The object has acting on it the force due to
gravity, so its acceleration is -9.8 ms^{-2}.

`v=int a dt`

`=int-9.8\ dt`

`=-9.8t+C`

Now at `t = 0`, the velocity = `15\ "ms"^-1`. So `C = 15`.

So the expression for velocity becomes:

`v=-9.8t+15`

Now, we need to find the displacement, so we integrate our expression for velocity:

`s=int v\ dt`

`=int(-9.8t + 15) dt`

`=-4.9t^2+15t+K`

Now, we know from the question that when `t = 0`, `s = 0`.

This gives `K = 0`.

So `s=-4.9t^2+15t`

At time `t = 2.5`, `s = 6.875\ "m"`.

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