# I Ball falling off a cliff.

1. Feb 24, 2017

### FallenApple

So a ball is rolling off a cliff. The red is the proper acceleration and the blue is the geometric acceleration. How is it in the rain frame that the surface is moving down initially?

Consider the situation where a rain drop falls straight down, with no ball rolling, just the ground right below. When the raindrop falls, since gravity is a pseudo force, we can consider the raindrop as stationary and the floor as accelerating upwards to meet the raindrop. This makes sense because the surface of floor will have an unbalanced pressure from the bottom pushing to the top, making the patch on the surface have a proper acceleration vector of up, accelerating it though spacetime to meet the rain drop. Hence from the rains perspective, the ground needs to be seen moving up.

So why all of the sudden, when there is a ball rolling off of a cliff, the situation is changed? What if the ball is just not rolling?, then the ground would be accelerating away from the raindrop forever, which doesn't make sense.

Is it because the water drop has to hit the floor of the top of the cliff after the ball has fallen off in order for the whole frame to be internal? So while the water is in free fall, the ball still needs to fall off first. Could this mean that the whole floor has to move away to make sure that the ball leaves the cliff before the floor converges with the raindrop in spacetime?

Here is the site

https://en.wikipedia.org/wiki/Proper_acceleration

Last edited: Feb 24, 2017
2. Feb 24, 2017

### Orodruin

Staff Emeritus
Did you read the note below the image?

3. Feb 24, 2017

### FallenApple

Yes. But it didn't explain why from the viewpoint of the rain, the floor must accelerate down first.

4. Feb 24, 2017

### Orodruin

Staff Emeritus
As said in the note, it is not the frame of a rain drop, rather it is a frame of a free falling ball that starts moving upwards in the Earth frame.