物理科之力學問題

Assume the initial speed as Vo(m/s), using a coordinate with y-coordinate upward as positive.

Since the air resistance is negligible, so when the ball comes down to the the edge of the a cliff, it will have the same speed as it left hand, but the direction is DOWNWARD (this part is important).

Using S = So + Vot + 1/2 (at^2)

==> -56 = 0 - Vo(4) + 1/2(-9.8)(4)^2 (the negative is due to
assuming the coordinate
upward to be positive)
==> −56 = −4Vo - 78.4
==> −56+78.4 = −4Vo
==> Vo = 5.6 m/s <== answer

remark: If you assume the downward coordinate as positive, then you will get 56 = 0 +Vo(4) + 1/2 (9.8)(4)^2 and get Vo= −5.6. The reason is because the initial speed of stone Vo is thrown upward.

When you solve the kind of kinematics problem, due to displacement, velocity and acceleration being vectors. You need to pay close attention to the direction of them related to the coordinate you use.