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Why falling objects have a high impact force?
The concepts of falling objects can be demonstrated by considering the following hypothetical examples. For each example, assume that the falling object is a bowling ball that weighs ten pounds.
In the first example, assume that the bowling ball is simply placed on your foot and permitted to rest there. You will feel the Force applied to your foot as a result of the ball’s weight. In fact, the Force applied would be ten pounds, the weight of the bowling ball. If we now raise the ball until it is waist high, or say two feet from the top of your foot, and then we drop the ball onto your foot, you will notice a significant increase in the Force applied to your foot. When the ball was resting on your foot, it was not falling, and therefore, did not possess Kinetic Energy. However, with the Kinetic Energy of 20 foot pounds (10.0 pounds x 2.0 feet) attained by the falling ball, the Force applied to the foot becomes quite painful if not injurious. Moreover, it is intuitively clear that progressively increasing the fall height of the ball creates progressively greater impact forces, although it is clear that the weight of the ball remains the same at ten pounds. This hypothetical verifies the supposition that increased Kinetic Energy through increased fall height will create a greater applied Force at impact.
In the second example let us assume that you are lying on your back on the floor and that we drop the same bowling ball from a height of one foot onto the front of your thigh. Of course, there would be a painful sensation as a result of the applied impact Force. However, it is doubtful that any major injury would occur from this experiment. But, if we drop the same ball from a height of one foot onto your forehead if your head is resting against the ground, the results would most probably be considerably more injurious. Aside from the fact that a person’s head is considerably more valuable in the scheme of things than a person’s thigh, the Force applied as a result of the head impact would be much greater than the Force of the same-ball-same-height impact against the thigh. Now, if we drop the same ball from a height of one foot above a foam pillow resting on your forehead, the Kinetic Energy at impact would be the same ten foot-pounds (10.0 pounds x 1.0 foot) as that of the experiment without the pillow, but clearly, the results would be different with resulting injuries either eliminated or significantly mitigated. This is because the Kinetic Energy was expended over a greater Work distance than when the ball strikes the head directly. Simply stated, the Force applied to the head is reduced by the pillow because much of the Work done involves compressing the pillow.
How do you calculate the impact force?
When an object falls, it attains Kinetic Energy, and a falling object’s Kinetic Energy can be calculated by using the following relationship if the weight of the object is known, and if the fall height is known:
Kinetic Energy = Weight x Fall Height
Thus, if a ten-pound object falls through a distance of ten feet before it strikes the floor, its Kinetic Energy at the point of impact would be 100 foot-pounds.
When a flexible surface is struck by a falling object, the Kinetic Energy of the falling object performs Work on the soft body. This Work is manifested by the deformation or compression of the of the surface. Furthermore, if the falling object is pliable, there will be deformation of the object, as well.
In physics, Work is defined as the Force required to move an object through a distance, or:
Work = Force x Distance
Also, from physics, we know that Work is equal to the amount of Kinetic Energy expended to perform the work, or:
Work = Kinetic Energy
And, the foregoing gives rise to the relationship:
Force x Distance = Weight x Fall Height
Finally, we can rearrange this relationship into a form that will yield the Force applied as the result of being struck by a falling object:
Force = Weight x Fall Height
1. Weight is the weight in pounds of the falling object.
2. Fall Height is the distance in feet through which the object falls prior to impact.
3. Distance is the composite compression/deformation distance in feet of the struck object and the striking object during the impact.