n balls each of mass m and velocity v collide with a wall, assume that the collisions are perfectly elastic. The pressure exerted per second on the wall is 

• 3 mnv

• mnv

• 2mnv

• none

The ground state energy of hydrogen atom is -13.6 eV. The kinetic energy of the electron
in this state is

• 27.2 eV

• 13.6 eV

• 6.8 eV

• 3.4 eV

A massless spring of natural length of 0.5 m and spring constant 50 N/m has one end fixed and the other end attached to mass of 250 g. The spring mass system is on a smooth floor. The mass is pulled until the length of the spring is 0.6 and then released from rest. The kinetic energy of the mass when the length of the spring is 0.5 m is

• 0.0176 J

• 2.25 J

• 6.25 J 

• 9 J

A particle performing uniform circular motion has angular momentum L. If its angular frequency is doubled and its kinetic energy halved, then the new angular momentum is

• $\frac{\mathrm{L}}{4}$

• 2L

• 4L

• $\frac{\mathrm{L}}{2}$

A spring scale is adjusted to read zero. Particles of mass 1 g fall on the pan of the scale and collide elastically and they rebound upward with the same speed. If the height of fall of particles is 2 m and their rate of collision is 100 particles per second, then the scale reading in grams will be (g =9.8 m/s)?

• 1000 g

• 1100 g

• 1200 g

• 1252 g

A smooth sphere of mass m moving with velocity u directly collides elastically with another sphere of mass M at rest. After collision their final velocities are V and v respectively. The value of v is

• $2\mathrm{u} \frac{\mathrm{M}}{\mathrm{m}}$

• $2\mathrm{u} \frac{\mathrm{m}}{\mathrm{M}}$

• $\frac{2 \mathrm{u}}{1 + {\displaystyle \frac{\mathrm{m}}{\mathrm{M}}}}$

• $\frac{2 \mathrm{u}}{1 + {\displaystyle \frac{\mathrm{M}}{\mathrm{m}}}}$

207.

A water film is made between two 10 cm long straight wires and at a distance of 0.5 cm. If distance between the wires is increased by 1 mm, then work done will be (T = 7.2 x 10^-2N/m)

• 7.22 × 10^-6 J

• 1.44 × 10^-5 J

• 2.88 × 10^-5 J

• 5.76 × 10^-5 J

The average kinetic energy of a hydrogen gas molecule at NTP will be (Boltzmann's  constant k = 138 x 10^-23 J/K)

• 0.186 × 10^-20 J

• 0.372 × 10^-20 J

• 0.56 × 10^-20 J

• 5.6 × 10^-20 J

The height of a water fall is 84 m. Assuming that the entire kinetic energy of falling water is converted into heat, the rise in temperature of the water will be

• 0.196°C

• 1.960°C

• 0.96°C

• 0.0196°C

A steel ball is dropped from a height h and it makes a perfectly elastic collision with a horizontal plane. Following its initial release, it will make periodic motion with frequency

• $2\mathrm{\pi }\sqrt{\frac{\mathrm{g}}{\mathrm{h}}}$

• $2\mathrm{\pi }\sqrt{\frac{\mathrm{h}}{\mathrm{g}}}$

• $\frac{1}{2}\sqrt{\frac{\mathrm{g}}{2\mathrm{h}}}$

• $\frac{1}{2}\sqrt{\frac{2\mathrm{h}}{\mathrm{g}}}$