# Transmission delay of pressure in a fluid

1. Apr 18, 2012

### Gh778

I would like to know if the transmission delay of pressure give a differential force on an object ? Example: an object in water, we put pressure with mass over water at right at t=0s. The pressure at right is faster on the right side than the left side (d1<d2). The object seems to move with a force in water. The delay is very small but exist. Is this true ?

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2. Apr 18, 2012

In water, there is effectively no delay since the medium is incompressible. For something like air, which is compressible, this can happen.

3. Apr 18, 2012

### Bob S

See section on speed of sound in water in http://en.wikipedia.org/wiki/Speed_of_sound
Although the sound pressure wave may arrive at the two sides of an object at different times, if it is a sine wave, the net force difference over a full cycle is zero.

4. Apr 18, 2012

### AlephZero

The speed of sound in water is about 1500 m/s compared with about 340 m/s in air. Whether you think that is fast enough to ignore depends on the situation. Sonar woudn't work in a truly incompressible fluid, for example.

5. Apr 18, 2012