2.11 - Terminal Velocity

Drag and air resistance

• Drag is the frictional force on an object moving through a fluid (a liquid or a gas).
• Air resistance is a type of drag.
• Drag increases as speed increases - the faster an object moves, the more drag acts against it.
• For example, a car has much more drag to work against at 70 mph than at 30 mph, so the engine must provide a larger driving force to maintain that steady speed.

Streamlining and reducing drag

• The most important way to reduce drag is to give an object a streamlined shape.
• A streamlined shape lets fluid flow easily past it, reducing drag.
• A parachute is designed to do the opposite - its shape produces as much drag as possible.

Practical - Terminal velocity of a toy parachute

Method

1. Mark regular, set heights on a vertical surface (for example, every 0.5 m).
2. Drop the toy parachute from a set height in front of the markings.
3. Record the time at which the parachute passes each marking.
4. Calculate the average velocity between each pair of markings using $\text{Velocity}=\frac{\text{distance}}{\text{time}}$.
5. Plot a graph of velocity against time.

Results

Velocity rises quickly at first, the curve becomes less steep as the parachute approaches terminal velocity, and it flattens into a horizontal line once terminal velocity is reached.

Improving accuracy

Film the fall and play the video back slowly to record the time at each marking more accurately.