Why are there two limiting equilibrium? Why does friction acts upwards? Why does friction acts downwards? Why is the object about to move up? Why is the object about to move down?
All these questions and concepts appear contradicting, but trust me, it’s easy once you get the hang of it.
This post is written with the assumption that you know the concept of forces and equilibrium. This post helps you to understand limiting equilibrium, but does not help you with calculations or problem solving.
Limiting equilibrium is the state a particle is in when it is on the point of sliding.
It’s best to learn with example.
Imagine a small ring threaded on a rough rod which is fixed vertically. The ring is pulled upwards with a string at an angle, and the ring is in equilibrium.
Play around with the interactive flash below. Explanations are below the flash.
Initially, with a small force, the ring is about to slip downwards, and thus by definition, the ring is in limiting equilibrium. Frictional force tries to oppose this by acting upwards.
As you increase the force applied, there will be an instance when the vertical component of the force balances the downward force due to gravity. Frictional force does not act during this instance.
As force increases, the ring has the tendency to move upwards now. It wants to move upwards, but the friction acts downwards and stop the ring from moving. (Previously the ring wants to move downwards due to gravity.)
As force increases, the ring will reach limiting equilibrium again, but this time, it is about to slip upwards.
Note that there are 2 limiting equilibrium in this example.
There is actually a range of force which when applied, will not cause the ring to move.
A photon is a packet (quantum) of electromagnetic energy carried by the electromagnetic waves.
1 electron volt (eV) is the energy gained by an electron when moving through a potential difference of 1 V.
The photoelectric effect/photoelectric emission/photoemission refers to the phenomenon that when some (clean) metal surfaces are illuminated by electromagnetic radiation, electrons are emitted from the surfaces.
The threshold frequency() is the minimum frequency of radiation for which photoelectrons can be emitted from the metal when radiation falls on it.
The specific heat capacity of a substance is the energy needed to raise the temperature of one kilogram of the substance by 1 K.
The numerical value of heat capacity of a body is the quantity of heat energy required to raise the temperature of the whole body by 1 K.
The numerical value of specific latent heat of fusion is the quantity of heat energy required to convert unit mass of solid to liquid without any change in temperature.
The numerical value of specific latent heat of vapourisation is the quantity of heat energy required to convert a unit mass of liquid to vapour without any change in temperature.
The internal energy (U) of any object is defined as the sum of all the microscopic kinetic and potential energies of the molecules within the object.
The first law of thermodynamics states that the internal energy of a system depends only on its state; the increase in the internal energy of a system is the sum of the work done on the system and the heat supplied to the system.