Many of us view gravity as a special force existing between objects. When we see a ball drop, we assume that there is an invisible bond between the ball and the Earth which pulls them together. However, no one can actually detect this bond directly.
Einstein began to change this perception of gravity when he introduced the principle of equivalence. To put it simply, this principle tells us that there is no difference between the forces of gravity and acceleration.
Acceleration is when an object picks up speed. Humans can sense acceleration. When you ride in an elevator you feel the force of acceleration. In the brief moment between when an elevator is at rest and when it is moving upward at a constant speed, passengers on board the elevator experience acceleration. They feel they are being pushed towards the floor. This push lasts only a second or two until the elevator reaches its regular traveling speed it no longer accelerates. If for some reason the elevator kept on picking up speed, the passengers would continue to feel the push of the elevator floor against their bodies. It would seem to them as if they had gained weight and in fact they would have. If you stand on a scale inside an upward accelerated elevator, you will actually weigh more than you normally do!