Why do the forces not net out to zero in a system in static equilibrium?

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In summary, for a system to be in static equilibrium, the sum of forces and torques must be equal to zero. While it is clear why torques must net out to zero, the explanation for why forces must also net out to zero may not be as obvious. However, this can be understood by picturing a wooden plank pivoted at the center with a small mass on one end and a very large mass on the other. In this scenario, while the torques balance out to zero, the forces do not. This is because the pivot itself exerts an upward force to support the plank and the two masses. Once this is taken into account, the system remains motionless.
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Gabe805
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I understand that for a system to be in static equilibrium the sum of the forces and torques must equal zero. I understand why the torques must net out to zero but not the forces. For example, if you picture a wooden plank pivoted at the center with a small mass on the far left end and a very large mass near the pivot such that the torques net out to zero, the system is not moving yet the forces do not net out to zero. if somebody can please explain, I would be very appreciative.
 
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Don't forget that the pivot itself exerts an upward force to support the plank and the two masses.
 
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Doc Al said:
Don't forget that the pivot itself exerts an upward force to support the plank and the two masses.

oh my gosh! How could I forget that! Thank you very much.. its been bugging me for three days now.
 

FAQ: Why do the forces not net out to zero in a system in static equilibrium?

1. What is static equilibrium?

Static equilibrium is a state in which all forces acting on an object are balanced, resulting in no net force or acceleration of the object.

2. What is the difference between static and dynamic equilibrium?

In static equilibrium, all forces are balanced and the object is not moving. In dynamic equilibrium, the object is moving at a constant velocity with all forces still balanced.

3. How is torque defined?

Torque is a measure of the force that causes an object to rotate around an axis. It is calculated by multiplying the force applied to an object by the distance from the axis of rotation to the point where the force is applied.

4. What is the principle of moments?

The principle of moments states that for an object to be in static equilibrium, the sum of the clockwise moments must be equal to the sum of the counterclockwise moments.

5. How do you calculate the center of mass for a system of objects?

The center of mass for a system of objects can be calculated by taking the average of the individual masses multiplied by their respective distances from a reference point.

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