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Speed of someone sitting in a car

  1. Nov 16, 2012 #1
    I was just doing a thought experiment..

    So when you are sitting in a car that is moving, your speed relative to yourself inside the car is zero, but to someone outside watching you, you are moving.

    However, when you put on the breaks really quick, your will lean forward. How can you have zero velocity, yet have inertia?
  2. jcsd
  3. Nov 16, 2012 #2


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    Because relative to you, the car suddenly got negative velocity. Your inertia, in that frame of reference, isn't tendency to keep moving. It's the tendency to stay at rest while the car is trying to accelerate you backwards.

    The question of why velocity is relative and acceleration is absolute is a much more interesting one. It made Einstein hesitant about publishing General Relativity. The alternative is Mach's Principle, which states that acceleration is relative to other massive bodies. That would mean that inertia is a tendency to carry on at the same speed relative to other massive bodies in the universe. There is no experimental evidence to support it, however.
  4. Nov 16, 2012 #3

    Simon Bridge

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    1. inertia is independent of speed... it's just mass.
    2. you have changed from an inertial to a non-inertial reference frame ... in you own frame, the car is accelerating backwards - and pushing on you, so you are also accelerating backwards (a little late so there is a difference in speed). The person on the road sees the car going forwards slower than you.
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