標題:
antelope moving
發問:
An antelope moving with constant acceleration covers the distance between two points 70.0 m apart in 7.00 s. Its speed as it passes the second point is 15.0 m/s. a) What is the speed at the first point? b) What is its acceleration? ans: a) 5m/s b) 1.43 m/s^2
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最佳解答:
Let u be the speed at the first point, and a be the acceleration of the antelope. Use equation of motion: v = u + at 15 = u + 7a i.e. 7a = 15 - u --------------- (1) Use equation of motion: v^2 = u^2 + 2a.s 15^2 = u^2 + 2 x 70a i.e. 140a = 225 - u^2 --------------- (2) (2)/(1): 140/7 = 225 - u^2/(15-u) u^2 - 20u + 75 = 0 hence, u = 5 m/s or 15 m/s (rejected, as this implies no acceleration) Therefore, the speed of the antelope at the first point is 5 m/s Substitute u = 5 m/s into (1) a = (15 - 5)/7 m/s^2 = 1.43 m/s^2
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