Terminal Speed
Physics: AQA A Level, topic 4.1.14
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.
,w_800.png)
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.
Test yourself on Terminal Speed
What is the relationship between weight and air resistance at terminal velocity?
Forces in Skydiving
When does a car reach its maximum speed?
How does drag depend on surface area?
Which of the following will increase the drag on an object?