Comparison of small objects and planets?

We know here on earth that any object which has mass exerts a weight on the surface it is resting and the surface in turn counterbalances it with a normal reaction such that the object remains in equilibrium. Why doesn't the same hold true for big size objects like planets. Can't earth be thought of an object resting on space which is the surface. Shouldn't there be a normal reaction coming from space to counterbalance the weight of the earth. But we know that space does not contain any tangible matter to generate normal reaction. How come then the earth stays in equilibrium?

3 Answers

  • 1 month ago

    If the moon was stationary resting on the earth then the normal reaction would hold it up.  But it isn't stationary.  It is continually accelerating.  This acceleration requires a force.  The force of gravity.  Gravity pulls the moon down and the moon pulls the earth up.  ( hence the tides) but it is in equilibrium.  It doesn't get closer.  The reason for that is kinetic energy.  If it gets closer it gains kinetic energy. Which means it moves faster.  If it is faster it NEEDS more force to keep it moving in this smaller circle so that the moon then moves out again.  In this way it maintains a stable radius unless energy is removed.  Of course energy is being altered. But so slowly that it is not making large variations.

    In fact the rotation of the earth is being decreased which is giving MORE energy to the motion of the moon.  Eventually the earth may rotate once every 35 days or so and the same face faces the moon at all times.

  • 1 month ago

    One way to look on a problem is that weight of the Earth that acts toward the center of orbit is counterballanced by centrifugal force acting in opposite direction. That's similar to what happens when you're upside-down on a roller coaster.

    Another explanation is that Earth is constantly falling to the Sun but because Earth has tangential component od velocity, it always overshoots and falls into empty space behind the Sun. And continues to fall over and over again and misses every time.

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  • 1 month ago

    space is NOT a surface. 

    weight depends on gravity. an object in space has no weight as it is not resisting the pull of gravity.

    "earth stays in equilibrium" because the forces on it are balanced. The pull of the sun (gravity) is exactly balanced by centrifugal force in the opposite direction due to it's orbit around the sun.

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