What is a curved line in velocity time graph represent?
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A curved line shows the slope is constantly changing. Think about it - an upward curve shows the slope is pretty flat at first but it gets very steep as time increases.

So what is the slope of a Vt graph represent? Well, slope is rise over run, aka delta V over delta t. That’s just the formula for acceleration.

We know the slope is changing, and the slope is acceleration, therefore we can say that the acceleration is changing.

It turns out that if you graph this changing acceleration with time, the acceleration changes at a constant rate - it’s linear, NOT a curve. (If you have specific values you could plot this, but if you’re just talking about general graphs, then just take my word for it).

In summary - if you’re velocity is changing at a non constant rate, then you’re acceleration must be changing (at a constant rate).

Hope this helped!