P2 - Terminal Velocity

Question 1

What happens initially to falling objects in a fluid?

Answer 1

Objects in a fluid or gas will initially accelerate due to a gravity but eventually this will be balanced by drag. This makes the resultant force zero and reaches a constant speed - it’s terminal velocity.

Question 2

Using the idea of forces, explain why a car reaches a terminal velocity and why opening a parachute on the back of the car reduces the terminal velocity.

Answer 2

Initially; thrust is bigger than drag so it accelerates as there is a resultant force.
Drag increases with speed.
Eventually drag is the same as thrust so there is no resultant force.
No resultant force = no acceleration = constant velocity called terminal velocity.

Opening a parachute increases the surface area, creating more drag.
You now have a resultant force so you ‘accelerate backwards’ but you actually slow down.
Drag now decreases until drag=thrust.
No resultant force, constant velocity, new terminal velocity.