The front of her sleeper wet from teething, a baby rejoices in the day by crowing and bouncing up and down in her crib. Her mass is 14.0 kg, and the crib mattress can be modeled as a light spring with a force constant 4.70 kN/m. She learns to use the mattress as a trampoline -- losing contact with it for part of each cycle -- when her amplitude exceeds what value?