Chapter 13
Applications of Aerodynamics in Sport
 

Page 435: Questions # 1 to 4 inclusive [answers in bold print]
 
1.    Name three different skills or activities in which aerodynamic drag is significantly detrimental.  If possible, how could the drag be minimized in each activity?

Bobsledding: The design of the bobsled is focused on streamlining the contours for less drag.  Cycling: Riders crouch to expose less surface area to the air flow.  The shape of racing helmets is streamlined with a rounded frontal area and a tapered pointed rear portion.  Discus throw: The discus should be thrown so that its thin edge encounters the flow as much as possible so that it can still act as a "wing".

2.    Tell which cyclist would encounter more resistive drag: (a) one riding at 20km/hr into a head wind of 5 km/hr, or (b) one riding at 30 km/hr with a tail wind of 10km/hr.  Explain why?

Cyclist (a) encounters more drag due to the greater relative air flow velocity past her [+20-(-5)] = 25km/hr

Cyclist (b) experiences only 20 km/hr relative flow velocity [+30-(+10)] = 20km/hr

3.    How could the cyclists in question 2 minimize the air resistance without changing speed?

Drag could be reduced by crouching low so that less surface area is exposed to the air flow.

4.    Describe how the air would flow around a volleyball that is hit with no spin.

The air flow would flow at differing speeds around surfaces of the volleyball because of the seams not encountering the flow symmetrically.  Random areas of low pressure result and the ball responds to the changing direction of drag force from one instant to the next.

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Page 442: Questions # 1 and 3 [answers in bold print]

1.    Explain the difference between a javelin's angle of projection and attitude angle.

A javelin's angle of release (angle of projection) is formed between the horizontal ground and the instantaneous release velocity vector of its center of gravity.  The attitude angle is formed between the horizontal ground and the long axis of the javelin; it is the angle that is visibly aparent to the observer.

3.    In which direction is an aerodynamic lift force applied to an airfoil mounted on the rear portion of a racing car if the airfoil has a negative angle of attack?  What would be the function of such a lift force?

A negative angle of attack will cause a downward lift force on the rear portion of  a racing car to give greater tire to road friction; it increases the normal force.

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Page 448: Questions # 1 to 3 inclusive [answers in bold print]

1.    Identify three different skills in which putting backspin on a ball would be desirable.  Do the same for top spin.

Backspin causes upward lift (Magnus) force on a ball moving with some horizontal velocity: golf drive, baseball pitch, soccer kick.  Top spin causes downward lift force on a ball iwth some horizontal velocity: top spin serve in tennis, baseball pitch, racquetball kill shot.

2.    For balls thrown with different types of spin, discuss the effects of spin direction (axis of rotation), spin velocity, and velocity of projection on the spatial path of the ball.  Discuss different kinds of baseball pitches relative to the same factors.

The path of a spinning ball will be determined by the projection velocity, the rate of spin and the spin axis.  Generally, a ball travelling at a high velocity with a given amount of spin will display less total deviation from the target than will a ball travelling at a lower veloctiy, because the faster ball reaches the target sooner.  The pitches that display greater change in their paths due to spin are those with reltiavely slow projection velocities.  The ball will deviate in a direction that is at right angles to the axis of spin, and therefore, the exact postion of the spinning axis needs to be extabilshed at the time of projection.  The greater the spin rate the greater the deviation will be from the prescribed parabolic path.

3.    Draw a top-spinning ball during its ascent and descent in its trajectory.  Draw in the Magnus force vector on each diagram.  Then, resolve each Magnus force vector into its horizontal and vertical components.  Compare the horizontal components on the two diagrams, and explain the effect of each one on the path of the ball.

Draw the picture large enough so that detail can be seen.  Refer to Figure 13.12 on page 443 for the initial Magnus forces.  Using Figure 13.13 on page 444 as a guide, resolve each Magnus force into its components.  It will be seen that the horizontal component during ascent acts with ball travel and during descent it acts against the ball's forward motion.