Science (English Medium) Class 11 - CBSE Question Bank Solutions

In figure, what will be the sign of the velocity of the point P′, which is the projection of the velocity of the reference particle P . P is moving in a circle of radius R in anticlockwise direction.

Sound waves of wavelength λ travelling in a medium with a speed of v m/s enter into another medium where its speed is 2v m/s. Wavelength of sound waves in the second medium is ______.

Speed of sound wave in air ______.

A sound wave is passing through air column in the form of compression and rarefaction. In consecutive compressions and rarefactions ______.

A string of mass 2.5 kg is under a tension of 200 N. The length of the stretched string is 20.0 m. If the transverse jerk is struck at one end of the string, the disturbance will reach the other end in ______.

A transverse harmonic wave on a string is described by y(x, t) = 3.0 sin (36t + 0.018x + π/4) where x and y are in cm and t is in s. The positive direction of x is from left to right.

1. The wave is travelling from right to left.
2. The speed of the wave is 20 m/s.
3. Frequency of the wave is 5.7 Hz.
4. The least distance between two successive crests in the wave is 2.5 cm.

Speed of sound waves in a fluid depends upon ______.

1. directty on density of the medium.
2. square of Bulk modulus of the medium.
3. inversly on the square root of density.
4. directly on the square root of bulk modulus of the medium.

At what temperatures (in °C) will the speed of sound in air be 3 times its value at O°C?

A steel wire has a length of 12 m and a mass of 2.10 kg. What will be the speed of a transverse wave on this wire when a tension of 2.06 × 10⁴N is applied?

If c is r.m.s. speed of molecules in a gas and v is the speed of sound waves in the gas, show that c/v is constant and independent of temperature for all diatomic gases.

Given below are some functions of x and t to represent the displacement of an elastic wave.

1. y = 5 cos (4x) sin (20t)
2. y = 4 sin (5x – t/2) + 3 cos (5x – t/2)
3. y = 10 cos [(252 – 250) πt] cos [(252 + 250)πt]
4. y = 100 cos (100πt + 0.5x)

State which of these represent

1. a travelling wave along –x direction
2. a stationary wave
3. beats
4. a travelling wave along +x direction.

Given reasons for your answers.

Light waves each of amplitude "a" and frequency "ω", emanating from two coherent light sources superpose at a point. If the displacements due to these waves are given by y₁ = a cos ωt and y₂ = a cos(ωt + ϕ) where ϕ is the phase difference between the two, obtain the expression for the resultant intensity at the point.

No physicist has ever "“seen"” an electron. Yet, all physicists believe in the existence of electrons. An intelligent but superstitious man advances this analogy to argue that ‘ghosts’ exist even though no one has '‘seen'’ one. How will you refute his argument?

The shells of crabs found around a particular coastal location in Japan seem mostly to resemble the legendary face of a Samurai. Given below are two explanations of this observed fact. Which of these strikes you as a scientific explanation ? 

(a) A tragic sea accident several centuries ago drowned a young Samurai. As a tribute to his bravery, nature through its inscrutable ways immortalized his face by imprinting it on the crab shells in that area.

(b) After the sea tragedy, fishermen in that area, in a gesture of honor to their dead hero, let free any crab shell caught by them which accidentally had a shape resembling the face of a Samurai. Consequently, the particular shape of the crab shell survived longer and therefore in course of time the shape was genetically propagated. This is an example of evolution by artificial selection. 

[Note : This interesting illustration taken from Carl Sagan’s ‘The Cosmos’ highlights the fact that often strange and inexplicable facts which on the first sight appear ‘supernatural’ actually turn out to have simple scientific explanations. Try to think out other examples of this kind].

A physical quantity P is related to four observables a, b, c and d as follows:

`P = (a^3b^2)/((sqrtcd))`

The percentage errors of measurement in a, b, c and d are 1%, 3%, 4% and 2%, respectively. What is the percentage error in the quantity P? If the value of P calculated using the above relation turns out to be 3.763, to what value should you round off the result?

Just as precise measurements are necessary in science, it is equally important to be able to make rough estimates of quantities using rudimentary ideas and common observations. Think of ways by which you can estimate the following (where an estimate is difficult to obtain, try to get an upper bound on the quantity):

the total mass of rain-bearing clouds over India during the Monsoon

`hati  "and"  hatj` are unit vectors along x- and y-axis respectively. What is the magnitude and direction of the vectors `hati+hatj` and `hati-hatj` ? What are the components of a vector `A = 2hati + 3hatj` along the directions of `hati + hatj` and `hati - hatj` ? [You may use graphical method]

Assuming the earth to be a sphere of uniform mass density, how much would a body weigh half way down to the centre of the earth if it weighed 250 N on the surface?

State if the following statement is true or false. Give a reason for your answer.

Work done in the motion of a body over a closed loop is zero for every force in nature.

A body constrained to move along the z-axis of a coordinate system is subject to a constant force F given by

`F = -hati+2hatj+3hatkN`

Where `hati,hatj,hatk` 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 ?