# AP Torque help!

1. Nov 13, 2007

### shotgunbob

1. The problem statement, all variables and given/known data
A lunch tray is being held in one hand, as the Drawing illustrates. The mass of the tray is .2 kg and its center of gravity is located at its geometric center (the tray is .4m long). On the tray is a 1 kg plate of food and a .25 kg cup of coffee. Obtain the force T exerted by the thumb and the force F exerted by the four fingers. Both forces act perpendicular to the tray, which is being held parallel to the ground.

2. Relevant equations
T=FX=Counterclockwise forces - clockwise forces = 0

3. The attempt at a solution
T= ((f)(.1)) - ( (t)(.06m) + (.2kg)(.2m)(10 m/s^2) + (1kg)(.24)(10) + (.25)(.38)(10m/s^2)) = 0

manipulated the equation so I got F= 625 + T

I Isolated T to be1656.25lbs of force, which is obviously wrong. Help!

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Last edited: Nov 13, 2007
2. Nov 13, 2007

### Chi Meson

no, ccw torques balance cw torques. "up" forces balance "down" forces.

looks like you're balancing torques, but what have you picked as a pivot point? I'd use the point of contact of the fingers, since that would make the torque there zero, leaving one unknown (T).

After finding T, go to balance the up and down forces to find F.

Last edited: Nov 13, 2007