Fluid mechanics

Question 1

Define ‘fluid mechanics’

Answer 1

The study of a body travelling through air or water

Question 2

Define ‘fluid friction’

Answer 2

The resistance to motion on a body travelling through fluid, resistance acts in opposition to the direction of motion and reduces velocity

Question 3

Name six factors that affects fluid friction

Answer 3

• Velocity
• Frontal cross-sectional area
• Shape
• Surface
• Temperature (the warmer the air, the less resistance because are density reduces)
• Altitude (altitude increase decreases air density and thus air resistance)

Question 4

Define ‘streamlining’

Answer 4

Shaping a body so that it causes the last drag when travelling through a fluid

Question 5

Define ‘aerofoil’

Answer 5

A streamlined shape with a curved upper surface and an under surface that is predominantly flat

Question 6

Describe a projectile

Answer 6

A body launched into the air subjected to weight and air resistance forces, projectiles are not in contact with the ground and are not affected by friction or reaction forces during flight

Question 7

What is the flight path of a projectile determined on?

Answer 7

Relative size of weight and air resistance

Question 8

Describe parabolic and non-parabolic flight paths

Answer 8

If weight is dominant (parabolic)

If air resistance is dominant (non-parabolic)

Question 9

When does a flight path become non-parabolic?

Answer 9

Air resistance takes dominance when a body travels through the air with high velocity, large frontal cross sectional are, rough surface or low mass e.g. golf ball/ shuttlecock/ tennis ball

Question 10

Define ‘parabola’

Answer 10

A uniform curve that is symmetrical about its highest point

Question 11

What forces act on a projectile in flight?

Answer 11

Air resistance and weight

Question 12

Describe the Bernoulli principle

Answer 12

A lift force is present when projectiles adopt an aerofoil shape
Angle of attack of the aerofoil will affect the size of the lift force
The greater the angle of attack up to 17 degrees with create a greater lift force
Lift force is created by presenting the projectile at the correct angle of attack
Thus air will travel at a higher velocity over the top of the projectile
Consequently producing a low pressure area above the projectile
The pressure gradient applies a lift force to the projectile

Question 13

How is spin created?

Answer 13

Applying an eccentric force to the projectile at the point of release, it is off centre and applied outside the centre of mass to generate torque

Question 14

Define ‘eccentric force’

Answer 14

A force whose line of application passes outside the centre of mass of a body causing angular motion

Question 15

Define the Magnus effect

Answer 15

The Magnus effect explains the deviations in flight paths of spinning balls in sport

Question 16

Describe the principle of the Magnus effect

Answer 16

Slower air creates an area of high pressure
Pressure moves from high to low using the ball to dip as it travels
Faster air creates an area of low pressure

Question 17

Describe the Magnus effect on top spin

Answer 17

Top of the ball:
Surface of the ball is travelling in the opposite direction to airflow
Air slows down = high pressure
Bottom of the ball:
Surface of the ball travelling in the same direction of airflow
Air speeds up = low pressure
Consequences:
Pressure difference = ball deviates towards area of low pressure
Ball dips so distance travelled is decreased from non-spin flight path

Question 18

Describe top spin and bounce

Answer 18

Bottom ball surface wants to slide backwards
Friction opposes motion acting forwards
Ball skims off the surface at a low angle
E.g. tennis top spin increases speed, reduces time for the ball to bounce and so the tim the opposition can react

Question 19

Describe the bounce of a backspin

Answer 19

Ball hits the ground wanting to move forward (BUT friction opposes)
Ball rebounds from the surface at a large angle
E.g. basketball sink shot from rebound off the backboard

Question 20

Describe the conditions for a perfect basketball shot with backspin

Answer 20

• 52 degrees launch angle
• 3 revolutions per second
• Aim 7cm from the basket centre on the backboard

Question 21

Describe the effect, view and bounce of sidespin

Answer 21

• Ball swerves, flight path deviates to the left (hook) or to the right (slice)
• View from the top
• Friction acts in the same direction as motion and ball will accelerate at a low angle in the direction of the swerve