Simple physics for horizontal force is F=M*A, and falling force is F=M8G.
If my math is correct a rig traveliong .5 mph would have an impact force of 2.933lbs, i.e. F=4000*0.73333333(.5mph in ft/second). However seeing how most trail impacts often from lurching, which could easily be 5mph, a plastic skid plate would have to stand up to at least 29,333lbs of force, or F=4000*5mph(7.3333333 ft/second).
Dropping onto a rock would be F=4000*9.8m/s/s(32ft/s/s). A 4000lb rig dropping for a 1/10 of a second would have 12,800 lbs of force, i.e. F=4000*3.2(32ft/s/s*.1).
My number was for the energy that it would see . Also your math is alittle off. The time part is not the speed of the object but how long it is under the force. Its hard to explain. Think of it as catching a egg if you just hold your hand there and try there is no time. But if you move your hands as you catch to lessen the force you add time. That lessens the force on the egg and allows it not to break.
\ I did say if it bounces back up. a ball hitting a wall will carry less force then a ball hitting a ball and rebounding back more. There are 2 different kinds of force.
http://hyperphysics.phy-astr.gsu.edu/hbase/flobi.html#c1%22
