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# What Is Acceleration?

### What Is Acceleration?

Acceleration is a change in velocity.

An example of **acceleration**

This woman is moving at $3 \space m/s$, so this image tells us the woman's _____________.

One second later the woman is moving at $7 m/s$. What happened?

In other words, the woman has "accelerated"

Over the course of **1 second**, she sped up from $3 \space m/s$ to $7 \space m/s$

Which one of these do you think has the fastest **acceleration**?

Speed and velocity describe how quickly an object travels. What best describes **acceleration**?

How to recognise acceleration

What type of graph is this?

This is a distance - time graph.

It tells you the position of an object at any point in time.

What does the gradient of this D-T graph tell us?

So the gradient or slope of a D-T graph shows us the velocity the object has. Is this particular object travelling at a constant velocity?

So if the speed of this object is not constant, in 1 word what is it doing?

So, acceleration is the change in speed or velocity of an object.

This means that a D-T graph with a changing gradient, or a curved line, represents an accelerating object.

Acceleration and deceleration

What is this cyclist doing?

Is this cyclist **accelerating**?

The cyclist's acceleration is...

So this cyclist has a negative acceleration.

This is the same as saying that this cyclist is **decelerating.**

*True or false:* **Accelerating***at* $-3 \space m/s^2$ *is the same as* **decelerating***at* $3 \space m/s^2$

So **acceleration** can be either positive (speeding up) or negative (slowing down). Do you think you can also talk about both positive and negative **deceleration**?

So an object getting slower and slower has a **negative** acceleration whereas an object getting faster and faster has a **positive** acceleration. What does that make acceleration?

So, what is the best description of acceleration?

Summary!

Acceleration is how fast the velocity of an object changes

Or its **rate of change of velocity.**

Acceleration can be shown by a curved line on a D-T graph

This image is a good example of an object that is accelerating

Acceleration can be both positive and negative

**Positive** values for acceleration means speeding up, for example $5 \space m/s^2$

**Negative** values for acceleration means slowing down, for example $-5 \space m/s^2$

You can also use the word "deceleration" for negative acceleration

For example, **accelerating** at $-5 \space m/s^2$ is the same as **decelerating** at $5 \space m/s^2$

You cannot have negative values for deceleration

Deceleration **always** means slowing down, so it doesn't make sense to have both negative and positive values for deceleration.