✔ 最佳答案
In theory, the only force acting on both 雞毛 and 鐵球 is the gravitational force in the vertical direction, so that there is only gravitational acceleration in the vertical direction.
s=ut+at^(1/2) => displacement = initial velocity * time + acceleration * square root of time
From the formulae above, time taken for those 2 objects to reach the ground is not depending on the mass or weight of the object.
It depends on the initial velocity and the acceleration of the object.
Since displacement and acceleration for both 雞毛 and 鐵球 is the same, there shouldn't be any difference between the time taken for them to travel from the same height to ground if the initial velocity of them are the same as well.
However, in reality, there is drag force or air resistance force acting on those 2 objects, so that they will reach the ground at different time.
Drag equation
Drag force = -1/2 *density * velocity^2 * Area * drag coefficient
Drag force is a force opposes the motion of the object when it moves through a fluid such as air or water. Drag force depends on the shape of the object, the properties of the fluid, and the speed of the object relative to the fluid.
Drag coefficients for rough unstreamlined objects can be 1 or more, for smooth object much less.
In this case, since the motion of the 2 objects is going downwards, therefore drag force is acting upwards. 鐵球 should be a more streamlined object compare to a 雞毛, so the drag coefficient of the 鐵球should be smaller than 雞毛. However, the area of a 雞毛 may be smaller than a 鐵球.
(You really need to consider these points when you think about this question.)
By assuming that the area of the 雞毛 and 鐵球 are the same. I'm quite sure that a 鐵球 would reach the ground first.
2006-11-21 08:59:46 補充:
the 1st formulae should be s = ut 1/2 at^2sorry that i've made such silly mistakes......