Interpreting direction of motion from velocity-time graph
About this video
Using a velocity-time graph, Sal decides whether an object is moving forward, backward or neither at several points. Positive velocity means forward, negative velocity means backward, and zero velocity means neither, even if the graph is rising while still below zero.
You'll be able to
- use the sign of velocity to tell the direction of motion.
- explain why a rising velocity graph below zero still means moving backward.
- tell the difference between reading a velocity-time graph and a position-time graph.
Key words
- velocity
- How fast and in which direction an object moves; positive is forward, negative is backward.
- velocity-time graph
- A graph showing the object's velocity at each time.
Transcript
- [Instructor] An object is moving along a line. The following graph gives the object's velocity over time. For each point on the graph, is the object moving forward, backward, or neither? So pause this video and see if you can figure that out. All right, now let's do this together.
And so we can see these different points on this velocity-versus-time graph. And the important thing to realize is is if the velocity is positive, we're moving forward. If the velocity is negative, we're moving backward. And if the velocity is zero, we're not moving either forward nor backwards, or neither forward nor backwards.
So right over here we see that our velocity is positive. It's a positive two meters per second. So that means that we are moving forward. Now, over here our velocity is zero meters per second. So this is neither. Now, over here our velocity is negative four meters per second.
So one way to think about it is we're moving four meters per second backward. So I'll write backward. Now, this is interesting, this last point. 'Cause you might be tempted to say all right, I'm oscillating. I'm going up. Then I'm going down. Then I'm going back up.
Maybe I'm moving forward here. But remember, what we're thinking about here, this isn't position versus time. This is velocity versus time. So if our velocity is negative, we're moving backwards. And here our velocity is still negative. It's becoming less negative, but it's still negative.
So we are still moving, we are still moving backward. If we were at this point right over here or at this point, then we would be moving forward if our velocity were positive.
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