Question

which kinematics equation do we use when we have constant acceleration?

Answer 1

There are several. The basic two are \[y(t) = y_o + v_o t + {1 \over 2} a t^2\]and\[v(t) = v_o + at\]These can be rearranged to come up with a couple more expressions.

Refer here for a list of these: http://en.wikipedia.org/wiki/Equations_of_motion#Constant_linear_acceleration

Answer 2

so these r used when acceleration is constanr

Answer 3

v^2 - u^2 = 2as