# I got question

ludi_srbin
In Friday we did simple lab in Physics. We took ball and dropped it measuring dropping height and rebound height. We did 10 trials and 10 times for every trial. We started from 100 cm and went all the way to 10 cm. Then we made a model y =0.75x+2 cm. Where x is dropping height in cm and y is rebound height, also, in cm. We could use this model to predict rebound height from any height. Well the accuracy wasn't really the priority. My question is if I "drop" the ball from height of zero cm I would get reboung height 2 cm, according to my model. How can that be?

Homework Helper
Hmm, in fact, since the ball never has the rebound height that's greater than the height it was dropped from. So:
$$\frac{3}{4}x + 2 \leq x \Leftrightarrow \frac{1}{4}x \geq 2 \Leftrightarrow x \geq 8$$
For any x < 8, it will return a false value.
So the formula can be true for x >= 8 cm. And that's why the smallest height in your work is 10cm, not 0 cm.
Viet Dao,

Gold Member
Apparantly either a linear model is no good for this situation, or your measurements were too imprecise or inaccurate.

Homework Helper
Up to which point did you measure the heights of the ball? It's bottom, middle or top?