# Recent questions and answers in Physics

Physics

### A total of $\$10000$is distributed among$150$person as gift. A gift is either of$ \$50$ or $\$100$. Find the number of gifts of each type. ### A 150 m long train is moving with uniform velocity of 45 km/h. The time taken by train to cross the bridge of length 850 m is ? ### The torque$\overrightarrow{z}$on a body about a given point is found to be$ \overrightarrow{A} \times \overrightarrow{L}$where$\overrightarrow {A}$is a constant vector and$\overrightarrow{L}$is angular momentum about that point . From this it follows that. 1)$\large\frac{dL}{dt}$is perpendicular to$\overrightarrow {L}$at all times 2) the component of$\overrightarrow {L}$in the direction of$\overrightarrow{A}$does not change with time. 3) the magnitude of$\overrightarrow {L}$does not change with time. 4)$\overrightarrow {L}$does not change with time. Which of the following is correct? ### The disintegration rate of a radioactive sample at a certain instant is 4750 dpm . Five minute later rate becomes 2700 disintegration per minute . Then the decay constant is ### A wire of length L and cross-sectional area A is made of material of Young's modulus Y. The work done in stretching the wire by an amount x is given by ### Electromagnetic waves are transverse in nature is evident by ### A negatively and uniformly charged disk is rotating in a clockwise direction as viewed from above. The direction of magnetic field at a point A is ### If level of modulation is increased ________ power is increased ### Two simple Harmonic motions of angular frequency 100 and 1000 rads$^{-1}$have the same displacement amplitude. The ratio of their maximum acceleration is : ### At$ 10^{\circ}C$the value of density of a fixed mass of an ideal gas divided by its pressure is x. At$110^{\circ}C$this ratio is : ### Two nuclei have their mass numbers in the ratio of 1 : 3. The ratio of their nuclear densities would be : ### The energy required to charge a parallel plate condenser of plate separation d and plate area of cross - section A such that the uniform electric field betwen the plates is E, is : ### The ground state energy of hydrogen atom is -13.6eV. When its electron is in the first excited state, its excitation energy is : ### Three closed vessels A , B and C are at the same temperature T and contain gases which obey maxwellian distibution of velocities . Vessel A contains only$\;O_{2}\;$. B only$\;N_{2}\;$and C a mixture of equal quantities of$\;O_{2}\;$and$\;N_{2}\;$. If the average speed of$\;O_{2}\;$molecules in vessel A is$\;V_{1}\;$, that of$\;N_{2}\;$molecules in vessel B is$\;V_{2}\;$the average speed of the$\;O_{2}\;$molecules in vessel C will be ### The main purpose of modulation is to ### The capacitance of parallel plate capacitor does not depend upon ### Three capacitors, each of capacitance C are connected to a battery of voltage V. When key K is closed, the charge which will flow through the battery is (see figure). ### A galvonometer of resistance$ 50 \Omega$is connected to a battery of 3V along with a resistance of$ 2950 \Omega$in series. A full scale deflection of 30 divisions is obtained in the galvonometer. In order to reduce this deflection to 20 divisions, the resistance in series should be : ### Two blocks are in contact on a frictionless table. One has mass m and another mass '2m'. A force F is applied on '2m' as shown. Now the force F is applied on 'm' from right. In the two cases respectively the ratio of force of contact between the blocks is ### A uniform circular disc of radius r is placed on a rough horizontal surface and given a linear$w_0$as shown. The disc com,es to rest after moving some distance to the right. It follows that ### A roller coaster is designed such that riders experience " weightlessness" as they go round the top of a hill whose radius of curvature is 20m. The speed of the car at the top of the hill is between ### A block takes twice as much time to slide down a rough$45^{\large\circ}$inclined plane as it takes to slide down an identical smooth$45^{\large\circ}$inclined plane.The coefficient of kinetic friction between the block and the rough inclined plane is ### A metallic resistor is connected across a battery. If the number of collisions of free electrons with the lattice is somehow decreased in the resistor, the current will ### Hot air balloon is an application of what law? ### As the temperature of a metallic conductor is increased, the product of resistivity and conductivity ### A particle is projected perpendicular to a uniform magnetic field. The area bounded by the trajectory of the particle is directly proportional to ### The motion of a sphere moving on a rough horizontal surface change from pure sliding (without rolling) to pure rolling (with out slipping) . In this process, the force of friction. 1) initially acts opposite to the direction of motion and later in the direction of motion. 2) causes linear retardation 3) causes angular acceleration 4) stops acting when pure rolling begins. Which of the following is correct? ### A solid sphere of mass m is lying at rest on a rough horizontal surface. The coefficient of friction between the ground and sphere is$\mu$. The maximum value of F, so that the sphere will not slip is equal to ### A body covers a certain distance x in equal thirds - the first third of distance with speed v$_1$, the second third with a speed of v$_2$and the last third with a speed of v$_3$. What is the average speed of the body over time? ### A spherical body of radius R rolls on a horizontal surface with linear velocity$V$. Let$L_1$and$L_2$be the magnitude of angular momentum of the body about the centre of mass and point of contact P respectively. a)$L_2=L_1$if radius of gyration$k=R$b)$L_2=2L_1$for all cases. c)$L_2 > 2L_1$if radius of gyration$K < R$d)$L_1> L_2$if radius of gyration$ K > R$Which of the following is correct? ### Which of the following does not represent Boyle's law? ### A ball is dropped from a height of h from ground. If the coefficient of restitution is e, the height to which the ball goes up after it rebounds for the nth time is ### A man spinning in free space changes of his body e.g by spreading his arms and curling up. By doing this, he can change his 1) moment of inertia 2) angular momentum 3) angular velocity 4) rotational kinetic energy in this Which of the following is correct? ### The velocity of electromagnetic radiation in a medium of permittivity$ \in_0$and permeability$ \mu_0$is given by : ### A person looks in to a spherical mirror . The size of image of his face is twice the actual size of his face. If the face is at distance 20 cm then find the radius of curvature of mirror. ### A horizontal disc rotates freely about a vertical axis through its centre. A ring, having the same mass and radius as the disc , is now gently placed on the disc. After some time, the two rotate with a common angular velocity. 1) some friction exits between the disc and the ring 2) the angular mometum of disc plus ring is conserved. 3) the final common angular velocity is$\large\frac{2}{3}$rd of the initial angular velocity of disc. 4)$\large\frac{2}{3}$rd of the initial kinetic energy is converted to heat. Which of the following is correct? ### From the top of a building 40 m tall, a ball a thrown vertically upwards with a velocity of$10\; ms^{-1}$After how long will the ball hit the ground? ### A person sitting in an open car moving at constant velocity throws a ball vertically up into air. The ball falls ### Minimum size of plane mirror, required to see full image of an observer is : ### Find the dimension of physical quantity$X$in the equation$Force=\large\frac{X}{Density}$### In the circuit as shown in the figure what is the potential difference between P and Q. ### a body is projected vertically upward from the earths surface such that it reaches a height equal to the radius of the earth above its surface.find the velocity of projection ### Six equal resistance are connected between point P,Q,R as shown in the fig. Then the net resistance will be maximum between ### A solenoid carrying a current supplied by a DC source with constant emf contains iron rod inside it. when the rod is pulled out of the solenoid , then the current in the solenoid will ### In an AC circuit V and I are given by$V=100 \sin (100 t) volt \; I =100 \sin (100 t +\large\frac{\pi}{3})amp\$ the power dissipated in the circuit is

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