Where is gravity the strongest?

27 March 2011

Question

Where on the Earth would you experience the strongest and weakest gravity?

Answer

We put this to Dominic Ford from the Department of Physics in Cambridge...

Dominic - The principle of physics that you need to work out the gravitational field around the distribution of matter is Newton's Law of Gravity. What that says is that every piece of matter in the universe attracts every other piece of matter in the universe with a force that decreases with the distance between the two masses, but it increases with the mass of those objects. So, when it comes to the whole Earth, you have to add up the forces from all of the little bits that make up the sphere of the Earth, to work out what the total net force is, and that's actually a mathematical problem that gave Newton quite a headache when he was formulating his Law of Gravity and it led him to pioneer a new mathematical technique that we call calculus to add up those little forces.

But even though the math itself is quite tricky, it's fairly easy to see roughly what the answer must look like because if you imagine that you burrow down into the Earth, you've then got some of the Earth above your head, and the rest of it below your feet whereas the before, the whole Earth was pulling you in one direction downwards. So when you burrow down into the Earth, the gravitational forces are cancelling out and that means there must be a weaker gravitational field down inside the Earth than there is on the surface.

Similarly, if you travel upwards into space or climb a high mountain, then the Earth is further away and that means its gravitational pull is weaker, and so, you will weigh less.

Diana - Standing at the top of Mt. Everest might make you feel a little bit lighter and that's in more than one way, I think! But what might make us feel a stronger pull?

Dominic - There are variations in the gravitational field across the surface of the Earth and that's actually a way that people look at the geology and the rocks that the Earth is made of. So, if you're looking for a particular kind of rock, you can look for variations in the strength of gravity that tell you that you've got denser rocks or less dense rocks, and that might tell you about the rock composition below your feet.

The Earth does bulge out at its equator, but in fact, the amount by which it bulges out is exactly the right amount to cancel out the centrifugal force from the Earth's rotation. So, in terms of the downward force that you feel, it's the same all over in the surface of the Earth.

Diana - Certain rocks give certain areas of the earth's stronger gravitational pull and the bulge at the earth's equator counteracts the centrifugal force of the earth's rotation, sounds simple. On Facebook, Steven Duncan said that you should place your bathroom scales on the roof rather than in the basement. Shawn Hoskins said that the Naked Scientists ought to fund a field trip for listeners to travel the globe to find out!

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