Page 138: Questions # 1 to 8 inclusive [answers in bold print]
 

1.    Stand with your heels and back against a wall.  Keeping your legs straight, reach down toward your toes without moving your base, and explain the results in terms of the CG, the line of gravity, and the base of support.

As you lean forward, your center of gravity moves forward and your vertical line of gravity also shifts forward.  When you have leaned forward far enough, your line of gravity falls beyond your foot base, thereby causing you to tip forward.  To prevent falling you step forward to form a new base within which your line of gravity falls and you regain your balance.

2.    Repeat the exercise in question 1, standing away from the wall.  Explain the difference in results.  What is the movement that occurs at the ankle joins as you reach toward your toes?

Standing away from the wall, your lower body shifts backward as your upper body moves forward and down.  With this compensatory backward shift, your center of gravity can be maintained over your base.  Ankle extension (plantar flexion) occurs.

3.    Stand with your side against the wall so that your legs are parallel to your body's longitudinal axis.  Slowly lift your outside foot.  Describe and explain the results.

When standing upright, your line of gravity falls between your two feet.  When you lift your outside foot, you remove half your base and the wall at your side prevents a shift of the body to move your center of gravity over the weight bearing foot.  Therefore, the body starts to fall.  Without the wall, the body shifts laterally to maintain a balance.

4.    State a principle relationg a body's line of gravity to its base of support
            a.    if balance is to be maintained
            b.    if minimum stability is desired in the forward direction,
            c.    if maximum stability is desired in the backward direction,
            d.    if optimum stability is desired in all horizontal directions.

a.    If balance is to be maintained, the body's line of gravity must be directed inside the base of support.

b.    For minimum stability forward, the body's line of gravity should fall near forward edge of base of support.

c.    For maximum stability backward, the line of gravity should fall near the forward edge of the base of support. 

d.    For optimum stability in all horizontal directions, the line of gravity should fall near the center of the base of support.
 
5.    Perform the following movements:
       a.    Stand with your feet, heel to toe, along a line directed from front to back.  Close your eyes, rise to your toes, and determine the direction in which you are least stable.
       b.    Stand with your feet spread laterally about 1 m apart, and repeat the steps in (a).  Determine the direction in which you are least stable.

a.  Stability is least in the side to side direction because that is the narrowest dimension of the base of support.

b.  The least stable direction is forward and backward.

6.    Using the experience gained in 5 (a) and (b), state a principle relating the body's gravity line to its base of suport for maximizing stability.

To maximize stability in a given direction, the base of support should be enlarged in that direction; also, if maximum stability is desired in, for example, the backward direction, the body's line of gravity should be located as far away from the back edge of the base as possible.

7.    Assume a push-up position.  What adjustments is necessary for balance if you remove one hand from the base of suport?  Explain why.

Upon removing one hand, your support base is reduced from a triangular shaped area to a thin strip bounded by the remaining hand and the two feet.  Your body must shift so that your center of gravity is located above that new base to maintain balance.

8.    Using the concept of CG location and noting how it changes iwth segmental movements, exxplain how "pumping" in a suspended swing can inititiate the swinging motion.

Initiating a swinging motion from a still position on a swing is analogous to the action of a gymnast initiating a swing on a horizontal bar: the body's center of gravity is raised and moved slightly forward as the hips flex and the hand support provides the external reaction force which makes the center of gravity's motion possible.  Following this initial forward and upward displacement, the center of gravity falls downward and backward, continuing backward past the initial position because of momentum.  The next forward swing is increased by repeating the "pumping" action at the top of the next forward swing.
 
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Page 143: Questions # 1 to 6 inclusive [answers in bold print]
 
1.    By throwing and catching a basketball with a partner eight strides away, experiment with various foot positions and determine which position enables you to throw the ball the fastest and to catch the ball most easily without losing your balance.

The eight strides can be varied according to the student's ability to throw the ball so that it reaches the catcher with enough speed to require the catcher to make segmental adjustments.  A lateral stance will be the least stable when throwing and catching in a forward-backward direction.  A common way to catch a fast ball with a lateral stance is to receive it off to one side and decelerate it gradually by twisting the upper body backward with the ball and hands.

2.    Stand facing a partner one stride away.  Placing your palms against your partner's, experiment with different foot positions and directions of force.  What determines maximum stability when you attempt to push each other off balance?  Explain the differences.

Maximum stability is determined by how well your body's segments can be shifted to compensate for the received force given by the other so that your line of gravity always falls within your base.  Also, if you are the pusher, you will receive a reaction force from your push, and that may be enough to force you off balance.

3.    Run at moderate speed in a circle approximately 6m in diameter.  Have an observer note the changes in inward body lean as you increase the running speed.  Explain.

Assuming the circular run is on a level surface, the body must lean toward the center of the circle so that the feet, upon pushing against the ground, may receive a ground reaction force with a horizontal component directed centripetally.  This centripetal force is responsible for the continuous change in direction of the runner; otherwise, the body's momentum would cause the body to move in a tangential direction at any instant.  As running speed is increased, but the circle remains the same, the body lean inward must increase to provide a greater centripetal friction component of ground reaction force which is necessary because of the greater tendency of the body to move off tangentially to the circular path.

4.    Repeat the steps in question 3, and increase the diameter of the circle without increasing the speed.  Have an observer note the changes in body lean.  Explain.

As the circle is increased, but running speed is not, the body lean inward will lessen and the runner will move in a more upright position.  With the more upright position, the centripetal force of the ground on the body is less, but adequate for the smaller change in direction.

5.    Run as fast you can to a line 20 m away and stop on it while maintaining balance.  Have an observer note the changes in body lean and foot placement as you approach and stop at the line.  Explain your success or failure to stop on the line without losing your balance.

Just as forward component of ground reaction force is necessary for forward acceleration, a backward component is necessary for deceleration.  As the line is approached, a greater backward body lean is associated with greater backward ground reaction force (friction).  If the body's center of gravity still has forward momentum at the line, deceleration was begun too late; consequently, if the feet are planted at the line, the remaining body momentum causes a forward fall.

6.    Explain the advantage in the baseball rules that allows a batter to ovrrun first base.  Explain why such a wide turn is made if the hitter continues on to second base.
 
If the batter could not overrun first base, he would have to start decelerating earlier to stop on the base, thereby increasing the time it takes to arrive at the base.  A wide turn is made so that the runner can maintain as much speed as possible.  With a smaller radius of curvature of the path, he must decrease his running speed so that he does not exceed the limits of the friction necessary for the centripetal force required for a sharp change of direction (spikes help).