I'd like to know when it is appropriate to use each formula. Im new to studying physics and its seems that there are certain situations when one formula leads to the answer, yet a derivative of that formula that should lead to the answer is incorrect.

Question

I'd like to know when it is appropriate to use each formula. Im new to studying physics and its seems that there are certain situations when one formula leads to the answer, yet a derivative of that formula that should lead to the answer is incorrect.

For example: the question was if i toss a ball in the air, and the Distance =0 (as a ball comes back to the ground and tossed from the ground), Time= 2 seconds, and acceleration = -10m/s^2. What is the initial velocity?

I tried using the Change in Velocity= Acceleration x Time. which should work because i am given acceleration and time. My working out is:

Vfin-Vini = -10m/s^2 x 2seconds
vfin=0 - Vini= -20
vini= 20m/s

But apparently the correct way to answer this question is to use this formula : Change in D= Vinitial x time + acceleration x Time ^2/2. Which works out to be 10m/s

If anyone could please tell me when it is appropriate to use each formula. OR where i have gone wrong in this please let me know

Answer 1

The final velocity (after 2 seconds) is not 0. It is zero after 1 second, at the top of the trajectory. That is why you got the wrong answer the first way.