Section 1
(Answers could very)
1. The purpose of the study was to examine reaction forces transmitted to the upper extremities
of high-level gymnasts during the round-off phase of the Yurchenko vault.
2. The two conditions were whether the elite-gymnast performed a Yurchenko vault, or a floor
exercise pass.
3. The two dependent variables were the peak vertical and peak anterior-posterior reaction
forces.
4. High reaction forces transmitted to the upper-extremities are far more dangerous than high
reaction forces transmitted to the lower-extremities because the lower-extremities are designed
for such weight bearing activities, whereas the upper-extremities have the force applied directly
to the small bones in the hand with little cushioning. Also the upper-extremities do not have
nearly the muscle mass of the lower-extremities.
Section 2
A. A golfer hits her tee-shot due north towards the fairway. Her shot has an initial velocity of 60
m/s. A 15 m/s wind is blowing in a northwesterly direction (45 degrees west of North).
1. Considering the initial velocity of the ball and the velocity of wind, what will be the
resultant velocity (m/s) of the golf ball assuming no other forces are acting on the ball?
First, draw a picture and combine the two velocity vectors using the tip to tail method. 15 m/s Resultant velocity 60 m/s Next, resolve the 15 m/s velocity vector into vertical and horizontal components using sin and cosine. Sin(45)*15 = 10.6 15 m/s 10.6 Cos(45)*15 = 10.6 10.6 Now, add the two vertical components together (10.6 and 60) and combine the new vertical vector (70.6) with the resolved horizontal vector (10.6) using the tip to tail method. 10.6 Resultant velocity 70.6 m/s Next use Pythagoreans equation to solve for the resultant velocity c = 71.4 2. What will be the resultant direction (º west of North) of the golf ball?
Use inverse tangent to solve for the resultant direction (angle). Tan-­‐1(10.6/70.) = 8.54° West of North B. A patient walks across the floor at a horizontal velocity of 1.3 m/s. During the stance phase of
walking, the peak vertical ground reaction force is 1000 N; at the same time the peak horizontal
force is 245 N. 1
What is the resultant magnitude and direction of the ground reaction force? 1000 N
First, draw a picture and combine the two ground reaction force vectors using the tip to tail method.Resultant GRF 245 N
Next use Pythagoreans equation to solve for the resultant velocity a2 + b2 = c2 2452 + 10002 = c2 c = 1029.6 N Next, use inverse cosine to solve for the resultant direction (angle). cos-­‐1(245/1000) = 75.8° from the horizontal 2 What will be the resulting vertical acceleration of the whole-body center of mass at
this instant in time?
You only need to worry about vertical forces since you’re finding vertical acceleration. Fy = GRFy – Fg = 1000 – 72*9.81 = 293.68 N F = ma 293.58 = 72(a) a = 4.08 m/s^2