You are in a car and there is a pendulum suspended in front of you. You have no idea of what is going outside, i.e., you are completely blind from the surroundings. You can see only the pendulum and the interior of the car which has nothing else. You find that the pendulum is at the beginning straight down and later you find that it is bending towards you. (I hope you understand) Can you conclude that the car is accelerating?
Hootenanny
Oct10-08, 12:54 PM
You are in a car and there is a pendulum suspended in front of you. You have no idea of what is going outside, i.e., you are completely blind from the surroundings. You can see only the pendulum and the interior of the car which has nothing else. You find that the pendulum is at the beginning straight down and later you find that it is bending towards you. (I hope you understand) Can you conclude that the car is accelerating?
No, the car could be travelling up an incline at constant velocity for example.
Georgepowell
Oct10-08, 01:30 PM
Can you conclude that the car is accelerating?
No, the car may be accelerating, but it is also possible it is stationary on a slope.
If you are in a windowless room, and the room tilts backwards, there is no possible scientific experiment you could do to discover weather it is gravity pushing you to the side of the room, or if you are simply accelerating.
There is symmetry between acceleration and gravity.
Naty1
Oct11-08, 09:01 AM
there is no possible scientific experiment you could do to discover weather it is gravity pushing you to the side of the room, or if you are simply accelerating
Actually there might be!...an effect discovered by Bill Unruh! Measurements of environmental temperature would reveal a gas of "hot" photons not present in a gravitational field. But this is very,very subtle.
The Unruh effect, which Unruh discovered in 1976, is the prediction that an accelerating observer will observe black-body radiation where an inertial observer would observe none. In other words, the accelerating observer will find himself or herself in a warm background.
http://en.wikipedia.org/wiki/Bill_Unruh