List of top Physics Questions asked in CBSE Class XI

In the previous problem, if 15.0 cm of water and spirit each are further poured into the respective arms of the tube, what is the difference in the levels of mercury in the two arms ? (Specific gravity of mercury = 13.6)

Explain why 

(a) The angle of contact of mercury with glass is obtuse, while that of water with glass is acute. 

(b) Water on a clean glass surface tends to spread out while mercury on the same surface tends to form drops. (Put differently, water wets glass while mercury does not.) 

(c) Surface tension of a liquid is independent of the area of the surface 

(d) Water with detergent disolved in it should have small angles of contact. (e) A drop of liquid under no external forces is always spherical in shape.

A U-tube contains water and methylated spirit separated by mercury. The mercury columns in the two arms are in level with 10.0 cm of water in one arm and 12.5 cm of spirit in the other. What is the specific gravity of spirit ?

A hydraulic automobile lift is designed to lift cars with a maximum mass of 3000 kg. The area of cross-section of the piston carrying the load is 425 cm2 . What maximum pressure would the smaller piston have to bear ?

The blades of a windmill sweep out a circle of area A. (a) If the wind flows at a velocity v perpendicular to the circle, what is the mass of the air passing through it in time t ? (b) What is the kinetic energy of the air ? (c) Assume that the windmill converts 25% of the wind’s energy into electrical energy, and that A = 30 \(m^2\) , v = 36 km/h and the density of air is 1.2 kg \(m^{-3}\). What is the electrical power produced ?

A vertical off-shore structure is built to withstand a maximum stress of 109 Pa. Is the structure suitable for putting up on top of an oil well in the ocean ? Take the depth of the ocean to be roughly 3 km, and ignore ocean currents.

Toricelli’s barometer used mercury. Pascal duplicated it using French wine of density 984 kg m–3. Determine the height of the wine column for normal atmospheric pressure.
A 50 kg girl wearing high heel shoes balances on a single heel. The heel is circular with a diameter 1.0 cm. What is the pressure exerted by the heel on the horizontal floor ?
A body of mass 0.5 kg travels in a straight line with velocity v =a x \(^{3/2}\) where a = 5 m\(^{-1/2}\) s–1 . What is the work done by the net force during its displacement from x = 0 to x = 2 m ?

Explain why 

(a) To keep a piece of paper horizontal, you should blow over, not under, it 

(b) When we try to close a water tap with our fingers, fast jets of water gush through the openings between our fingers 

(c) The size of the needle of a syringe controls flow rate better than the thumb pressure exerted by a doctor while administering an injection 

(d) A fluid flowing out of a small hole in a vessel results in a backward thrust on the vessel 

(e) A spinning cricket ball in air does not follow a parabolic trajectory.

Fill in the blanks using the word(s) from the list appended with each statement: 

(a) Surface tension of liquids generally ... with temperatures (increases / decreases) 

(b) Viscosity of gases ... with temperature, whereas viscosity of liquids ... with temperature (increases / decreases) 

(c) For solids with elastic modulus of rigidity, the shearing force is proportional to ... , while for fluids it is proportional to ... (shear strain / rate of shear strain) 

(d) For a fluid in a steady flow, the increase in flow speed at a constriction follows (conservation of mass / Bernoulli’s principle) 

(e) For the model of a plane in a wind tunnel, turbulence occurs at a ... speed for turbulence for an actual plane (greater / smaller).

A trolley of mass 300 kg carrying a sandbag of 25 kg is moving uniformly with a speed of 27 km/h on a frictionless track. After a while, sand starts leaking out of a hole on the floor of the trolley at the rate of 0.05 kg \(\text{s}^{-1}\). What is the speed of the trolley after the entire sand bag is empty ?
The bob of a pendulum is released from a horizontal position. If the length of the pendulum is 1.5 m, what is the speed with which the bob arrives at the lowermost point, given that it dissipated 5% of its initial energy against air resistance ?
A pump on the ground floor of a building can pump up water to fill a tank of volume 30 \(m^3\) in 15 min. If the tank is 40 \(m\) above the ground, and the efficiency of the pump is 30%, how much electric power is consumed by the pump ?
A molecule in a gas container hits a horizontal wall with speed 200 \(m\,s^{-1}\) and angle 30° with the normal, and rebounds with the same speed. Is momentum conserved in the collision ? Is the collision elastic or inelastic ?
An electron and a proton are detected in a cosmic ray experiment, the first with kinetic energy 10 \(\text {keV}\), and the second with 100 \(\text {keV}\). Which is faster, the electron or the proton ? Obtain the ratio of their speeds. (electron mass = \(9.11\times 10^{-31}\) kg, proton mass = \(1.67\times 10^{-27}\) kg, 1 \(\text {eV}\) = \(1.60\times 10^{-19}\) J)
A body constrained to move along the z-axis of a coordinate system is subject to a constant force F given by \(\text F\) = \(-\hat{i}+2\hat j+3\hat k\) where \(\hat i,\hat j,\hat k\) are unit vectors along the x-, y- and z-axis of the system respectively. What is the work done by this force in moving the body a distance of 4 m along the z-axis ?
A body of mass 2 kg initially at rest moves under the action of an applied horizontal force of 7 N on a table with coefficient of kinetic friction = 0.1. Compute the 
  1. work done by the applied force in 10 s, 
  2. work done by friction in 10 s, 
  3. work done by the net force on the body in 10 s, 
  4. change in kinetic energy of the body in 10 s, and interpret your results.
A truck starts from rest and accelerates uniformly at 2.0 \(ms^{-2}\). At t = 10 s, a stone is dropped by a person standing on the top of the truck (6 m high from the ground). What are the 
  1. velocity, and 
  2. acceleration of the stone at t = 11s ? 
    (Neglect air resistance.)

Explain why 

(a) The blood pressure in humans is greater at the feet than at the brain 

(b) Atmospheric pressure at a height of about 6 km decreases to nearly half of its value at the sea level, though the height of the atmosphere is more than 100 km 

(c) Hydrostatic pressure is a scalar quantity even though pressure is force divided by area.

A stone of mass 0.25 kg tied to the end of a string is whirled round in a circle of radius 1.5 m with a speed of 40 rev./min in a horizontal plane. If, the speed of the stone is increased beyond the maximum permissible value, and the string breaks suddenly, which of the following correctly describes the trajectory of the stone after the string breaks -
For any arbitrary motion in space, which of the following relations are true : 
  1. \(v_{average}\)= \(\bigg(\frac{1}{2}\bigg)\) (\(v\) (\(t_1\) ) + \(v\) (\(t_2\) )) 
  2. \(v_{average}\) = \(\frac{[r(t_2)-r(t_1)]}{(t_2-t_1)}\)
  3. \(v(t)\) = \(v(0)\) + \(at\) 
  4. \(r(t)\) = \(r(0)\) + \(v(0)t+\bigg(\frac{1}{2}\bigg)at^2\)
  5. \(a_{average}\) =\(\frac{[v(t_2)-v(t_2)}{(t_2-t_1)}\) 
    (The ‘average’ stands for average of the quantity over the time interval \(t_1\) to \(t_2\) )
A shell of mass 0.020 kg is fired by a gun of mass 100 kg. If the muzzle speed of the shell is 80 \(m\,s^{-1}\), what is the recoil speed of the gun ?
A stone of mass 0.25 kg tied to the end of a string is whirled round in a circle of radius 1.5 m with a speed of 40 rev./min in a horizontal plane. What is the tension in the string ? What is the maximum speed with which the stone can be whirled around if the string can withstand a maximum tension of 200 N ?
Two billiard balls each of mass 0.05 \(\text {kg}\) moving in opposite directions with speed 6 \(\text m \,\text s^{-1}\)collide and rebound with the same speed. What is the impulse imparted to each ball due to the other ?