Kinematics: velocity time graph concept of acceleration?

On your graph, deceleration (slowing down) only occurs for t=0 to t=1s. But acceleration occurs for the whole period from t=0 to t=2s.

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The problem is that in common-use, 'acceleration' means getting faster, and the opposite, deceleration, means getting slower.

For physics and maths, put this out of your mind!!!

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Acceleration in physics (and maths) is a speed-change (getting faster *or* slower) and/or a direction change. It is a vector, meaning it has a direction as well as a magnitude.

(Technically acceleration is the rate of change of velocity.)

But the term deceleration (in common-use *and* in physics) simply means decreasing speed. In physics 'deceleration' is NOT the opposite of acceleration.

So 'deceleration' should only ever be used when the object is slowing down - i.e. the magnitude of velocity (i.e. speed) is decreasing. Or better still, never use it unless you really have to!

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NOTES

A positive value of acceleration does *not* mean speeding up. A negative value of acceleration does not mean slowing down. The positive (or negative) sign tell you only the direction.

For example, a 2kg mass has v = -6m/s (taking negative to mean 'to the left'). So the mass is moving left with a speed of 6m/s.

If you apply a force of -3N (3N to the left) to the mass; this speeds the mass up (because the force is in the same direction as the velocity). a = F/m = -3/1 = -3m/s². The direction of acceleration (a vector) is the same as the direction of the force.

So here is an example of a negative acceleration giving a speed increase.

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The same sort of thing happens when you throw an ball up - the acceleration is always 9.8m/s² downwards, but the speed can decrease (when the ball is rising) or increase (when the ball is falling) even though the acceleration is unchanged.

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The video on v-t graphs may help.