# What does it mean by 'a is constant' in motion..........'?

• HCverma

#### HCverma

Motion in a straight line has three formulas
1. v = u + at
2. v^2 - u^2 = 2as
3. s = ut + 1/2 at^2
These three formulas are applicable when a is constant. Now my question is 'what does it mean by 'when a is constant'? Could you please explain it?

It means the acceleration is constant over time.

Now my question is 'what does it mean by 'when a is constant'?
About the only 'constant acceleration' situation we encounter is when an object falls from a height of a few tens of metres and when it has a big enough mass that we ignore the effect of air resistance. I must have had my attention elsewhere when those formulae were first presented to me and I spent a lot of time worrying about how that could possibly apply to motor cars accelerating ("Pay attention you little oik!"). It was much later that I actually cottoned on properly.
Cars of course do not accelerate at a constant rate and neither do space rockets because they lose fuel constantly so their mass reduces throughout the burn. But we all need to learn to use those formulae because we have to start somewhere.

Now my question is 'what does it mean by 'when a is constant'? Could you please explain it?

It means that the acceleration always has a single value and does not vary. The real world almost never follows this simplification though.

Cars of course do not accelerate at a constant rate and neither do space rockets because they lose fuel constantly so their mass reduces throughout the burn.
Or the classic example of an object that often undergoes non-constant acceleration...

https://c8.alamy.com/comp/E5P2E2/a-...h-fighter-aircraft-performs-aerial-E5P2E2.jpg

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