Newton's Second Law
Forces and Motion

Terminal speed - the skydiver's story

Physics Narrative for 11-14 Supporting Physics Teaching

The skydiver's story

The same ideas about resultant forces can be used to explain the motion of a skydiver falling through the air. The skydiver story is included here so you can check your own understanding. It is a challenge to follow the description of forces but getting to the end will provide you with a sense of achievement! The skydiver's story is told in four stages. Match the stages to the diagrams.

Skydiving stage 1 – the leap

Once committed, the skydiver leaps out of the plane and starts to descend. Gravity provides the downward force on the diver and as a result down they go. During this stage they speed up just as any falling object does. The downward force, the pull of gravity, is the dominant force. There is a drag force – as a result of moving through the air. During this first phase the driving force is larger than the retarding force. The resultant force is not zero. Here, the forces acting on the skydiver do not add to zero.

We'll say more about such non-zero resultant forces in episode 03.

Skydiving stage 2 – terminal speed

As the skydiver falls and speeds up, the air rushing past them exerts a greater drag force. At a certain, very high, speed the upward drag force is equal to the pull of gravity on them. Once this speed is reached there is no more speeding up. The skydiver continues to fall at this terminal speed. The resultant force on the skydiver is zero: equilibrium.

Skydiving stage 3 – the parachute opens

At a predetermined height above the ground the parachute is opened. The huge canopy area results in a much larger drag force, as the area of the falling object suddenly increases. The upward force acting on the skydiver increases. The diver will sense this sudden change. Their downward progress is slowed down due to the resultant force acting on them. As the diver slows down, the upward drag force reduces until…

Skydiving stage 4 – a new equilibrium

… at a new, much slower speed, the diver reaches a new equilibrium where the driving force is once again opposed by an equal but opposite retarding force. The diver continues to fall steadily to Earth under the action of these balanced forces.

Newton's Second Law
is expressed by the relation F=ma
can be used to derive Kepler's First Law

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