# Friction and Terminal Velocity

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## Friction and Terminal Velocity

#### Friction

Friction is caused by rubbing. It can be the surfaces between two solids rubbing, a solid surface and a liquid or a gas, etc. Anything! When friction is caused by fluids (liquids or gases) we tend to call it drag or air resistance.

A good Physics phrase to use in exams is - "friction dissipates energy." That means energy changes from kinetic energy to heat energy, where it is lost to the surroundings.

It is important to remember that friction (or drag) increases as speed increases.

Here's a thought experiment for you...

Then:

Stick your head out of a car window (carefully!) when you're moving.

Then:

Stick your head out of a plane window next time you go to Spain or somewhere for your holidays.

Did you notice the increase in drag with speed?

#### Terminal velocity

One particular example of this is that when objects accelerate towards the ground due to gravity they experience an increasing air resistance force up. When the air resistance force up has grown so big that it matches the weight down there is no resultant force and therefore no acceleration. The object will travel at a constant speed. This is called the terminal velocity.

It's not just falling objects that have a terminal velocity. You have one when you run! Think about it.