How does an atomic clock work?

The term atomic clock is the general name used to describe any variety of time keeping devices based on the regular vibrations associated with atoms.

One of the first atomic clocks or as it is also known the ammonia clock. Was developed by the National Bureau of Standards, and was based on the measurements of the vibrations of atoms of nitrogen, oscillating back and fourth in ammonia molecules at a rate of 23,870 vibrations per second.

The modern day atomic clocks are based on caesium atoms. The spectrum of caesium includes a feature corresponding to radiation with a very precise frequency of 9,192,631,770 cycles per second. One second is now defined as the time it takes for that many oscillations of the radiation associated with this feature in the spectrum of caesium. This type of clock is known as a Caesium clock and it is accurate to one part in 10,000 billion, or one second in 316,000 years.

Even more accurate clocks have been developed using radiation from hydrogen atoms. They are known as Hydrogen Maser Clocks, and one of these clocks, at the US Naval Research Laboratory in Washington, DC, is estimated to be accurate to within one second in 1.7 million years. In principle, clocks of this kind could be accurate to one second in 300 million years!

Difference between evaporation and vaporization

Vaporization of an element or compound is a phase transition from the liquid phase to gas phase. There are two sorts of vaporization: evaporation and boiling.

Evaporation is a phase transition from the liquid phase to gas phase that occurs at temperatures below the boiling temperature at a given pressure.

Boiling is a phase transition from the liquid phase to gas phase that occurs at or above the temperature the boiling temperature.

Centripetal Force and Centrifugal Force

Centripetal force is the force present in circular motion. Centripetal force is always pointed inward, towards the center of rotation. A centripetal force causes a centripetal acceleration. Any acceleration causes a change in velocity. Since the centripetal force is always pointed inward, radially along the circular path, it is always pointed perpendicularly to the velocity vector (which is always tangent to the circle), which makes it possible to have a constantly changing velocity with a constant speed. 


          Centrifugal force is the reaction to the centripetal force. This is equal in magnitude to the centripetal force, directed away from the center of rotation, and is exerted by the rotating object upon the object which exerts the centripetal force. As it is an actual force, it is always present, independent of the choice of reference frame.

The fictitious/virtual centrifugal force appears when a rotating reference frame is used for analyzing the system. The centrifugal force is exerted on all objects, and directed away from the axis of rotation.

Both of the above can be observed in action on a passenger riding in a car. If the car swerves around a corner, the passenger's body pushes against the outer edge of the car. This is the reactive centrifugal force, which is called a reaction force because it results from passive interaction with the car which actively pushes against the body.

Difference between Kinematics and Dynamics

Kinematics is a branch of dynamics which describes the motion of objects without consideration of the circumstances leading to the motion. Kinematics is concerned with the space-time relationship of a given motion without considering the origins of forces.

On other hand, Dynamics deals with forces responsible for origin of motion of objects.

Why do people sink in water?

It is because of what is called 'buoyant force'.

If you take a mug and try to push it inside water, you can feel a force that pushes the mug upwards. That is buoyant force.

F = v x d x g, where
F = buoyant force,
v = volume of the body,
d = density of the liquid,
g = acceleration due to gravity

Thus, in this case which you have mentioned, buoyant force primarily depends on the volume and density. More the volume, more the force. But more the volume, less the density.

Ships are in fact made of iron, steel and other metal alloys which are in fact much denser than water. But the air spaces in them increases the volume to such an extent that the overall volume increases, and the density drops (more the volume, less the density). Thus, the buoyant force also decreases, and ships float.

Human body density is in fact less than water. But the problem is that when people fall into water, they consume a lot of water which keeps the volume steady but increases the overall density. Thus, they sink. If one doesn't consume water at all, then he/she stays afloat in water.