## Principle of equivalence

The starting point for general relativity is a statement called the principle of equivalence, which states that a uniform gravitational field in some direction is indistinguishable from a uniform acceleration in the opposite direction. Remember, an accelerating reference frame introduces pseudoforces in the direction opposite to the true acceleration of the reference frame. For example, if you are driving in a car and step on the brakes, the car has a backward acceleration. Inside the car, you have a forward acceleration relative to the car.

We can illustrate the principle of equivalence by looking at the forces on a person standing on a scale in a elevator, as illustrated in Fig. 8.4. In the first case, we have the elevator being supported so there is no acceleration, but there is gravity. We take the acceleration of gravity to be —g. (Upward forces and accelerations are positive; downward forces and accelerations are negative, and we have

Real Forces Gravity Acceleration

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