# How does a the vertical force of gravity end in horizontal motion?

## Main Question or Discussion Point

When a stream of water falls perfectly vertical, some water hitting the surface will disperse horizontally. What is the force that causes this? References would be awesome!

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A.T.
What is the force that causes this?

berkeman
Mentor
some water hitting the surface will disperse horizontally. What is the force that causes this?
To help you see why the reply by @A.T. is correct, you can picture two squishy rubber balls, dropped side-by-side touching each other. When they impact the ground at the same time, they both deform during the bounce and bulge outward on the sides. This causes the two balls to push on each other during the bounce, causing them to bounce apart instead of straight back up.

To help you see why the reply by @A.T. is correct, you can picture two squishy rubber balls, dropped side-by-side touching each other. When they impact the ground at the same time, they both deform during the bounce and bulge outward on the sides. This causes the two balls to push on each other during the bounce, causing them to bounce apart instead of straight back up.

Wow that really helped me understand the concept. Now when I searched for radial pressure, I only found engineering topics. Is radial pressure not encountered in a physics curriculum? How would this apply to water? Do the individual atoms bounce apart from each other too?

A.T.
Do the individual atoms bounce apart from each other too?
On the atomic level that's what happens. On the macroscopic level we use the concept of pressure which is the average result of all this bouncing.

I would prefer to express it as Radial external pressure differences.

In other words, among others, It is the boundary conditions that determine the movement of the droplet, or of the parts of it.

A.T.