# Vacuum Question

1. Jan 30, 2012

### wilha

Try to picture this.

When moving through space (i mean any space, not necessarily outer space) , does one create vacuum just by moving through it ?

i.e
When a car goes by at a high speed, does it create short vacuums as it passes by, and the air around it fill that void behind the car after it goes by ?

2. Jan 30, 2012

### Staff: Mentor

A slight vacuum, yes. A moving car creates an area of high pressure in front and low pressure behind.

3. Jan 30, 2012

### wilha

Lets say I am in a car and it is stopped at a red light, and a bunch of cars zip by next to my car but at really high speeds, the shaking experience of my car, is it cause by a "slight" vacuum, or something different ?

4. Jan 30, 2012

### bobc2

A number of years ago NASA put a structure in orbit called "Wake Shield." It was shaped like a large concave disk and its purpose was to create a better vacuum (lower particle density) behind the shield as it orbited the earth. They could use the volume behind the shield as a high vacuum laboratory in which they could do things like vacuum deposition on a target substrate or growing crystals. I never did hear of results from the experiments.

[Edit: I just found this link.]

http://ipp.nasa.gov/innovation/Innovation15/InnovWelcome.html

Last edited: Jan 30, 2012
5. Jan 31, 2012

### Dr_Morbius

The shaking happens because as the other car rushes past you there is lower pressure behind it. Your car is interfering with some of the surrounding air and preventing it from flowing into the low pressure area. As the surrounding air tries to rush into the area it flows around your car and pushes on it. As the other car drives off the low pressure area dissipates and the shaking stops. This happens very rapidly and doesn't last long. That's why you feel a sudden jolt and not a continuous jostling.

This effect can be quit pronounced behind a big rig truck and can be startling and dangerous. That's why I stay away from trucks when I'm on the freeway. Even when you're driving next to the back of a truck on the highway the turbulence caused by this effect can be severe.

6. Jan 31, 2012

### Danger

This is time to introduce a caution of the "Don't try this at home" sort. There is a very dangerous practice called "draughting", wherein someone tries to save a lot of fuel by tucking in behind a tractor-trailer on the highway. There is a very high chance of that resulting in death, so don't do it.

7. Jan 31, 2012

### Lsos

Essentially yes...but depends what you mean by vacuum. There's definitely an area of low pressure behind a moving car, and air does rush in to fil the "void". But it's by no means a complete absence of air. Just less of it. Even the best vacuums we can ever make have some molecues floating around, and the area behind a car is far from that.

8. Jan 31, 2012

### mozartkart

Hey bobc2, thanks for that link. Really interesting :D

9. Jan 31, 2012

Thanks guys