Describe several situations in which an object is not in equilibrium even though the net force on it is zero.

I can't think of a single one. I can think of the opposite (equilibrium with net force not being zero) but I don't get it the other way around. Can someone explain?

What about a car rolling at constant velocity?

What about a top spinning at constant velocity?

Certainly! It can be quite confusing to understand how an object can be in a state of equilibrium even when the net force acting on it is zero. The key is to keep in mind that equilibrium involves both forces and torques acting on an object.

Equilibrium occurs when both translational and rotational forces balance out, resulting in a state of no acceleration. The net force acting on an object can indeed be zero, but if the net torque is not zero, the object will not be in rotational equilibrium and will be considered to be in an unbalanced state.

Here are a few examples of situations where an object is not in equilibrium even though the net force on it is zero:

1. Leaning Tower of Pisa: The famous tower in Italy visibly leans to one side. The net force acting on it is vertically balanced, but due to the asymmetrical distribution of mass, the torque caused by the weight of the tower causes it to lean.

2. Tightrope Walker: A tightrope walker maintaining balance on a rope is not in equilibrium, despite having a net force of zero acting on them. The tension in the rope prevents them from falling, but they require constant adjustments to maintain rotational equilibrium.

3. Unbalanced Mobile: A mobile hanging from the ceiling appears stationary, but it is not in equilibrium. The forces acting on each hanging object may cancel each other out, resulting in a net force of zero, but the torque caused by the different lengths of the strings causes the mobile to rotate.

In summary, equilibrium is not solely determined by the net force acting on an object but also involves the net torque. Even with a zero net force, an object may still be unbalanced if the net torque is non-zero, resulting in rotational motion or instability.

Certainly! While it may seem counterintuitive, there are indeed situations where an object is not in equilibrium even though the net force on it is zero. This occurs when the object is experiencing other forces or factors that cause it to be in a state of dynamic equilibrium or unstable equilibrium. Here are a few examples:

1. Tension in a rope: Consider a rope being stretched horizontally between two poles. If the tension in the rope is not uniform along its length, even though the net force on the rope is zero, the rope can experience internal tension forces that result in an overall bending or curving shape.

2. Unbalanced torques: Torque is the rotational equivalent of force. If an object is subjected to various torques that do not cancel each other out, the object can have a net torque of zero but still be rotating or undergoing angular acceleration. For example, think of a see-saw where a heavier person sits closer to the fulcrum than a lighter person. The torques due to their respective weights do not balance, causing the see-saw to rotate.

3. Unstable balance: In some cases, an object may be in unstable equilibrium, meaning that it can remain momentarily balanced but the slightest disturbance will cause it to lose balance. An example of this is a pencil standing on its sharpened tip. Initially, the net force on the pencil is zero, but the slightest disturbance will cause it to fall.

4. Elastic deformations: Objects made of elastic materials, like springs or rubber bands, can be subjected to internal forces even when the net force on them is zero. For instance, if you stretch a rubber band and then let it go, it will undergo oscillating motions even though the net force is zero at its equilibrium position.

Remember, equilibrium refers not only to the absence of net force but also the absence of net torque and stability. These factors can contribute to an object not being in equilibrium even when the net force on it is zero.