Acceleration: How fast how fast is changing.

We can change the state of motion of an object by changing its speed, direction, or both. Any of these changes is a change in velocity. Sometimes we are interested in how fast the velocity is changing. The rate at which the velocity is changing is called the acceleration. Another way to say it is acceleration is how fast the velocity is changing.

Acceleration = (change in velocity) / (change in time)

The key idea that defines acceleration is change. Whenever we change our state of motion, we are accelerating. If we speed up, we are accelerating. If we slow down, we are accelerating (this is often called deceleration, or negative acceleration). If we change directions we are accelerating. Acceleration is all about change. Just because something is moving, it doesn’t mean it is accelerating. Acceleration is a change in how fast it is moving.

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Which car(s) are accelerating?

Which car(s) are not accelerating?

Which car undergoes the greatest acceleration?

What does this mean as far as the graph is concerned? Remember that when we graph distance vs. time, the slope is the velocity. If the velocity is constant, then the slope (steepness) stays the same. It the slope (steepness) is changing, that must mean the velocity is changing. If the velocity is changing, then the object is accelerating.

Look at the graph below.



Which line(s) shows no acceleration?

Which line(s) shows acceleration?

Which line(s) shows the greatest acceleration?