I fill a bucket with water from a tank by gravity, but what happens to the gravity?
@donferrell333, via Twitter
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Nothing happens to the gravity. It just keeps pulling. The darn bottom of the bucket just got in the way.
Pat French
Telford, Shropshire, UK
Throughout the process of filling your bucket, gravity doesn’t change. The same gravitational force pulls in a straight line from the centre of Earth to its surface, to the bucket, to the water in the tank, even to the moon and as far as we know to the edge of the cosmos. It has been acting on Earth since it formed and is related to the mass of the planet.
Gravity was there before you opened the tap. It imparted the energy that caused the water to fall into your bucket, but was unchanged in the process.
One way to conceive of this is to think of gravity as a riverbed. The water comes and goes, but the riverbed remains.
Alex Wilkins
91av reporter
A force field – in gravity’s case, a gravitational field – is something that extends throughout the universe and interacts with any particle that can interact with that field. A gravitational field interacts with all known particles.
This force field can’t be depleted. It will always move a particle according to the field’s strength at a particular point. This strength is dictated by the distribution of mass in space. So, if water moves from a tank to a bucket, this will infinitesimally change the distribution of mass in and around Earth, and very slightly change the strength of the gravitational field.
Earth is so much more massive than the amount of water moved that there has essentially been a negligible change in the force field. The field will always be strong enough to move water from the tank to the bucket. If the tank and bucket were big enough to move sufficient water, that might change the gravitational field, but you would need a lot of water!
Richard Swifte
Darmstadt, Germany
Gravity is one of the fundamental forces of nature and a property of everything composed of matter. Nothing ever “happens” to it – it simply exists. It might be better to ask what the effect of gravity is in this situation. The water falls downwards into the bucket. Therefore, to preserve the centre of gravity of Earth, the planet will move upwards by an immeasureably tiny distance.
Ron Dippold
San Diego, California, US
There are two answers to this. The first is that you did change the gravity field a teeny bit when you filled the bucket with water. Gravity emerges from mass, and you moved a minuscule bit of mass lower, making an infinitesimal dimple in Earth’s gravity well larger at the expense of the tank’s teeny gravity well.
The second view is that you spent gravitational potential energy – the amount of energy it takes to lift something in a gravitational field. As an example, 12 litres of water that is 3 metres above the ground has about 350 joules of potential energy. If you pour this water into a bucket on the ground, you have spent that 350 joules (enough to keep a 1600 lumen LED bulb lit for about 15 seconds).
But where did that energy go? Into water turbulence, friction and heat. The gravitational energy has now gone, but if you had a little turbine, you could have extracted maybe 80 per cent of that. This is how hydropower works and why pumping water higher is a popular energy storage method.
Nick Canning
Coleraine, County Londonderry, UK
Because the mass of water in your bucket is negligible in comparison with the mass of Earth, the change in the gravitational field associated with bucket filling is undetectable.
Swap the water for a planet-sized object, however, and the change in the field with time becomes significant. The dance of two colliding planets is complicated to calculate, but it can be done using Isaac Newton’s law of universal gravitation. Make the objects even more massive – by turning them into black holes, perhaps – and the dance requires Albert Einstein’s general relativity to calculate the effect of gravity with time.
But what is gravity? A force, a thing that mass has, a field, the result of the stress-energy tensor, curvature of space-time? All of the above? It is mysterious.
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