✔ 最佳答案
Strictly speaking, impact force doesn't include the weight of the falling object. It is the excessive force (or net force) that acts on the falling body in order to bring it to stand still.
Hence, in the example given,
since the velocity of the falling object before hitting the ground = square-root[2 x 9.8 x 2.8] m/s = 7.408 m/s
use impluse = change of momentum
(impact force) x 0.02 = 0.7 x 7.408
i.e. (impact force) = 259 N
The following two web-pages gives a calculator that works out the "impact force" from falling objects. Try to input the figures from your example. Using "distance travelled after impact" = average velocity x time = (7.408/2) x 0.02 m = 0.07408 m
You would get the same result of 259 N as calulated above
http://www.livephysics.com/tools/classical-mechanics/solve-problem-related-to-impact-force-from-falling-object.html
http://hyperphysics.phy-astr.gsu.edu/hbase/flobi.html#c1"
In fact, when the weight of the object is taken into account, the force is termed as the "normal reaction force", or "reistance force" experienced by the object.
