Question

Comparing gancoli question 2-10 and question 2-11

So The first question asks how far does ball drop, and the second 2-11 says what is the position of the ball.

Both question ignore the fact that the tower is 70 m high and instead set y to zero. I thought for question 2-10, this was because we were thinking bout how far e ball dropped. Pls see other note about thos question in Physics concepts. But i realize

That is not the case, since quest 2-11 asks for position, but y

Is still zero and not 70m. I worked it out and figured out With 70 both times and figured out that if i determined diplacement i get the same

Answer, but how do you know when you can just set a y value

Like 70 m in this case to zero. ???? Pls see my own calculations on

Following page.

Answer 1

Taking y=0m is not a necessasity as you have already figured out.Its only meant to make calculations simple.Its just a reference.

example say a meter scale: You can start from 0 mark and measure till 12cm and say the length is 12cms of this line segment or you could have started from 2cm mark and saw the final reading as 14 cm and then said that the line segment measures 14-2=12 cms. Answer is same but in one case you take 0 as reference ,in other 2cm as reference.