# Homework Help: Question about energy absorption

1. Jul 10, 2008

### huskiehenne

Ok, so I just took part in this lab for my Kinesiology class and I am having a little trouble figuring out the answers. It is my first time on this board and was just wondering if anyone could help. Here is the scenario.

Energy absorption is a critical component of injury prevention, in many sports such as, catching a baseball, landing in gymnastics, and breaking a fall in Judo. Energy absorption for the purpose of injury prevention as in catching a baseball depends upon the athletes’ ability to control or manipulate the kinetic energy of the ball. This is commonly known as “Soft hands” or “Work-Energy” relationship in scientific terms. This lab will examine the concept of “Soft hands” as it relates to work and energy.

1) You will throw a ping-pong ball to your partner.
2) Your partner will attempt to catch the ball using the hard, flat surface (of a hardbound book, for example) without letting the ball bounce off the surface.

Question - 1. What two things (mechanical variables) were altered or controlled to allow the ping-pong ball to be caught on the book without the ball bouncing? Explain why these two things induce the desired effect (1 point).

I suppose this is rather long, but I am a little confused to what the two variables would be....any help would be great.

2. Jul 11, 2008

The only way I can think of to do this is to have the book moving (accelerating more like) with the direction of the ping pong ball so that the acceleration is less on the ball. This is just like when you catch a ball with your hands. If someone throws a basketball to you very hard, you want to start catching it out in front of you and let your arms move toward you as your catch the ball. The effect of this is to spread out the time of the impact so that there is less of a force exerted on your hands at a given instant. This comes from the concept of impulse=force*time. I wouldn't say that it has to do with the Work-energy relationship at all, but maybe I'm missing the boat on this one. I didn't nail down your variables exactly, but hopefully this gives you an idea or two.

3. Jul 11, 2008

### LowlyPion

Kinesiology is about Human kinetics, but it also is subject to laws of physics.

The choice of a hard ping pong ball being "caught" by a hard surface is another way of saying you have two objects that if they impact will have an inelastic collision and there will be a rebound from the collision - that is in the case of the ping pong ball it would "bounce".

Now the ping pong ball in motion has what properties? Which of those properties can you expect to change? And what is the desired result? That it come to rest on the book without bouncing perhaps?

A question for you is what physical equation covered in class covers "work" and the kind of resulting changes you want to achieve? Have you discussed the "work-energy principle" perhaps?