Facts on Inertia



  • Everything that is standing still has inertia, which means that it will not move unless forced to.
  • Everything that moves has momentum, which means that it will not slow down or speed up or change direction unless forced to.
  • There is no real difference between inertia and momentum, because everything in the Universe is moving. Things only appear to be still because they are not moving relative to something else.
  • An object’s momentum is its mass times its velocity.
  • Something heavy or fast has a lot of momentum, so a large force is needed to slow it down or speed it up.
  • A ball moves when you kick it because when a moving object strikes another, its momentum is transferred. This is the law of conservation of momentum.
  • Angular momentum is the momentum of something spinning. It depends on its speed and the size of the circle.
  • When a spinning skater draws his arms close to his body, the circle he is making is smaller yet his angular momentum must be the same. So he spins faster.
  • For the same reason, a satellite orbiting close to the Earth travels faster than one orbiting farther out.
  • A spinning top stays upright because its angular momentum is greater than the pull of gravity.
  • If a spinning top is given a knock it will go round at a slant. If spun with a slant at the start, it will quickly right itself until halted by friction.