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Torque and equilibrium
Intermediate
Balance both forces and torques for static equilibrium.
Στ = 0A rigid body in equilibrium needs ΣF = 0 and Στ = 0 about any point. Choose a pivot that eliminates unknown forces to simplify.
A 4 m plank, 20 kg, supported at its ends with a 10 kg load at 1 m: reaction at the far end by moments.
Moments must be taken about the same point for all forces.
In more detail
For an object to be in rotational equilibrium, the net torque about any point must be zero. Combined with zero net force, this gives full equilibrium.
Real-world uses
- Ladders and cranes are checked for torque balance to avoid tipping.
- Seesaws balance when torques from each side are equal.
- A seesaw or a ladder leaning against a wall must balance both the net force and the torques to stay still.
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Frequently asked questions
Can I choose any pivot point?
Yes, for a body in equilibrium, the net torque is zero about any point.
What causes a ladder to tip?
The torque from its weight about the base exceeds the torque from the wall or floor support.
Why must torques balance as well as forces?
Forces can balance while still producing a turning effect, so both conditions are needed for full static equilibrium.
Related topics in Newton's Laws & Forces
Springs in series and parallel · Hooke's law limits · Equilibrium of concurrent forces · Center of mass · Motion of center of mass · Normal force on curves