L7 - Ball-surface interactions

Question 1

Why apply backspin to a golf shot?

Answer 1

Increases drive distance through enhanced lift, and decreases stopping distance.

Question 2

How can we measure the coefficients of restitution and friction for a golf ball?

Answer 2

e can be measured from a ball drop test.

µ can be measured from the force required to drag a weighted sled supported by golf balls.

Question 3

Relate the horizontal and vertical rebound velocities to inbound velocity (in x and y), coefficient of restitution and coefficient of friction for a golf ball rigid ball - rigid surface impact

Answer 3

Question 4

In actual tests, a softer floor gives a larger rebound angle. Why? What else does this affect about the impact.

Answer 4

The ball is rebounding off the front of a curved depression, caused by the ball. This depression affects spin

Question 5

What measuring equipment do the ITF use to characterize a court’s surface pace?

Answer 5

2 light gates either side of the impact position.

Question 6

Actual rebound is _______ than predicted due to ____-___________ and _____ _______. The first of these reasons can be simulated using a ________________ model.

Answer 6

Actual rebound is steeper than predicted due to ball-deformation and surface friction. The first of these reasons can be simulated using a mass-spring damper model.

Question 7

Making a suitable assumption, we can explore the effect of friction on rebound angle and speed via a _______________ model. State the assumption and mechanism of the effect.

Answer 7

Rigid ball - rigid surface
Most courts are a similar hardness, therefore e value can be assumed constant.

Increasing friction, reduces horizontal component of velocity after impact, whilst vertical component is unaffected; reducing the resultant velocity and increasing the rebound angle.

Question 8

State the SPR equation and a simplified version of it.

Answer 8

Question 9

Why is a mass - spring damper model required for ball-surface interactions in cricket? (3)

Answer 9

Conflicting effects:

The ball is slightly less rigid than the surface, so pitch deformation effect is less

A softer pitch leads to a large depression

A harder pitch has a greater ‘e’ (and thus higher vertical rebound velocity)

Question 10

Draw the diagrams of combined experiments to measure stiffness and damping properties of both the ball and the pitch.

Answer 10

Question 11

Why do we carry out the experiments from the diagram?

Answer 11

Stiffness for both the ball and the pitch is non-linear and damping varies with contact area, so we need measurements to validate each model with.