Work out instantaneous velocity instantly with clear inputs, formula shown and shareable results.
Under constant acceleration the velocity at any instant is v = u + at, the first of the SUVAT relations. Starting at 5 m/s and accelerating at 2.5 m/s2 for 4 s reaches 15 m/s. The displacement is shown alongside because velocity alone does not say how far the body has moved.
Velocity at time t
v = u + a x t
Displacement
s = ut + 0.5 a t^2
No. It assumes acceleration is constant over the interval; otherwise you need the integral of acceleration or a numerical method.
Because it is a vector along the chosen axis. A negative result means the body is moving opposite to the direction you called positive.