Fluid Mechanics and Projectile Motion

Question 1

Define drag

Answer 1

The resistance to motion of a body travelling through fluid

Question 2

What is fluid mechanics?

Answer 2

The study of objects or bodies that travel through a fluid

Question 3

What is drag?

Answer 3

the resistance to motion on a body travelling through fluid. It acts in the opposite direction to motion

Question 4

What is air resistance?

Answer 4

the force acting in the opposite direction to the motion of a body travelling through air

Question 5

Identify the factors that affect air resistance

Answer 5

velocity of the body
cross-sectional area of the body
shape and surface characteristics

Question 6

How does velocity affect air resistance?

Answer 6

the greater the velocity of a body the greater the air resistance

Question 7

How does cross-sectional area of a body affect air resistance

Answer 7

the larger the cross-sectional area the greater the air resistance

Question 8

How do shape and surface characteristics of a body affect air resistance?

Answer 8

the smoother or more aerofoil a body, the less air resistance
rougher / or bits sticking out = more air resistance

Question 9

What is an aerofoil?

Answer 9

a streamlined shape with a curved upper surface, and an under-surface that is predominantly flat - like the cross-section of the wing of an aircraft

Question 10

What is a projectile?

Answer 10

human bodies or objects launched into the air that are subject to forces of weight and air resistance only

Question 11

what does the term parabola mean?

Answer 11

a uniform curve that is symmetrical about its highest point

Question 12

What is lift force?

Answer 12

force that acts perpendicular to the direction of travel for a body moving through a fluid (or air)

Question 13

what is meant by the term angle of attack?

Answer 13

the angle at which a projectile is tilted from the horizontal.
Lift force will increase as the angle of attack is increased up to a certain point (usually about 17 degrees)

Question 14

Describe the Bernoulli principle

Answer 14

molecules in a fluid exert less pressure the faster they travel and more pressure the slower they travel

Question 15

Describe how you can create an upward lift force in a discus

Answer 15

if a discus is thrown at the correct angle or attack, the air that travels over the top of the discus has to travel a further distance than the air that travels underneath. The air above the discus travels faster, therefore, according to the Bernoulli principle it has a lower pressure.
The air below the discus doesn’t have to travel as far, so it travels slower, creating a higher pressures.
Therefore the discus is lifted upwards

Question 16

Describe how they produce a downward lift force in motor racing

Answer 16

the cars are created so the air has further to travel underneath the car than on top.
The air going faster underneath (as more distance to travel) has a lower pressure
the air on top that has less distance to travel is a higher pressure, which pushes the car downwards
this helps it keep on the track, creating more friction so it can grip better.

Question 17

What is an eccentric force?

Answer 17

a force whose line of application passes outside the centre of mass of a body, causing the resulting movement to be angular

Question 18

Describe the Magnus effect

Answer 18

it uses Bernoulli’s principle to explain how spinning balls generate a lift force that changes their flight path, as the spinning surface affects the airflow around it.

Question 19

How does topspin affect the flight of a ball?

Answer 19

Topspin
- the air underneath the ball is travelling in the same direction as the spin, therefore it goes faster, creating a lower pressure
- the air on top of the ball is travelling in the opposite direction to the balls spin, therefore it goes slower, creating a higher pressure
- higher pressure on top causes a downwards lift force, the ball dips in flight and travels less distance than a non-spinning ball
when the ball hits the floor it will accelerate quickly off the surface at a smaller angle to that of a non-spinning ball

Question 20

How does backspin affect the flight of a ball?

Answer 20

Backspin

• the air underneath is going in the opposite direction to the spin of ball, so it goes slower, creating a higher pressure
• the air on top of the ball is going in the same direction as the spin of the ball, so it goes faster, creating a lower pressure
• this causes an upwards lift force, therefore the ball floats in flight and travels further than a non-spinning ball hit with the same force

Question 21

How does side-spin affect flight of a ball?

Answer 21

Sidespin

• the air travelling on the side of the ball in the same direction as the spin will travel faster = lower pressure
• the air travelling on the side of the ball in the opposite direction as the spin will travel slower = higher pressure
• this causes the ball to swerve to the side where there is lower pressure
• when the ball hits the floor it will accelerate even further in the direction of the spin