Terminal Velocity

Air Resistance (Drag)

When an object moves through the air, it experiences a friction force which will oppose the movement. We know that this force is called air resistance or drag. 

Like friction, the amount of drag depends on the speed of the object, but drag is also affected by the shape of the object; streamlined objects experience less air resistance. This is why high-speed trains and sports cars all have streamlined shapes so they do not experience as much drag. It is particularly important with fast-moving objects because drag increases with speed.

Terminal Velocity

If an object is falling through the air, it experiences two significant forces: the pull of gravity and the opposing drag force.

When the object first starts falling, the drag force will not be very large compared to gravity. The resultant force is downwards so the object will accelerate.

As the objects velocity increases, so does the drag force. Eventually, the drag force will be equal to the force of gravity so the forces are balanced. At this point there is no resultant force and so the object will no longer accelerate; it will travel at a steady speed. This is the Terminal Velocity and although the object is still falling, it will not get any faster.

Parachutes

A parachute can be used to reduce the terminal velocity of a falling object. When the parachute opens, it will suddenly increase the drag force. At certain speeds, this is greater than the force of gravity and so there will be resultant upwards force which will cause a negative acceleration and slow the object down.

As the object slows down, the drag force will decrease. Eventually it will be the same as the force of gravity and the two forces will be balanced again. At this point the object will stop decelerating and move at its new terminal velocity.

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