Answer to Essential Question 2.1: The magnitude of the net displacement is always less than or equal to the total distance. The two quantities are equal when the motion occurs without any change in direction. In that case, the individual displacements point in the same direction, so the magnitude of the net displacement is equal to the sum of the magnitudes of the individual displacements (the total distance). If there is a change of direction, however, the magnitude of the net displacement is less than the total distance, as in Example 2.1.
2-2 Velocity and Speed
In describing motion, we are not only interested in where an object is and where it is going, but we are also generally interested in how fast the object is moving and in what direction it is traveling. This is measured by the object’s velocity.
Average velocity: a vector representing the average rate of change of position with respect to time. The SI unit for velocity is m/s (meters per second).
Because the change in position is the displacement, we can express the average velocity as:
. (Equation 2.2: Average velocity)
The bar symbol ( _ ) above a quantity means the average of that quantity. The direction of the average velocity is the direction of the displacement.
“Velocity” and “speed” are often used interchangeably in everyday speech, but in physics we distinguish between the two. Velocity is a vector, so it has both a magnitude and a direction, while speed is a scalar. Speed is the magnitude of the instantaneous velocity (see the next page). Let’s define average speed.
In Section 2-1, we discussed how the magnitude of the displacement can be different from the total distance traveled. This is why the magnitude of the
average velocity can be different from the average speed.
Average Speed = (Equation 2.3: Average speed)
EXAMPLE 2.2A – Average velocity and average speed
Consider Figure 2.6, the graph of position-versus-time we looked at in the previous section. Over the 50-second interval, find:
(a) the average velocity, and (b) the average speed.
SOLUTION
(a) Applying Equation 2.2, we find that the average velocity is:
.
Figure 2.6: A graph of your position versus time over a 50-second period as you move along a sidewalk.
Chapter 2 – Motion in One Dimension
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