



 
Hi Kobina. The formula for potential energy is simply PE = mgh. When 40% of it has been lost, then that energy has become kinetic energy, so KE = (0.40)mgh. But KE is related to the speed of the object and its mass: KE = (1/2) mv^{2}. We're interested in the velocity, not the amount of KE or PE, so we can set these two expressions equal to each other: (0.40)mgh = (1/2)mv^{2} And when we solve for the velocity, we notice that mass doesn't event matter, thus v = sqrt(0.8(gh)) g is a constant: 9.8 m/s^{2} and h is given: 20 m. So you can complete the calculation and find the answer. Cheers,  


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