Question

A skilled volleyball player uses elbow motion (as well as shoulder motion) in a jump serve. With her shoulder positioned and maintained in flexion (about 100 degrees), a server initially starts the preparatory phase by flexing the elbow. At this time, the angular velocity of the elbow is 800 deg/s in flexion (positive) direction. As the server moves to strike the ball, the elbow undergoes extension, and immediately before the strike with the ball, the elbow’s angular velocity is 1500 deg/s in extension (negative direction). This change in angular velocity at the elbow joint occurs in 0.3 seconds. Which of the following statements is/are true about the angular acceleration of the elbow joint? Select statement(s) that is/are true. [1 pt.]

Group of answer choices

The elbow's angular acceleration is a positive value

The elbow's angular acceleration is a negative value

The elbow joint movement is speeding up

The elbow joint movement is slowing down

Answer 1

I have indicated the figure of server while he is going to the ball.The figure have three parts which indicates the complete process happening during the serving of the ball. We want to to focus on the third part of figure where elbow gets the jerk to move in opposite direction. To feel the question what is going on please observe your hand while doing the same procedure, you will find some jerk in your hand after the ball is thrown away.

Now, since the elbows initial motion is in positive direction and suddenly in time interval of 0.3 second it get changed to negative direction this means that acceleration will have negative value.

Coming to the speed of elbow which is simply a scalar quantity, We finds that initially the elbow speed has to get slow down so that its angular speed from 800 deg/s decreases to 0 and after that from 0 to 1500 deg/s in negative direction which means that it is speeding up.  

I have explained the things, But if are still in doubt please comment below.