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course Phy 121
20/Jul/2010 20:37
Determine the acceleration of an object whose velocity is initially 18 cm/s and which accelerates uniformly through a distance of 55 cm in 4.2 seconds. Determine v_Ave:
55 cm / 4.2 s = 13.09 cm/s
18 cm/s + 13.09 cm/s = 31.09 cm.s
Determine vf:
31.09 cm/s * 2 = 62.18 cm/s
Determine a:
62.18 cm/s - 18 cm/s / 4.2 s = 11.05 cm/s^2
Initial velocity 18 cm/s and final velocity 62 cm/s imply average velocity 40 cm/s, which would cover the distance in less than 2 seconds. So final velocity 62 cm/s is not consistent with the given time interval.
Neither is 31.09 cm/s, for a similar reason.
In any case there is a condition on the idea of doubling average velocity to get final velocity. This requires that acceleration be uniform and initial velocity be zero.
Note that average velocity is the average rate of change of position with respect to clock time. So your first step does give you the average velocity.
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