Interpreting direction of motion from position-time graph
About this video
Using a position-time graph, Sal decides whether an object is moving forward, backward or neither at several points. A downward-sloping graph means moving backward, an upward slope means moving forward, and a horizontal tangent line means neither, at that instant.
You'll be able to
- read position and time from a position-time graph.
- use the slope of the tangent line to tell whether an object is moving forward, backward or neither.
- explain why position and direction of motion are different things.
Key words
- position-time graph
- A graph showing where an object is, relative to its starting point, at each time.
- slope of the tangent line
- How steep the graph is at one point; on a position graph it tells you the direction and rate of motion.
- starting point
- Position 0 on this graph; positive positions are ahead of it, negative positions are behind it.
Transcript
- [Instructor] An object is moving along a line. The following graph gives the object's position, relative to its starting point, over time. For each point on the graph, is the object moving forward, backward, or neither? So pause this video and try to figure that out.
All right, so we see we have position in meters versus time, so for example, this point right over here tells us that after one second, we are four meters ahead of our starting point. Or for example, this point right over here says that after four seconds, we are almost it seems, almost four meters behind our starting point.
So let's look at each of these points and think about whether we're moving forward, backward, or neither. So at this point right over here, at that moment, we're about 2 1/2 meters in front of our starting point. We're at a positive position of 2 1/2 meters. But as time goes on, we are moving backwards closer and closer to the starting point.
So this is we are moving backward. One way to think about it, at this time we're at 2 1/2 meters. If you go forward about half a second, we are then back at our starting point. So we had to go backwards. And if we look at this point right over here, it looks like we were going backwards this entire time while our curve is downward sloping.
But at this point right over here, when we are about it looks like five meters behind our starting point, we start going forward again. But right at that moment, we are going neither forward nor backwards. It's right at that moment where we just finished going backwards and we're about to go forward, and one way to think about it is, what would be the slope of the tangent line at that point, and the slope of the tangent line at that point would be horizontal.
And so this is neither. So we can use that same technique to think about this point. The slope is positive. And we see that all right, right at that moment, it looks like we are at the starting point, but if you fast forward even a few, even a fraction of a second, we are now in front of our starting point.
So we are moving forward. We are moving forward right over here. And at this point, we are at our starting point, but if you think about what's going to happen a moment later. A moment later we're gonna be a little bit behind our starting point, and so here we are moving backward.
And we're done.
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