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S E C T I O N 2 . 3 • Acceleration 33 Conceptual Example 2.4 Graphical Relationships between x, v x , and a x (a) (b) (c) x t
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x Figure 2.7 (Example 2.4) (a) Position–time graph for an ob- ject moving along the x axis. (b) The velocity–time graph for the object is obtained by measuring the slope of the position–time graph at each instant. (c) The acceleration–time graph for the object is obtained by measuring the slope of the velocity–time graph at each instant. The position of an object moving along the x axis varies with Solution The velocity at any instant is the slope of the A , the slope of the x -t graph increases uniformly, and so the velocity increases linearly, as shown in Figure 2.7b. A and t B , the slope of the x -t graph is constant, and so the velocity remains constant. At t D , the slope of the x -t graph is zero, so the velocity is zero at that instant. D and t E , the slope of the x -t graph and thus the velocity are negative and decrease uniformly in this inter- E to t F , the slope of the x-t graph is still negative, and at t F it goes to zero. Finally, after t F , the slope of the x -t graph is zero, meaning that the object is at F . The acceleration at any instant is the slope of the tan- gent to the v x -t graph at that instant. The graph of accelera- tion versus time for this object is shown in Figure 2.7c. The A , where the slope of the v x -t graph is positive. It is zero be- tween t A and t B and for t & t F because the slope of the v x -t graph is zero at these times. It is negative between t B and t E because the slope of the v x -t graph is negative during this interval. Note that the sudden changes in acceleration shown in Figure 2.7c are unphysical. Such instantaneous changes can- Quick Quiz 2.3 Make a velocity–time graph for the car in Figure 2.1a. The speed limit posted on the road sign is 30 km/h. True or false? The car exceeds the Therefore, the average acceleration in the specified time in- B # t A ! 2.0 s is The negative sign is consistent with our expectations— (B) Determine the acceleration at t ! 2.0 s. # 10 m/s 2 ! a x ! v xf # v xi t f # t i ! v x B # v x A t B # t A ! (20 # 40) m/s (2.0 # 0) s v x B
! (40 # 5t B 2 ) m/s ! [40 # 5(2.0) 2 ] m/s ! % 20 m/s The velocity of a particle moving along the x axis varies in x ! (40 # 5t 2 ) m/s, where t is in seconds. (A) Find the average acceleration in the time interval t ! 0 to t ! 2.0 s. Solution Figure 2.8 is a v x -t graph that was created from the velocity versus time expression given in the problem x -t curve is nega- tive, we expect the acceleration to be negative. We find the velocities at t i ! t A ! 0 and t f ! t B ! 2.0 s by substituting these values of t into the expression for the v x A
! (40 # 5t A 2 ) m/s ! [40 # 5(0) 2 ] m/s ! % 40 m/s Example 2.5 Average and Instantaneous Acceleration |