SUVAT Problems: Why t-x and Not t+x?

[nokia8650]

When doing SUVAT problems, why does the item which begins motion later given a time "t-x" where x>0. For example, if dropping a ball, and then dropping another ball 5 seconds later, the first ball's time can be expressed as "t" and the second ball's time as "t-5". Why is it not t+5?

Thanks

 

[tiny-tim]

Hi nokia8650!

Because the SUVAT formula you're using (I assume) only works if the ball is stationary at t = 0.

Imagine each ball has its own clock, the first showing t1 and the second showing t2.

So t1 = 0 when the first ball starts.

And t2 = 0 when the second ball starts, which of course is when t1 = 5.

So t2 = t1 - 5.

 

[nokia8650]

Thanks a lot!