Video: Dot Diagrams, Velocity, and Acceleration

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Dot Diagrams, Velocity, and Acceleration

Video Transcript
How can a dot diagram describe the motion of an object?
And how can you use a dot diagram with other info to determine the direction of the velocity and acceleration vectors?
I'm Mr. H and I have some answers for you.

Constant Speed, Increasing Speed, and Decreasing Speed
These are dot diagrams. They use a dot to represent the position of an object at constant intervals of time. For instance, these dots might represent object positions for every one second of time. By inspecting the spacing between dots, you can determine the whether it is a constant speed, speeding up, or slowing down motion. In this dot diagram, the dots are spaced an equal distance apart. This indicates a constant speed motion. For an increasing speed, the spacing between consecutive dots increases over the course of time ... like this. And for decreasing speed, the dot spacing decreases over the course of time. They become closer and closer together. What if an object is at rest? Well, there's one big pile of dots located at the resting position.
Some beginning Physics students mis-interpret dot diagrams for leftward-moving objects. When the object moves from right to left, the first dot is located on the right side of the diagram and the last dot on the left side. You read the dot diagram from right to left.
Vector Directions
Vector quantities like velocity and acceleration have a direction associated with them. And dot diagrams can be used to determine the direction of velocity and acceleration. But first, you need some rules. It's a simple rule for velocity: the direction of the velocity is the same as the direction the object is moving. The falling object has a downward velocity. This rising object has an upward velocity.
Acceleration directions are not so simple. That's because the direction of acceleration depends on which way the object is moving AND whether it is speeding up or slowing down. Here's the simplest way to put it:
If an object is slowing down, then the direction of the acceleration is in the opposite direction that the object is moving.  
Since this upward moving object is slowing down, it's acceleration direction is downward ... opposite the direction that it is moving. Of course you would need another rule for speeding up objects: the direction of acceleration is in the same direction that the object is moving. So since this downward moving object is speeding up, the acceleration vector is directed downwards ... the same direction that the object moves.
Hey - You Got This!

In the Description section of this video, you will find links to some awesome interactive exercises on our website. The best way to ensure understanding is to apply the concepts you're trying to understand. So give one of them a try. 
I'm Mr. H. Thanks for listening!  



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