A pole-vaulter is nearly motionless as he clears the bar, set \(4.2 \mathrm{~m}\) above the ground. He then falls onto a thick pad. The top of the pad is \(80 \mathrm{~cm}\) above the ground, and it compresses by \(50 \mathrm{~cm}\) as he comes to rest. What is his acceleration as he comes to rest on the pad?