# Homework Help: Rotational Dynamics of a tray

1. Nov 13, 2006

### tmurphy12

Hello, I know the rules state to show work to recieve help but my problem is that i dont know where to begin so i am lookign for a push in the right direction...here is the question...
A lunch tray is being held in one hand, as the figure illustrates. 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 1.04-kg plate of food and a 0.248-kg cup of coffee. Obtain (a) the force exerted by the thumb and (b) the force exerted by the four fingers. Both forces act perpendicular to the tray, which is being held parallel to the ground.
http://edugen.wiley.com/edugen/courses/crs1507/art/qb/qu/c09/ch09p_16.gif
the only thing i did was multiply the masses by gravity to get their weight
again any help would be greatly appreciated.

2. Nov 13, 2006

### tmurphy12

i also figure that the lever arm will be the length given in the picture...i am just confused as to how to find the force without knowing the torque

3. Nov 13, 2006

### physics girl phd

Draw a free body diagram for the system... with the two unknown forces they ask for. Then use two facts -- that the net force must be zero, and that the net torque must be zero. Two equations, two unknowns, so you can solve for the unknowns.