Hope it helps u ☺️ plz mark it as brainiest dear..
sounds like a homework problem
the method you should use is work done = force x distance.
assume the resitive force is constant when piercing the plank and the same for both planks.
work out energy to pierce plank/energy to pierce plank. use energy or work = fx d but remeber the f values are the same so you can cancel.
now you know the energies of the kes: ke to pierce /ke to pierce = same ratio as in 2. above.
ke /ke = (1mv^2) ()/(1mv^2) now assume mass is constant and cancel.
ke /ke = v^2 ()/(v2)
you know thr artionof the kes, you know v ^2()
you should be home and dry.
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