Representing Velocity

Section 2.7 Representing Velocity

You now have two major quantities for describing motion: position and velocity. You want to make sure both kinds of quantities can be encoded in a motion diagram.

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Representation 2.7.1. Motion Diagram.

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A motion diagram is a physical representation in which an object is represented at different positions that are separated by equal time intervals. For each instant, you can draw and label a vector indicating the velocity of our object of interest.

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The simplest possible motion is motion along a straight-line path with a constant, unvarying speed. Let’s imagine a car that is driving to the right at a constant speed. Suppose you take images at a regular time interval, once every second:

Six pictures of a car arranged horizontally. A blue arrow above each car points to the right. — 🔗
Figure 2.7.2. A stroboscopic diagram of a car moving at constant speed.

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You can apply the particle model and represent the car as a point object, creating a motion diagram showing the position of the car at each instant of time and the corresponding displacement vectors.

Six horizontally spaced dots, each dot separated by a labeled arrow. — 🔗
Figure 2.7.3. A motion diagram of a car moving at constant speed.

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Notice that the displacement vectors are equal in length when the car moves at constant speed.

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Exercises Practice Activities

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1. Extending Your Previous Work.

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Recall the motion diagram you made for the ball rolling across the floor: Figure 2.5.1.

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At each point on your diagram, add a vector representing the instantaneous velocity at that instant. You should be able to explain how you used the position vectors you have already drawn to determine both the magnitude and the direction of your new velocity vectors.

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Use the strobe diagram below for the remaining exercises. The diagram shows a ball moving from left to right. A total of