f.4 phys,,,why there are 2 ans?

Take upwards to be positive.

u = 5 m s-1
s = 50 cm = 0.5 m
a = -10 m s-2

s = ut + (1/2)at2
(0.5) = (5)t + (1/2)(-10)t2
5t2 - 5t + 0.5 = 0
10t2 - 10t + 1 = 0
t = [10+√(102-4x10x1)]/(2x10) or t = [10-√(102-4x10x1)]/(2x10)
t = 0.887 or t = 0.113
Time taken = 0.887 s or 0.113 s


There are two cases that s = 0.5 m:

The first case: The coin is going up and at the position of 50 cm above the ground. This takes 0.113 s.

The second case: The coin has reached the highest position and is going down at position of 50 cm above the ground. This takes 0.887 s.

Therefore, there are two answers.

first , you have to let which direction is positive (very important)

Step 1:
Let the upward direction be positive

Step 2:
since displacement include magnitude and direction, if the coin above the
starting pt 50cm, that means displacement S = 50cm
Reason:
the direction of displacement can be found from the motion of the coin
from the beginning point to the final point --> upward direction-->+ve
--> s = 50cm but not -50cm

step 3: using equation of motion (list the direction can make you better understanding and do less error)

u = +5ms^-1 (upward)
s= 0.5m (upward)
a =-10 ms^-2 (gravity always downward)

by using s = ut + 1/2at^2
0.5 = 5t - 5t^2
t = 0.113s and 0.887s

2008-10-20 01:16:45 補充：
return means: return to the floor so it has 2 answers

No. There should only be ONE answer, which is 0.113 second.

The question asks "........ When will the coin be 50cm above the starting point BEFORE IT RETURNS?"

The time of '0.887 second' is the time AFTER the ball returns from the maximum height, and hence should not takened as answer to this question.