- #1
schwiffty
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Homework Statement
You are holding a sign as shown below. The sign (including the horizontal bar it hangs from) has a mass of 2.20 kg and is 40.5 cm wide. The sign is hanging from a 1.03 m tall, 4.46 kg vertical post. The sign is symmetric with a uniform mass distribution as implied by the drawing
What external force (+ if up, - if down) and torque (+ for CCW, - for CW) do you have to apply with your hand to keep the sign in static equilibrium? [Use g = 9.80 m/s2 and ignore the width of the vertical post.]
Homework Equations
Net Forces = 0
Torques (counter clockwise) = Torques (clockwise)
The Attempt at a Solution
I think that the only forces acting on the sign are vertical so for the net forces equation I have:
Forces up = Forces down =====> FHand = Mg + mg
Where M is the mass of the vertical post and m is the mass of the sign.
For the conservation of torques I have: d * FHand = d*Mg + d*mg
Where d is the lever arm in meters.
(The axis of rotation is at the left side.)
0.405 * FHand = 0.405 * (4.46kg*9.8) + 0.2025 * (2.20kg*9.8)FHand = (17.7017 N*m + 4.3659 N*m) / 0.405 m
FHand = 54.488 N Torque = 22.0676 N*m
This answer is wrong and I do not know why.