Question

how to know where to use equatiin of motion​

Answer 1

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The three equations of motions are:

1) v = v0 + aΔt

2) x = x0 + v0Δt + ½aΔt^2

3) v^2 = v0^2 + 2a(x − x0)

These are the equations you learn in the physics class at school to calculate the velocity or end position of an object given an acceleration and a time or a distance.

Let me tell you a little secret: there are no 3 equations of motion. The three things you see above follow directly from the definition of a constant acceleration in one direction. But you need a bit of calculus to see that. The 3 above are an easy way to calculate things without needing calculus.

To know which one you need requires a close examination of your physics problem. E.g.

Equation 1) you use when the physics teacher asks you to calculate a speed, given some actions during a period of time.

Equation 2) you use when the physics teacher asks you to calculate a position, given some actions during a period of time.

Equation 3) you use when the physics teachers asks you to calculate a speed, given some actions over some distance.

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