Fluid mechanics: lift

Question 1

Define Bernoullis principle

Answer 1

An increase in the speed of a fluid leads to a decrease in static pressure (and vice versa)
OR
Fast air leads to low pressure and slow air leads to high pressure.

Question 2

What does Bernoulli’s principle relate to?

Answer 2

How a lift force is generated

Question 3

What direction is a lift force?

Answer 3

Can be either upwards or downwards

Question 4

What happens if a lift force is in an upwards direction?

Answer 4

If the lift force is in an upward direction, it will keep an object in the air for longer, therefore increasing its horizontal displacement

Question 5

Give an example of an upwards lift force

Answer 5

Discus

Question 6

What happens if a lift force is in a downwards direction?

Answer 6

A lift force being in a downward direction forces an object towards the ground, therefore increasing the amount of friction generated with the ground.

Question 7

Give an example of a downwards lift force

Answer 7

Cyclist

Question 8

When is lift achieved?

Answer 8

When an object is tilted into the air flow (release point of discus)
When an object is tilted it changes the flow of air around an object
If the air has to travel further due to this change, it will increase in velocity
This increase in velocity leads to low air pressure - generating lift

Question 9

What happens if an objects flight is parallel to the ground?

Answer 9

It will not be forced upwards or downwards because the air flowing over and under the object is moving at the same speed. As there is no difference between the speed of the air, air pressure remains the same.

Question 10

Give a way to remember the fast air/slow air Bernoulli’s principle?

Answer 10

Immi and caldwell race - has to be a draw.
Caldwell ran fast, immi walked slow

Question 11

Describe the lift force achieved with cyclists

Answer 11

With cyclists, the low streamlined body position over the handle bars means that the air that travels over the top of the cyclist has a shorter distance to travel that the air underneath. This results in the air above the cyclist travelling at a slower velocity - therefore creating a higher pressure. The higher pressure above the cyclist creates a downward lift force and allows the tyres of the bike to maintain a firm grip on the track.