Terminal speed

Objects moving in water or air

When objects move faster in air (or water), air resistance or drag forces increase. As the drag forces increase, the resultant force is reduced. When the drag forces become the same as the weight or the thrust, then the forces become balanced (equal and opposite). There is now no resultant force and the object stops accelerating, but continues to move at constant speed. This constant speed is called terminal speed or terminal velocity.

Question

As a skydiver moves through the air, what stays constant, weight or drag?

Weight. The drag changes with the speed of the skydiver.

Question

Look at pictures A, B, and C. Work out the resultant force of the skydiver, and the acceleration if the skydiver has a mass of 60 kg.

3 skydivers labelled A, B and C. Each skydiver weighs 600 newtons. The drag on A is 0, the drag on B is 100 newtons, and the drag on C is 600 newtons.

Picture A

The resultant force is 600 N. The acceleration will be

a = F ÷ m

= 600 N ÷ 60 kg

= 10 m/s2

This is called the acceleration due to gravity.

Picture B

The resultant force is 600 – 100 = 500 N.

The acceleration will be

a = F ÷ m

= 500 N ÷ 60 kg

= 8.3 m/s2

Notice that the acceleration is getting less as the drag starts to increase.

Picture C

The resultant force is 600 – 600 = 0 N.

The acceleration will be zero, as there is no resultant force.