Question

A child (25 kg) walks along a 4 m plank of uniform density (m=8kg) that is laying with one end extending 1 m off the end of a dock.
1. Where is the ceanter of mass located in the plank?
2. What is the magnitude of the torque of the plank about the edge of the dock?
3. How far from the edge of the dock can the boy walk on the plank befopre the plank starts tipping into the water?
4. How could the boy test this conclusion without taking the risk of falling in the water?