College Physics with MasteringPhysics 7th Edition by Jerry Wilson,Anthony Buffa,Bo Lou

Exercise 4

A person on a bathroom scale (not the digital type) stands on the scale with his arms at his side. He then quickly raises Iris arms over Iris head, and notices that the scale reading increases as he brings his arms upward. Similarly, there is a decrease as he brings his arms downward. Why does the scale reading change (Try this yourself.)