How long will it take for ball to hit ground?
2015-09-04 6:23 am
Standing on the roof of a 70 m building, you throw a ball straight up with the initial speed of 13.5 m/s. If the ball misses the building on the way down, how long will it take to land on the ground?
回答 (2)
2015-09-04 7:02 am
✔ 最佳答案
s = ut + ½at²downwards +tive
u = initial velocity = - 13.5 m/s
a = g = +9.81 m/s²
=> 70 = -13.5t + ½9.81t²
=> 4.905t² - 13.5t - 70 = 0
or t² - 2.75t - 14.27 = 0
solving t ~= 5.395 s
hope this helps
2015-09-04 7:00 am
Take all downward quantities to be positive.
Take g = 10 m/s²
(You may take g = 9.8 m/s² or 9.81 m/s²)
initial velocity, vₒ = -13.5 m/s
acceleration a = 10 m/s²
displacement, s = 70 m
s = vₒt + (1/2)at²
70 = (-13.5)t + (1/2)(10)t²
70 = -13.5t + 5t²
14 = -2.7t + t²
t² - 2.7t - 14 = 0
t = 5.3 or t = -2.6 (rejected)
Time taken = 5.3 s
Take g = 10 m/s²
(You may take g = 9.8 m/s² or 9.81 m/s²)
initial velocity, vₒ = -13.5 m/s
acceleration a = 10 m/s²
displacement, s = 70 m
s = vₒt + (1/2)at²
70 = (-13.5)t + (1/2)(10)t²
70 = -13.5t + 5t²
14 = -2.7t + t²
t² - 2.7t - 14 = 0
t = 5.3 or t = -2.6 (rejected)
Time taken = 5.3 s
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