been popped must be gravity, and since the water stays in the same shape for a while must mean that the water "doesn't know" that it's gravity it should "listen" to now during that time. What does this say about the speed of gravity? How slow/fast is it and what happens with larger/smaller objects?

One thing holding the water together could be air pressure, but what happens with if you do the exact same thing in vaccuum? Will it stay in the same shape for a few moment or will it dissolve at the "speed of gravity"?

Let's say it stays the same, what would that mean for earth if the sun disappeared? Would we feel the gravitational reaction way after the light went out?

• You may also like to watch this video where a very long "slinky" toy has a similarly counterintuitive behavior when you look at it in slow motion, but ultimately in both cases you can reason out that it's simply obeying Newton's laws, there is no "speed of gravity" that affects these situations. (The speed of gravity is in fact the same as the speed of light and is much, much faster than any of the things happening in these videos.) Commented Dec 20, 2016 at 23:13
• It's worth noting that the frame rate of the video (during the slow motion segment) changes, so the last section is much less slowed-down than the section before it. That's what gives this weird effect of the water hanging there for a bit a d then starting to fall suddenly.
– user107153
Commented Dec 21, 2016 at 1:04

Inertia is what it's called. It says that bodies do not start moving instantaneously. There are many good answers here that I will not repeat, but I suggest looking at those in this question.

Note also that the movie shows artificial slow-downs in order to distinguish very fast phases of the movement.

Unfortunately, this video has nothing to do with the speed of gravity, since only a negligible amount of mass is displaced as the rubber of the balloon retracts. Instead, you can explain this entirely from a Newtonian standpoint.

The key here is that the force keeping the balloon from popping (i.e. the restoring force in the rubber of the balloon) is much stronger than gravity, and the rubber in question is thin, and therefore much lighter than the water. Applying $F=ma$, we can see that the acceleration experienced by the rubber of the balloon is much greater than the acceleration of the water (as there is higher force and lower mass in the former case).

The other key is that the video (or at least the part you're talking about) is shot in extremely slow motion. Applying kinematics, we see that for zero initial velocity, the position of the water in the balloon should be $y=y_0-gt^2$. For very small $t$ (as happens in slow motion), we see that $y\approx y_0$, so the balloon should not move in such a short timespan.

If you do the exact same thing in vacuum, you will get slightly different results, mainly because the balloons would have already popped before you could start due to the lack of outside air pressure to balance the pressure of the rapidly-boiling water inside. If you managed to get stronger balloons, the water would begin to rapidly boil once the balloons popped and the surface contacted the vacuum. It would take a while for the water to completely vaporize, though.

The sun disappearing is a very different situation - in that case, a significant mass (somewhere around $10^{30}$ kg) is being removed from the situation, so general relativity applies here. In this case, we need to take into account that, as recently confirmed by LIGO, gravitational disturbances propagate at the speed of light. This means we would feel the gravitational reaction exactly as the lights went out.

Gravity is already continuously working on the water even before the balloons are popped. Otherwise they would not hang down. There is no lag in communication. Communication would have been necessary if the gravity did not act already before they were popped. Popping the balloons only frees up the water which starts accelerating right away. This does not say anything about speed of gravity.