81.

A truck accelerates from speed v to 2v. Work done in during this is

• three times as the work done in accelerating it from rest to v

• same as the work done in accelerating it from rest to v

• four times as the work done in accelerating it from rest to v

• ess than the work done in accelerating it from rest to v

The kinetic energy of a body ofmass 4 kg and momentum 6 N-s will be

• 4.5 J

• 2.5 J

• 5.5 J

• 3.5 J

A stone of mass 0.05 kg is thrown vertically upwards. What is the direction and magnitude of net force on the stone during its upward motion ?

• 0.98 N vertically downwards

• 0.49 N vertically downwards

• 9.8 N vertically downwards

• 0.49 N vertically upwards

A 12 kg bomb at rest explodes into two pieces of 4 kg and 8 kg. If the momentum of 4 kg piece is 20 Ns, the kinetic energy of the 8 kg piece is 

• 25 J

• 20 J

• 50 J

• 40 J

A motor pump lifts 6 tonnes of water from a well of depth 25 m to the first floor of height 35 m from the ground floor in 20 minutes. The power of the pump (in kW) is [g = 10 ms^-2]

• 3

• 12

• 15

• 6

The energy spectrum of a black body exhibits a maximum around a wavelength λ₀. The temperature of the black body is now changed such that the energy is maximum around a wavelength $\frac{3{\mathrm{\lambda }}_0}{4}$. The power radiated by the black body will now increase by a factor of

• 64/27

• 256/81

• 4/3

• 16/9

n identical bulbs, each designated to draw a power P from a certain voltage supply. The total power which they will draw is

• $\frac{\mathrm{P}}{\mathrm{n}}$

• $\frac{\mathrm{P}}{{\mathrm{n}}^2}$

• nP

• P

Two similar springs P and Q have spring constants K_P and KQ, such that K_P>K_Q. They are stretched, first by the same amount (case a), then by the same force (case b). The work done by the springs W_P and W_Q are related as, in  case (a) and case (b), respectively

• W_P =W_Q ;W_P> W_Q

• W_P =W_Q ;W_P= W_Q

• W_P > W_Q ;W_Q> W_P

• W_P > W_Q ;W_Q> W_P

A block of mass 10 kg, moving in the x-direction with a constant speed of 10 ms^-1 , is subjected to a retarding force F= 0.1x J/m during its travel from x = 20 m to 30 m. Its final KE will be

• 475 J

• 450 J

• 275 J

• 275 J

Two particles of masses m₁,m₂ move with initial velocities u₁ and u₂. On collision, one of the particles gets excited to a higher level, after absorbing energy (E). If final velocities of particles be v₁ and v₂, then we must have