The slope at any point on a displacement-versus-time graph is the instantaneous velocity at that point. The slope of the curve becomes steeper as time progresses, showing that the velocity is increasing over time. The graph of displacement versus time in Figure 3 (a) shown above is a curve rather than a straight line. (c) Acceleration has the constant value of 5.0 m/s 2 over the time interval plotted. t graph is constant for this part of the motion, indicating constant acceleration. Instantaneous velocity at any point is the slope of the tangent at that point. This is shown at two points, and the instantaneous velocities obtained are plotted in the next graph. Graphs of motion of a jet-powered car during the time span when its acceleration is constant. Time starts at zero for this motion (as if measured with a stopwatch), and the displacement and velocity are initially 200 m and 15 m/s, respectively. The graphs in Figure 3 below represent the motion of the jet-powered car as it accelerates toward its top speed, but only during the time when its acceleration is constant. Graphs of Motion when α is constant but α≠0 This is an impressively large land speed (900 km/h, or about 560 mi/h): much greater than the typical highway speed limit of 60 mi/h (27 m/s or 96 km/h), but considerably shy of the record of 343 m/s (1234 km/h or 766 mi/h) set in 1997. Thus a graph of displacement versus time gives a general relationship among displacement, velocity, and time, as well as giving detailed numerical information about a specific situation. Substituting these symbols into y = mx + b gives Using the relationship between dependent and independent variables, we see that the slope in the graph above is average velocity or \boldsymbol and the intercept is displacement at time zero-that is, x 0. Graph of displacement versus time for a jet-powered car on the Bonneville Salt Flats. It shows a graph of displacement versus time for a jet-powered car on a very flat dry lake bed in Nevada. Figure 2 shown below is just such a straight-line graph. A graph of displacement versus time would, thus, have x on the vertical axis and t on the horizontal axis. Time is usually an independent variable that other quantities, such as displacement, depend upon. The equation for a straight line is y = mx + b. The letter b is used for the y-intercept, which is the point at which the line crosses the vertical axis. Here m is the slope, defined to be the rise divided by the run (as seen in the figure) of the straight line. If we call the horizontal axis the x-axis and the vertical axis the y-axis, as in Figure 1 a straight-line graph has the general form When two physical quantities are plotted against one another in such a graph, the horizontal axis is usually considered to be an independent variable and the vertical axis a dependent variable. The two PHET simulations mentioned in the previous chapter are embedded here, so you can play with them now.įirst note that graphs in this text have perpendicular axes, one horizontal and the other vertical. Please note that this uses Flash so it might not run on all computers. y=bx) to see how they add to generate the polynomial curve. View the curves for the individual terms (e.g. The shape of the curve changes as the constants are adjusted. PHET EXPLORATIONS: Simulations to help you understand the concept Graphing Slope-Intercept Graphing Slope Intercept.ĭirect link: Graphing Straight Lines Graphing Straight Lines.
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