A racing car accelerates uniformly through three gear changes with the following average speeds:20 ms-1 for 2.0s ;40ms-1 for 2.0 s and 60 ms-1 for 6.0s. What is the overall average speed of the car?
Correct answer: D. 48 ms-1
- A. 12 ms-1
- B. 13.3 ms-1
- C. 40 ms-1
- D. 48 ms-1
Explanation
To find the overall average speed, we use the formula: Average speed = Total distance / Total time. First, calculate the individual distances traveled during each gear change:For the first gear change: Distance = 20 ms-1 x 2 s = 40 mFor the second gear change: Distance = 40 ms-1 x 2 s = 80 mFor the third gear change: Distance = 60 ms-1 x 6 s = 360 mThe total distance is 40 m + 80 m + 360 m = 480 m. The total time is 2 s + 2 s + 6 s = 10 s. Thus, the overall average speed is 480 m / 10 s = 48 ms-1, making Option D the correct answer.Options A, B, and C are incorrect because they do not correctly apply the formula for average speed, either due to incorrect calculations or by using incorrect values.
This question appeared on the ETEA MDCAT 2013 paper, which you can sit online with every answer explained.
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