# Acceleration in Water compared to Air

1. Jul 2, 2009

### Rich M

If an object is dropped in air through a relatively short known height (say 2m) on earth, then ignoring any air resistance its velocity on impacting the ground can be calculated from standard laws of motion. Its kineteic energy at impact can then be calculated if its mass is known. My question is what would be the approach for calculating the velocity and/or kinteic energy for the same object dropped through the same height in water?

2. Jul 2, 2009

### fatra2

Hi there,

As a matter of fact, your object dropped into water will be subject to the same gravitational pull. The only difference is that water will resist the movement much more. But the equations are the same, with the added water resistance factor.

Ok for the simple theory behind it. Fact is that the resistance of air/water or any other medium cannot be considered constant, like we tend to do so in physics 101. The amount of friction developed by the surrounding material is dependant on the velocity of the object (dr/dt) and on a friction coefficient, which is not the same as $$\mu$$.

3. Jul 2, 2009

### maverick_starstrider

Comparing acceleration in air by "ignoring air resistance" to acceleration in water seems kinda like a cheat. Both air and water are fluids and the equations and approach one would use to model the resistance due to air or water are identical. However, in water these resistance effects are simply a lot more pronounced.

4. Jul 2, 2009

### Bob S

Need to add (subtact actually) buoyancy force. A ping pong ball will float.

5. Jul 2, 2009

### infomax

Adding the forces like vicous force , upthurst force , Gravitational force and any other force if present vectorically will help youfind the velocity , then the kinetic energy