# Q & A: Water Friction?

Q:
We had a project on Friction so far. Well i know what causes it, but can you explained Body travelling through water. What happenes at the body’s surface? What effect does the depth of water have?
- Jian (age 15)
Glen Waverley
A:
Jian -

When an object (say a boat) moves through water, there certainly is a type of friction. The layer of water near the boat at least partly moves along with the boat, and rubs against other parts of the water. The water exerts a net drag force on the boat, and tends to make the boat move along with the average water flow, or stay put if the water isn't going anywhere. The water will heat up slightly as energy from the big motion gets lost in energy of the jiggling water molecules.

This friction in liquids is different from friction between solids in an important way. Between solids, friction can be big when they aren't sliding and drops some when they start to slide. In a liquid, the faster something moves through the liquid the more friction there is.

If the boat isn't streamlined, it also has to push the water out of its way, increasing the drag force. The shape of the boat is really important here. The more streamlined the boat is, the less will need to be pushed aside. When the boat starts going fast, the force needed to push the water aside can become much larger than the
force needed to drag the water along the sides of the boat.

The deeper you get underwater, the denser the water gets. That is, further down, the water is more packed together - the molecules are more closely crowded. I guess a little increase in density should affect the water's viscosity- how hard it is to drag
things through it. If you go really deep, that packing increase is enough even to noticeably increase the water's density itself, so there's more to push aside. So the deeper down you get, the more force there is resisting the boat's motion. To find out more about the math involved here, you can look at the answer to Calculating Underwater Pressure.

-Tamara

(published on 10/22/2007)

## Follow-Up #1: measuring friction underwater

Q:
how to calculate the frictional force acting upon a body which is moving completely underwater ? please do reply to my doubt !!
- keshav (age 22)
bangalore, karnataka , india
A:

There are simple ways to measure friction even deep under water. Most objects are different densities than water, so gravity exerts either an upward or downward force on them. The speed they go then will depend on how much friction there is. To measure the friction at greater speeds, one might give the object a big push and then measure how it gradually slows down.

Mike W.

(published on 10/30/2013)