4.1.14

# Terminal Speed

Test yourself

## Terminal Speed

An object travelling through a medium such as air will reach a maximum speed where air resistance equals the driving force. This maximum speed is called terminal speed (or terminal velocity).

### Acceleration

• Let's imagine a skydiver jumping out of a plane.
• To begin with, he accelerates at 9.81 m/s².

### Increasing speed, increasing resistance

• As the skydiver speeds up, the number of air particles he collides with per second increases.
• So, the air resistance increases.

### Balancing of forces

• The weight of the skydiver remains the same, so the downward force remains constant.
• As the skydiver gets faster and faster, the air resistance increases more and more until the resistive force is equal to the weight of the skydiver.
• At this point, the skydiver has reached terminal speed.
• He will remain at this maximum speed unless something changes.

### Parachute

• When the skydiver deploys his parachute, the air resistance increases due to the extra surface area.
• The resistive force is now larger than the weight of the skydiver.
• The skydiver slows down (decelerates).

### New terminal speed

• As his speed decreases, so does the force due to air resistance.
• The speed decreases until the resistive force is equal to the weight of the skydiver.
• This gives a new, smaller, terminal speed.
• This new terminal speed is small enough for the skydiver to hit the ground safely.

## Drag and Maximum Speed

The amount of drag an object experiences is proportional to its speed and its surface area. Amount of drag also depends on the object's shape.

### Speed

• The faster an object is travelling, the larger the drag force will be.

### Surface area

• The larger the surface area of an object, the larger the drag force.

### Shape

• The shape of an object can affect the drag forces considerably.
• The more streamlined an object is, the smaller the drag force will be.
• Jet aircraft and Formula 1 cars are examples of very streamlined vehicles.
• They are designed to be streamlined so that they can reach higher speeds with the same driving force.

### Projectile motion

• In reality, projectiles travelling through air experience air resistance.
• This means that, in a real experiment, we expect the projectile to travel smaller distances and reach lower speeds than predicted.

### Maximum speed

• The maximum speed of a vehicle is the vehicle's terminal velocity through the surrounding air.
• This speed is reached when the driving force of the vehicle is equal to the resistive forces.
• To increase the maximum speed, we can:
• Reduce the drag forces, by making the vehicle more streamlined.
• Increase the driving force, by making the engine more powerful.