Chapter 12
 Analysis of Projectile-Related Activities

 

Page 395: Question # 3 [answers in bold print]

3.    Perform a vertical jump and reach, and mark the height of reach.  Compare the difference in height attained when both arms remain overhead at the height of the jump and when on arm is extended downward quickly after takeoff.  Explain the difference.  How do these findings relate to the technique used for (a) a volleyball spike, (b) a basketball jump, and (c) a basketball rebound?  What effect does the repositioning of the body segments have on the path of the CG of the body?  on the path of the hand that remains above the head?  How does this relate to clearing the bar in a high jump?

When both arms are kept overhead at the peak of height of the jump, the mark should be lower than when one arm is extended downward after takeoff, if all other factors remain the same.  The reason is that the center of gravity of the body (system) is projected to a predetermined height at takeoff, depending on the projection velocity attained at takeoff.  If both arms are overhead at the instant of takeoff, the body's center of gravity is located approximately just above the waistline.  At the moment before the center of gravity reaches its predetermined height, if one arm is brought down, the rest of the body shifts upward relative to the center of gravity, and the center of gravity is now below the waist, say, at the iliac crest level, that part of the body, rather than the waistline, will rist to the predetermined height.  The touching hand, therefore, is that much higher than it would have been if the body's center of gravity remained at waistline level withing the body.

Using this same principle, the reaching hand in a), b), and c) attain more height to touch the ball sooner and/or higher.

The repositioning of the body segments (withing the system) has no effect on the predetermined path of the center of gravity of the system.  If the arm descent is timed properly, the other hand will be able to achieve greater reaching height.  For maximum jump height over the bar in a high jump, each body part should reach its maximum height just as it goes over the bar and drop as low as possible just after it clears the bar.  This will cause the body parts that follow to be raised higher relative to the center of gravity of the body, thus facilitating bar clearance of subsequent body parts.

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Page 405: Questions # 2, 3, 7 [answers in bold print]

2.    Draw a trajectory illustrating the path of a softball that is projected at 45 degrees with no air resistance (takeoff and landing heights are equal).  Draw a trajectory illustrating the same object traveling with air resistance.  What is the difference between the two trajectories?  Why does the difference exist?

The trajectory of the softball, or any projectile, will be a parabola if there is no air resistance to change it (see Figure 12.3).  Traveling in air, there is the external force of aerodynamic drag in addition to gravity affecting the motion of the ball.  The drag slows both its vertical and horizontal component velocities so that it does not go as high nor as far as it would in a vacuum (see Figure 12.3).

3.    Draw an arbitrary trajectory for each of the following situations:

a.    Projection velocities are equal at a 35-degree projection angle from a takeoff height of 1.50m and a takeoff height of 2.00m.  Both land on the ground level.

b.    Takeoff heights are equal at a 35-degree projection angle with projection speeds of 20 m/sec and 25 m/sec.  Both land at the same level.

c.    Takeoff and landing heights are equal, and speeds are equal.  The projection angles are 30 degrees and 40 degrees.

With all other factors remaining unchanged and air resistance considered negligible:

a. The one with the greater takeoff height will travel farther when the landing heights are the same for both

b. The one with the greater projection speed will travel farther when the landing heights are the same for both

c. The one projected closer to an angle of 45 degrees will travel farther when the takeoff and landing heights are the same

7.    Perform a standing long jump, keeping the shoulders flexed forward on landing.  Perform a standing long jump, hyperextending the shoulders during flight.  Is there a difference in jump distances?  To what could you attribute the difference?

One would expect a long jumper to contact the ground with the fet a few inches farther if the shoulders were hyperextended immediately prior to contact.  The hyperextension of the shoulders during unsupported conditions brings the arm masses behind the center of gravity.  Since the center of gravity position relative to the ground cannot change its prescribed path, the hyperextension of the arms will cause the remaining body parts to move forward, thus the feet will be farther forward relative to the ground when the jumper lands.

Page 412: Questions # 3, 4 [answers in bold print]

3.    Consider three volleyball servers of various heights.  What differences would there be in the ways they should be coached to serve?  Draw the possible trajectories.

Disregarding the differences in serving velocites and/or skill, one would expect that the ability to serve overarm would be most crucial to the shortest server since the takeoff height of the serve would be the lowest for that player.  The tallest player may be able to serve in a sidearm fashion, or possibly underarm, without sacrificing too much takeoff angle, that is to still keep the angle fairly low.

4.    Consider three volleyball servers, one beginning, one intermediate, and one advanced.  (All are of equal height, but ability may affect velocity of impact. )  What factors should be considered in coaching each?  Draw three possible trajectories.

The beginning server would probably lack the ability to generate velocity on the overarm and sidearm serving styles and thus would have to serve those technizues with an angle that would be great enough to cancel the advantages of serving overarm or sidearm.  The underarm would be the best for the beginner.  The advanced player would be able to impact velocity to the ball sufficiently so that the angle of the overarm serve would not have to be increased.  The intermediate server would have a lwoer trajectory underarm or a higher trajectory overarm serve than the beginning or advanced players, respectively.