Physics 2a - Velocity-Time Graphs

In an earlier post, I talked about distance-time graphs, which was one of the types of graphs you needed to know for the further science exam. You can read more about that type of graph if you click here! Here, I will talk about the other way to describe and object's journey: the velocity-time graph!

How to Read Velocity-Time Graphs

This type of graph is very similar to the distance-time graph except that the Y-axis is the velocity (m/s). You may notice that the unit is the same as the unit for speed. This is true! Both SPEED and VELOCITY measure how fast something is going. The only difference is that VELOCITY has a direction in addition to a value. For example, where a speed might be 50mph, the velocity could be 50mph west.

Below is an example of a velocity-time graph. Let's look at how to read it.

The graph has been split into 5 sections. By doing this, we can explain what the different parts mean.

If you get confused with the descriptions at any point, just like last time, imagine the object to be a car :D

A: Straight + Positive Correlation = ACCELERATION

Unlike distance-time graphs, if the line has a positive correlation, it shows us that the the velocity is increasing. This is unlike the distance-time graph where it meant that the object is traveling at a steady speed - don't get mixed up!

B & D: Straight + Flat = STEADY SPEED

If the line is flat, then it means that all parts of the line are at the same velocity; you are always at the same speed; you are at a steady speed! If we look at line B on the graph, we can tell that the object is traveling constantly at about 8m/s.

C: Curve + Positive Correlation = INCREASING ACCELERATION

This is still accelerating as we can tell from the fact that the line has a positive correlation; the velocity is increasing. However, this time it is a curve which means that the rate of acceleration is not constant/directly proportional, but increasing/decreasing (over-curves are decreasing but under-curves are increasing).

E: Straight + Negative Correlation = DECELERATING 

This time, the line has a negative correlation which means that the object is decelerating. If we follow different points of the line across to the Y-axis, we will see that the velocity is decreasing, proving that the object is decelerating.

How to Find the Distance

To find the distance traveled, you must calculate the area under the graph. You can do this using simple geometry skills. 

Eg: Find the distance traveled in the first 20secs.

(using the graph above)

Area = 1/2 x base x height

Area = 1/2 x 20 x 8

1/2 x 20 x 8 = 80m