Set 6 Problem number 1

• Problem

• Solution

• Generalized Solution

• Explanation in terms of Figure(s), Extension

• Figure(s)

Problem

On a graph of position vs clock time, with position in meters and clock time in seconds, we find the points ( 14 , 20 ) and ( 18 , 2 ).

• What do the rise and run between these points represent?
• What does the slope between these points represent?

Solution

Position will be plotted on the vertical axis with the independent variable time on the horizontal axis.

The rise will therefore indicate a change in position while the run will indicate a change in time. The rise represents a displacement in position from 20 meters to 2 meters or a displacement of

• rise = `ds = -18 meters.

The run represents a change in time from 14 seconds to 18 seconds, which implies a time interval of

• run = `dt = 4 seconds.

The slope is

• slope = rise/run = `ds / `dt =  -18 meters / ( 4 sec) = -4.5 meters/second.

The units of this result are units of velocity, suggesting that the slope represents velocity.

• In fact the slope was found by dividing a displacement by a time interval, which is congruent with the definition of velocity as rate of change of position (i.e., change in position divided by elapsed time).

Generalized Solution

On a graph of position s vs. clock time t, two points will have coordinates (t1, s1) and (t2, s2).

• The rise between these points is from s1 to s2, a rise of `ds = s2 - s1. This rise represents the difference in position, or displacement, between position s1 and position s2.
• The run is from t1 to t2, a run of `dt = t2 - t1. This run represents the difference in clock time between t1 and t2, or the time interval between t1 and t2.
• The slope is the rise divided by the run, which is `ds / `dt, the position change divided by the time interval. This is the average rate at which position s changes with respect to clock time t, or the average velocity of the object whose position is represented.

Explanation in terms of Figure(s), Extension

The graph below shows two points (t1, s1) and (t2, s2) on a graph of position vs. time. The rise is seen to be `ds = s2 - s1, representing the change in position. The run is seen to be `dt = t2 - t1, the time interval between the points.

The slope `ds / `dt therefore represents the position change divided by the time interval, which is the average rate at which the position changes. This average rate of change is generally called the average velocity.

Figure(s)