Question

The mass of the tray itself is 0.200 kg, and its center of gravity is located at its geometrical center. On the tray is a 1.03-kg plate of food and a 0.256-kg cup of coffee. Assume L1 = 0.0620 m, L2 = 0.110 m, L3 = 0.231 m, L4 = 0.381 m and L5 = 0.401 m. Obtain the force T exerted by the thumb (enter first) and the force F exerted by the four fingers. Both forces act perpendicular to the tray, which is being held parallel to the ground.

Answer 1

The lunch tray is being held in one hand, as the drawing above illustrates

Answer 2

So we know that the tray is static, so the sum of the torques about the point of rotation is zero, so the first question is! What's the point of rotation?

Answer 3

That is, if the thumb weren't pressing down, where would the tray pivot?