Air resistance

Air resistance

If you drop a stone and a feather, the feather falls much slower than the stone. This is because of air resistance. If there is no air resistance, they will fall at the same rate.

A stone would also experience air resistance. This is not so obvious as a feather, because the weight W of the stone is normally much larger than the force F from the air resistance. However, the air resistance increases with velocity. As the stone accelerates, it moves faster, and so the air resistance increases.

Imagine dropping a stone from a great height, like from an aeroplane. As the stone falls over this large distance, its velocity can become very large. So the air resistance also becomes very large - even as large as the weight of the stone. When this happens, we have two forces: the weight that is downwards, and the air resistance that is upwards. Since they are equal and opposite, they cancel and the resultant force is zero.

If the resultant forces is zero, the acceleration a is also zero. So the velocity v stops increasing, and the stone moves at a constant velocity after that. This constant velocity is called the terminal velocity.

A graph of v is plotted against the time t would look like the figure on the left. The velocity starts from zero, reaches the terminal velocity gradually, and does not increase any further.

The corresponding graph for the acceleration a is shown on the figure on the right. It starts at g, the acceleration of free fall. This is the resistance when there is no air. This is because at zero v, there is no air resistance. As v increases, the air resistance increases. So the resultant force, and therefore the acceleration a, decreases. When terminal velocity is reached, it the resultant force is zero, so the acceleration a is zero.

The Physics Notes