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### Course: Physics library>Unit 1

Lesson 3: Acceleration

# What are acceleration vs. time graphs?

See what we can learn from graphs that relate acceleration and time.

## What does the vertical axis represent on an acceleration graph?

The vertical axis represents the acceleration of the object.
For example, if you read the value of the graph shown below at a particular time, you will get the acceleration of the object in meters per second squared for that moment.
Try sliding the dot horizontally on the graph below to choose different times, and see how the acceleration—abbreviated Acc—changes.

Concept check: According to the graph above, what is the acceleration at time ?

## What does the slope represent on an acceleration graph?

The slope of an acceleration graph represents a quantity called the jerk. The jerk is the rate of change of the acceleration.
For an acceleration graph, the slope can be found from $\text{slope}=\frac{\text{rise}}{\text{run}}=\frac{{a}_{2}-{a}_{1}}{{t}_{2}-{t}_{1}}=\frac{\mathrm{\Delta }a}{\mathrm{\Delta }t}$, as can be seen in the diagram below.
This slope, which represents the rate of change of acceleration, is defined to be the jerk.
$\text{jerk}=\frac{\mathrm{\Delta }a}{\mathrm{\Delta }t}$
As strange as the name jerk sounds, it fits well with what we would call jerky motion. If you were in a ride where the acceleration was increasing and decreasing significantly over short periods of time, the motion would feel jerky, and you would have to keep applying different amounts of force from your muscles to stabilize your body.
To finish up this section, let's visualize the jerk with the example graph shown below. Try moving the dot horizontally to see what the slope—i.e., jerk—looks like at different points in time.

Concept check: For the acceleration graph shown above, is the jerk positive, negative, or zero at ?

## What does the area represent on an acceleration graph?

The area under an acceleration graph represents the change in velocity. In other words, the area under the acceleration graph for a certain time interval is equal to the change in velocity during that time interval.
$\text{area}=\mathrm{\Delta }v$
It might be easiest to see why this is the case by considering the example graph below which shows a constant acceleration of 4 for a time of 9 s.
If we multiply both sides of the definition of acceleration, $a=\frac{\mathrm{\Delta }v}{\mathrm{\Delta }t}$, by the change in time, $\mathrm{\Delta }t$, we get $\mathrm{\Delta }v=a\mathrm{\Delta }t$.
Plugging in the acceleration 4 and the time interval 9 s we can find the change in velocity:
Multiplying the acceleration by the time interval is equivalent to finding the area under the curve. The area under the curve is a rectangle, as seen in the diagram below.
The area can be found by multiplying height times width. The height of this rectangle is 4, and the width is 9 s. So, finding the area also gives you the change in velocity.
The area under any acceleration graph for a certain time interval gives the change in velocity for that time interval.

## What do solved examples involving acceleration vs. time graphs look like?

### Example 1: Race car acceleration

A confident race car driver is cruising at a constant velocity of 20 m/s. As she nears the finish line, the race car driver starts to accelerate. The graph shown below gives the acceleration of the race car as it starts to speed up. Assume the race car had a velocity of 20 m/s at time .
What is the velocity of the race car after the 8 seconds of acceleration shown in the graph?
We can find the change in velocity by finding the area under the acceleration graph.
But this is just the change in velocity during the time interval. We need to find the final velocity. We can use the definition of the change in velocity, $\mathrm{\Delta }v={v}_{f}-{v}_{i}$, to find that
The final velocity of the race car was 44 m/s.

### Example 2: Sailboat windy ride

A sailboat is sailing in a straight line with a velocity of 10 m/s. Then at time , a stiff wind blows causing the sailboat to accelerate as seen in the diagram below.
What is the velocity of the sailboat after the wind has blown for 9 seconds?
The area under the graph will give the change in velocity. The area of the graph can be broken into a rectangle, a triangle, and a triangle, as seen in the diagram below.
The blue rectangle between and is considered positive area since it is above the horizontal axis. The green triangle between and is also considered positive area since it is above the horizontal axis. The red triangle between and , however, is considered negative area since it is below the horizontal axis.
We'll add these areas together—using $hw$ for the rectangle and $\frac{1}{2}bh$ for the triangles—to get the total area between and .
But this is the change in velocity, so to find the final velocity, we'll use the definition of change in velocity.
The final velocity of the sailboat is .

## Want to join the conversation?

• I don't get why the slope is shown to be negative in the two examples, while both objects are gaining speed. Seems like the slope should be positive... What's the reasoning behind this?
• These are acceleration vs time graphs. Any line ABOVE the time axis (a=0) indicates positive acceleration. and a line below the time axis indicates negative acceleration (Slowing down)
BUT The slope of the lines says NOTHING about the amount of acceleration.
The slope is only a measure of 'jerkiness' (or rate of change) of acceleration.
• Are there quantities like acceleration of jerk, jerk of jerk and so on?
• They keep referring to straight lines as curves. (i.e. "But multiplying the acceleration by the time interval is equivalent to finding the area under the curve. The area under the curve is a rectangle, as seen in the diagram below.") Why is this? Is there a reason?
• Area under a curve generally talks about the area under a specific geometric shape, be it a line or a curvature(sometimes known as concavity). Some teachers also teach it as 'area under the graph', so as long as you know what you're calculating, you should be fine.
• The area under the acceleration-time graph is velocity.
That under the velocity-time graph is displacement (or may be distance).

What about that under the displacement-time graph, what would it be?
• I believe it represents a negative derivative of displacement. This is called Absement and is essentially the "total" displacement. Essentially, the derivative of this is displacement, the "change" in Absement, and velocity would the derivative of displacement, the "change" in displacement, the acceleration being the second-order derivative, and so on.

The area under the curve is the anti-derivative, and in lay terms moving upwards. For instance, the area under acceleration-time graph is the velocity, moving upwards.

For reference, I located a list of the derivatives of displacement.

-1. Absement
0. Displacement
1. Velocity
2. Acceleration
3. Jerk
4. Jounce (snap)
5. Crackle
6. Pop
7. Lock
8. Drop
9. Shot
10. Put
• ok, but how do you go about making a acceleration time graph?
• Or you could measure acceleration at each point in time with an accelerometer.
• How can we calculate the jerk using only the information given by a velocity-time graph ?
• Take the second derivative of the v(t) graph to get J(t).
J(t) = da(t)/dt = d²v(t)/dt²
• in Example 1: Race car acceleration it is said that the driver had a constant velocity of 20 m/s and again she had it at 0s. but in the graph it is shown that at 0s she had a acceleration of 6 m/s2. so my question is if the graph is mistaken or it is right. since its clear that constant velocity means 0 acceleration and she had a constant velocity of 20 m/s at 0s, so it means the acceleration at 0s would also be 0m/s2 . am i right?