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A Few Problems from Kinematics

  1. A body falling freely from rest has velocity v after it falls a height h . Calculate the distance it should fall down further for its velocity to become double
  2. A particle in uniform acceleration in a straight line has a speed of v m/s at position x meter is given by √(25-16x). What is the acceleration of the particle

Momentum and Kinetic Energy

Two bodies one light and other heavy have equal momentum.Which of them has higher kinetic energy?

(Aajma Manoj asked)


The lighter body will have greater kinetic energy.


Momentum, p=mv

squaring both sides, p2 = m2v2

Kinetic Energy = 1/2 mv2 = p2/2m

Therefore, for bodies of equal momenta, kinetic energy is inversely proportional to mass.

Derive an expression for the minimum horizontal velocity to be given to a ball hanging vertically from a point so that it is able to just complete a vertical circular path.

While pushing, we are giving the ball some kinetic energy which will be converted to potential energy as it moves upward. If the whole kinetic energy is converted to potential energy at the top most point, it will fall straight down, resulting in only a semicircle. For the ball to continue the path, it should have a centrifugal force equal to weight of the body when it is at the topmost point.

let V be the initially applied velocity and v be the velocity at the topmost point

mv²/r = mg

v²/r = g

v² = rg

v = √(rg)

Potential Energy at the topmost point , Ep = 2mgr

Ep is the difference in kinetic energy between initial point(Ei) and at topmost point(Et).

                Ei – Et =  Ep

½mV² – ½mv² = 2mgr

      ½m(V²-v²) = 2mgr

            V²-v² = 4gr

                           V² = 4gr + v²

                          V² = 4gr + rg

                V² = 5rg


Apparent weight in an elevator

You are standing on a bathroom scale in an elevator the is moving upward at constant speed, when suddenly the cable breaks. from just before to just after the cable breaks, the reading on the scale will be?

When the elevator is moving up with constant speed, the bathroom scale (weighing scale) shows the correct weight, because the mechanics involved will be the same if the frame is at rest or moving with a constant velocity.
When the cable breaks, it is a case of freely falling body and the scale reads zero.

Electric Flux

From: Nancy James

Subject: Electric flux
Message Body:
Could some one please explain what is written in the ncert book of PHYSICs class XII about ‘electric flux’?




Save the images, open and zoomin to read

Buoyant force

What is buoyant force? I’m unable to understand practically from book. please give vast and practical explanation.. (Raj asked)


Whenever a body is immersed in a fluid, say water, it experiences an upward force acting on it. The upward force exerted by a fluid on an object immersed in it is called Buoyant force.

When an object is immersed, it displaces the fluid already present there. The upward force (buoyant force) is the result of the fluid’s attempt to reoccupy the space.

If the object placed in a fluid is denser than the fluid, it will sink. Though it experiences an upthrust, its weight will be more than the upthrust.

The upthrust is equal to the weight of liquid displaced by the immersed part of the body.

If the object is of less density than the fluid, it will come up and float.

For a floating body, the net weight is equal to zero, as the upthrust balances the actual weight of the body.

Restoring force and Newton’s law

We know that whenever a body is deformed a restoring force originates and tries to restore the shape of the body and according to newtons third law of motion whenever a force acts on a body an equal and opposite force acts on the other body . So is the restoring force an opposite reacton to the deforming force and if it is not then why do we consider its direction equal and opposite to the deforming force while solving problems .

(Asked Akshat )

Answer: Restoring force comes into play only when a deforming force is exerted. When no external force acts on a body, the molecules are in equilibrium. No net force acts on it. If an external force is applied on it, it will try to  increase or decrease the intermolecular distance giving rise to a state of inequilibrium to teh molecules and they will tend to go back to their original positions to retain their state of equilibrium. This gives rise to the RESTORING FORCE.

Restoring force the the internal force that comes into play whenever an external force tries to change the inter-molecular distance. The restoring force at any instant (within limits) is equal in magnitude and opposite in direction to the deforming force

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March 2021

Schrodinger’s Cat in Daily Life

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