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AP®︎/College Physics 1
Course: AP®︎/College Physics 1 > Unit 1
Lesson 4: Velocity and speed from graphs- Instantaneous speed and velocity
- Why distance is area under velocity-time line
- Average velocity and average speed from graphs
- Instantaneous velocity and instantaneous speed from graphs
- Finding displacement from velocity graphs
- Instantaneous velocity and speed from graphs review
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Instantaneous velocity and speed from graphs review
Review the key terms and skills related to analyzing motion graphs, such as finding velocity from position vs. time graphs and displacement from velocity vs. time graphs.
Key terms
| Term | Meaning | |
|---|---|---|
| Instantaneous velocity | Velocity at a given moment in time. Has SI units of start fraction, start text, m, end text, divided by, start text, s, end text, end fraction. | |
| Instantaneous speed | Speed at a given moment in time. Equal to the magnitude of the instantaneous velocity. Has SI units of start fraction, start text, m, end text, divided by, start text, s, end text, end fraction. |
| Equation | Symbol breakdown | Meaning in words |
|---|---|---|
| v, with, \bar, on top, equals, start fraction, delta, x, divided by, delta, t, end fraction | v, with, \bar, on top is average velocity, delta, x is displacement, and delta, t is change in time. | Average velocity is displacement divided by time interval of displacement. |
| v, start subscript, start text, a, v, g, end text, end subscript, equals, start fraction, d, divided by, delta, t, end fraction | v, start subscript, start text, a, v, g, end text, end subscript is average speed, d is distance, and delta, t is change in time . | Average speed is distance divided by time interval for the distance traveled. |
Analyzing motion graphs
Velocity is the slope of position vs. time graph
The equation for the slope of a position vs. time graph matches the definition of velocity exactly.
To calculate the average velocity between two points P, start subscript, 1, end subscript and P, start subscript, 2, end subscript, we divide the change of position delta, x by the change in time delta, t.
The instantaneous velocity at point P, start subscript, 1, end subscript is equal to the slope of the position graph at point P, start subscript, 1, end subscript.
Displacement is the area under the curve on a velocity vs. time graph
To find the displacement between two points P, start subscript, 1, end subscript and P, start subscript, 2, end subscript on a velocity vs. time graph, we find the area under the curve between the two points.
The change in time delta, t will be the width of the area, and the height v is on the vertical axis.
Common mistakes and misconceptions
Some people think instantaneous velocity and speed are the same as average velocity and speed. When people use the words speed or velocity, they usually mean instantaneous velocity or instantaneous speed. Average velocity and speed account for motion occurring over a time period, and instantaneous velocity and speed describe motion at a given moment in time.
Learn more
For deeper explanations of velocity and speed see the videos on
instantaneous speed and velocity and
position vs. time graphs.
To check your understanding and work toward mastering these concepts, check out these exercises:
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can you show example and exercise how to solve instantaneous velocity or speed involving curve graph.. i very like this website to learn things that i missed in class bcs of the way of explanation process very clear, fun, and easy to understand.. i hope you can improve the 'instantaneous' topics and notify me please.. ^_^(26 votes)
I have difficulty conceptualizing a displacement as an area. The math makes sense. I just can't "see" it.(7 votes)
How do you you find the Instantaneous velocity speed from graphs review?(2 votes)
The slope of a given point is the instantaneous velocity. For example if you had a position vs. time graph, then if you were to find the instantaneous velocity of a moment, point P, then the slope of point P is the instantaneous velocity. Was this helpful?(6 votes)
- What does it mean when it says that instantaneous speed is the magnitude of instantaneous velocity?(2 votes)
- It's the quantity without the sign. So if the instantaneous velocity is, for example, -2m/s, the instantaneous speed is 2m/s.(2 votes)
- Exercise number 4: finding displacement from velocity graphs really challenge me: 4 attempt until i can find the right answer.(1 vote)
How do you know how and what to do with specific graphs??(1 vote)
It depends on what you want to know, Typically, just read what the problem is asking for(1 vote)
Can someone tell me why the area under the slope is the displacement?(1 vote)- Displacement is basically the vector form of distance, and we know distance is speed x time. So displacement is velocity (vector form of speed) x time, which is what’s plotted on the graph. The area under the graph will thus be velocity x time, giving you the displacement.(1 vote)
Can you explain to me show the solution how to find second when they give t=0 I'm so wrong on this point I did it again and again but I'm still wrong(1 vote)- Please can you explain what they meant in the
Common mistakes and Misconceptions.
And please secondly what does constant slope really mean in an x vs t graph. Does it mean average velocity(or slope)?(1 vote) 
Is this a trick question?(1 vote)