antelope moving

2013-05-23 5:18 am

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

回答 (1)

天同

2013-05-23 5:48 am

✔ 最佳答案

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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