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Sashwat’s Questions-misc.

On 26th Feb Sashwat Ghai sent us some of his problem. The questions and their answers are featured below:

Q1.What is meant by law of conservation of momentum? Please explain with an example.

Answer 1)

Momentum is the product of velocity and mass. The law of conservation of momentum says that the total momentum in a closed system (which is not interacting with external forces) before and after any change is constant
A common problem in physics that requires the use of this fact is the collision of two particles. Since momentum is always conserved, the sum of the momenta before the collision must equal the sum of the momenta after the collision:

mim2where u1 and u2 are the velocities before collision, and v1 and v2 are the velocities after collision.
Numerically explaining,
Suppose a 1 kg object moving with a velocity of 10m/sec collides with a stationery object of 1 kg and if they start moving together then according to this principle the two will move together with a velocity of 5 m/sec.
Momentum before collision = m1*v1+m2*v2=1*10+1*0=10kgm/sec
Momentum after collision should be = 10kgm/sec
Mass of the system = m1 + m2 =1 + 1 = 2kg
Therefore velocity of the system = 10/2 = 5m/sec

Q2.Can we say that a rocket has internal force acting on it or do we consider the upthrust to be external force? If the force is internal then it means that there is no origin of force in a rocket?

Answer 2)

The mass of rocket keeps on changing as the fuel is lost. Had we taken into account the effect due to burning of fuel we would have found that the momentum is conserved. The upthrust is produced as a reaction of the exhaust gas trying to move out. Thus the upthrust acts as an external force on the rocket helping it to move up.

Q3 How does a cathode ray tube work?


It works on the following principles: (i) thermionic emission (ii) deflection of the electron beam by the electric and magnetic field (iii) fluorescence produced by the electron beam on a fluorescent screen


The electrons are emitted as the temperature of the heater becomes high. The heat energy is converted to the kinetic energy of the electrons. The electrons being charged particles (negatively charged) they can be deflected by magnetic fields. The path of the electron beam is controlled by the magnetic fields produced by the focusing coils and the deflecting coils.

When the electrons fall on the fluorescent screen, the energy of the kinetic energy of the electrons is converted to light energy thus giving the visual effect.

Q4 Give the diagram and working of a car battery.

Answer 4

A Car battery is composed of simple galvanic cells. There is an acid (Sulphuric acid) which is the electrolyte and metal (lead) which is the negative plate or cathode. Lead oxide acts as the anode.


While starting the vehicle the lead is being converted to lead oxide and the chemical energy from the reaction is being converted to electrical energy which helps to start the engine.

Once the vehicle is in motion the battery gets recharged. A part of the circuit current of the car comes back to the battery converting lead oxide to lead again.


Working of a car battery

Q5.You must learn to be more responsible for your actions.
[Begin: Responsibility…]

Answer 5:
You must learn to take responsibility for your actions.

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