^Internal force
^Examples of Newton’s third law
other team is pulling the other end of the rope. No force can be exerted if the other end is free. One team exerts the force of action and the other team provides the force of reaction. 
^Newton’s third law
3rd law is called action reaction law.
^Newton’s second law
NSL defines force. External force is required to accelerate a body (change speed or direction or both).
When a body is seen from inertial frame of reference, the time rate of change of linear momentum is equal to the net external force acting on the body & takes place in the direction of net force
As all the three laws can be explained only from second law, that is why the 2nd law is called real law of motion.
^Newton’s first law
^Newton’s law of motion
Isaac Newton (1643 – 1727) published Principia Mathematical in 1687. In this work, he proposed three “laws” of motion.
^Pseudo force
Imaginary forces experienced in non – inertial frame are called pseudo forces. A pseudo force experienced due to increase or decrease of speed is called tangential pseudo force.
Following drawings show direction of pseudo force acting on a block in different situations:
^Rydberg’s formula
Let ‘E’ be the energy & λ be the wavelength of the photon released when an electron jumps from a higher quantum state of principal quantum number n2 to a lower quantum state having principal quantum number n1, then
Note Rydberg’s constant depends on mass of
Electron, thus it is not a universal constant.
In deriving the above value the nucleus is assumed to be at rest. However if nucleus is not assumed stationary then the Rydberg constant depends on both mass of electron & nucleus & is given by
^Inertial & non inertial frame of reference
Accelerated frames are called non inertial & frames having zero acceleration are called inertial or Newtonian or Galilean frame of reference, thus any frame of reference at rest or moving with constant velocity with respect to a stationary frame of reference is also inertial. A frame of reference attached to an accelerating body is called non inertial frame of reference.
Suppose you are observing something which is at rest w.r.t. ground e.g. a tree, a building etc. If you observe it while staying at rest or while moving with uniform velocity w.r.t. ground, then your acceleration is zero & you are inertial frame observer. But while observing your velocity w.r.t. ground decreases or increases or changes in direction then you are said to be non inertial frame observer.
As earth is spinning about its own direction as well around the sun its speed & direction both vary over the time, hence for astronomical observations the Earth is non – inertial. However all the objects on earth have same acceleration & the relative acceleration of any one object w.r.t. other is zero, hence earth can be treated as inertial frame of reference in laboratory frame.
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