• Question: Is it true that if we stopped revolving around the sun, that we would fall into it?

    Asked by ibityourfinger to James, Marcus, Martin, Rob, Suzanne on 18 Mar 2012.
    • Photo: Suzanne McEndoo

      Suzanne McEndoo answered on 18 Mar 2012:

      I think so. The sun is the biggest thing anywhere near us, so it puts out a huge gravitational force, constantly pulling us towards it. If the earth was just sitting here, not moving, it would just get dragged closer and closer to the sun until we fell in. (Luckily(?), we would have moved out of the habitable zone of the solar system long before that, so we would already be long extinct.)

      Because we’re moving around the sun, the pull of gravity is always at a right angle to our direction of movement. So we feel the sun’s gravity force, and it changes our velocity with is speed in a particular direction (this is what newton’s second law tells us will happen), but instead of changing our speed, it changes our direction.

      Because of this circular motion, we experience a centrifugal force that pushes us away from the sun. If you want to see this in motion, take a bucket of water and swing it in a circle fast. Even if the bucket is upside down for part of the swing, the water stays in it, because of the force pushing it away.

      (Don’t try this indoors, or near any windows! Outside in summer, close to a change of clothes and a towel would be best!)

    • Photo: James Boone

      James Boone answered on 18 Mar 2012:

      Planet earth is effectively in a balance of two forces, one that is in a direction away from the sun caused by our orbit around the sun. The equation that tells us this force is F=(mv^2)/2. This is balanced by the force of the sun’s gravitational pull on earth which is given by F=GMm/(r^2), where big M is the mass of the Sun and little m is the mass of the earth and r is the distance between sun and earth. So if you take away the force away from the sun caused by our orbit, we’d be left with only he one pulling us towards the sun and we’d head straight towards it.

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