You are here:

Physics/motion in straight line


Ball A is thrown up vertically from the ground with speed 10 m/s. At the same instant another ball B is released from the rest at height h. At time t, the speed of A relative to B is ?

Hello saroj,

Formulas for both balls will use the same value of t. I'll let up be the positive direction. Using the kinematic formula
Vf = Vi + a*t

Ball A: Va = Via + a*t = 10 m/s - 9.8 (m/s^2)*t
Ball B: Vb = Vib + a*t = - 9.8 (m/s^2)*t

Va - Vb = (10 m/s - 9.8 (m/s^2)*t) - (- 9.8 (m/s^2)*t) = 10 m/s
When t = 0, it works. If you round g up to 10 m/s^2, after 1 second, A will have stopped at its highest point. And B has been accelerating downward at g for 1 second, so B's velocity will be -10m/s. So it also works when t = 1 s. And it will continue to work till they hit the ground.

I hope this helps,


All Answers

Answers by Expert:

Ask Experts


Steve Johnson


I would be delighted to help with questions up through the first year of college Physics. Particularly Electricity, Electronics and Newtonian Mechanics (motion, acceleration etc.). I decline questions on relativity and Atomic Physics. I also could discuss the Space Shuttle and space flight in general.


I have a BS in Physics and an MS in Electrical Engineering. I am retired now. My professional career was in Electrical Engineering with considerable time spent working with accelerometers, gyroscopes and flight dynamics (Physics related topics) while working on the Space Shuttle. I gave formal classroom lessons to technical co-workers periodically over a several year period.

BS Physics, North Dakota State University
MS Electrical Engineering, North Dakota State University

©2017 All rights reserved.