Friction and terminal velocity

In a nutshell

Friction is a force that resists the sliding or rolling of one solid object over another. Terminal velocity is the maximum possible velocity an object has as it travels through a fluid.

Friction

Friction is a force that resists sliding or rolling of one solid over another. Friction will always act in a direction to oppose the motion of a moving object, and will act to slow objects down and stop. It is therefore necessary to have a driving force to keep the object in motion.

There are a number of conditions for the driving force which dictate the movement of the object:

If the driving force is equal to the friction force, the object will move at a constant speed.
If the driving force is greater than the friction force, the object will accelerate.
If the driving force is less than the friction force, the object will decelerate.

Friction generally occurs between two solid surfaces in contact, such as the tyres of a moving vehicle and the road. Drag occurs when an object passes through a fluid, such as a boat moving through water.

Note: Air resistance is a type of drag.

Example

A ball thrown in the air will experience drag or air resistance as it passes through the air, causing it to slow down.

Terminal velocity

Terminal velocity is the maximum possible velocity an object has as it travels through a fluid. An example of terminal velocity is that of a skydiver jumping out of a plane:

1	As soon as the skydiver jumps out of the plane, the skydiver begins to accelerate as their force due to gravity (weight) is much larger than the air resistance.
2	The skydiver's downward velocity rapidly increases, which increases the air resistance. This causes the downwards acceleration of the skydiver to slowly decrease.
3	The downwards acceleration of the skydiver decreases gradually, until the point where the driving force (weight) is equal to the air resistance. The skydiver has reached terminal velocity.
4	The skydiver opens their parachute. This causes the air resistance acting on the skydiver to sharply increase. The force due to gravity is now much less than the air resistance, so the skydiver begins to decelerate.
5	As the skydiver's velocity decreases, so does the air resistance acting on them. The air resistance decreases until it is equal to the skydiver's weight. The skydiver has reached a new, lower terminal velocity.
6	The skydiver falls slowly to the ground.

In summary, skydivers use parachutes to greatly increase the drag acting on them, which acts to decrease their terminal velocity to a value which is safe enough to fall to the ground to.

Moving vehicles can also have terminal velocity as they travel through the air. For a car, the driving force is generated by the engine, and the resistive forces are friction and air resistance.

Note: The amount of drag that is generated depends on the cross-sectional area of the object.