Question

Sally Leadfoot was pulled over on her way from Syracuse to Ithaca by an officer claiming she was speeding. The speed limit is 65 mi/hr and Sally had traveled 97 km in 102 minutes. How fast was Sally’s average speed? Does she deserve a ticket?

Answer 1

Sally's average speed is 35.3 mi/h.

Average speed of a body is the total distance traveled in the given time interval.

Express the distance  d traveled in miles.

$d=\frac{97 km}{1.6 km/m\\i} \\ = 60 mi$

Express the time t traveled in hours.

$t =\frac{102 min}{60 min/hr}\\ = 1.7 h$

Calculate the average speed v.

$v =\frac{d}{t} \\ =\frac{60 mi}{1.7h} \\ = 35.3 mi/h$

Her average speed is 35.3 mi/h, which is less than the speed limit of 65 mi/h.

However, the average speed of an object is different from its instantaneous speed. It could be possible that at the time when the officer apprehended her, Sally could have been travelling at a speed greater than the prescribed speed limit, which would have prompted the officer to issue a speeding ticket to her.

Thus, the average speed of a person cannot be considered as a bench mark for speeding offences, since her instantaneous speed could have been higher than the speeding limit and yet she could have had an average speed less than the speeding limit.