List of top Physics Questions asked in CBSE Class XI

A man of mass 70 \(\text {kg}\) stands on a weighing scale in a lift which is moving 
  1. upwards with a uniform speed of 10 \(\text m \,\text s^{-1}\) , 
  2. downwards with a uniform acceleration of 5 \(\text m \,\text s^{-2}\) , 
  3. upwards with a uniform acceleration of 5 \(\text m \,\text s^{-2}\) . What would be the readings on the scale in each case? 
  4. What would be the reading if the lift mechanism failed and it hurtled down freely under gravity ?
A ball is dropped from a height of 90 m on a floor. At each collision with the floor, the ball loses one tenth of its speed. Plot the speed-time graph of its motion between t = 0 to 12 s.
A bob of mass 0.1 \(\text {kg}\) hung from the ceiling of a room by a string 2 \(\text m\) long is set into oscillation. The speed of the bob at its mean position is 1 \(\text m\,\text s^{-1}\). What is the trajectory of the bob if the string is cut when the bob is 
  1. at one of its extreme positions, 
  2. at its mean position.
A metre-long tube open at one end, with a movable piston at the other end, shows resonance with a fixed frequency source (a tuning fork of frequency 340 Hz) when the tube length is 25.5 cm or 79.3 cm. Estimate the speed of sound in air at the temperature of the experiment. The edge effects may be neglected.

A wire stretched between two rigid supports vibrates in its fundamental mode with a frequency of 45 Hz. The mass of the wire is 3.5 × 10–2 kg and its linear mass density is 4.0 × 10–2 kg m–1. What is (a) the speed of a transverse wave on the string, and (b) the tension in the string?

A rocket with a lift-off mass 20,000 \(\text {kg}\) is blasted upwards with an initial acceleration of 5.0 \(\text \,m \text s^{-2}\). Calculate the initial thrust (force) of the blast.

Given below are some functions of x and t to represent the displacement (transverse or longitudinal) of an elastic wave. State which of these represent (i) a travelling wave, (ii) a stationary wave or (iii) none at all: 

(a) y = 2 cos (3x) sin (10t) 

(b) \(y=2\sqrt{x-vt}\)

(c) y = 3 sin (5x – 0.5t) + 4 cos (5x – 0.5t) 

(d) y = cos x sin t + cos 2x sin 2t

(i) For the wave on a string described in Exercise 15.11, do all the points on the string oscillate with the same (a) frequency, (b) phase, (c) amplitude? Explain your answers. (ii) What is the amplitude of a point 0.375 m away from one end?
The driver of a three-wheeler moving with a speed of 36 km/h sees a child standing in the middle of the road and brings his vehicle to rest in 4.0 s just in time to save the child. What is the average retarding force on the vehicle ? The mass of the three-wheeler is 400 kg and the mass of the driver is 65 kg.

The transverse displacement of a string (clamped at its both ends) is given by 

y(x, t) = 0.06 sin \((\frac{2π}{3 }x)\) cos (120 πt) 

where x and y are in m and t in s. The length of the string is 1.5 m and its mass is 3.0 ×10–2 kg. 

Answer the following : 

(a) Does the function represent a travelling wave or a stationary wave? 

(b) Interpret the wave as a superposition of two waves travelling in opposite directions. What is the wavelength, frequency, and speed of each wave?

(c) Determine the tension in the string

For the travelling harmonic wave 

y(x, t) = 2.0 cos 2π (10t – 0.0080 x + 0.35) 

where x and y are in cm and t in s. Calculate the phase difference between oscillatory motion of two points separated by a distance of 

(a) 4 m, 

(b) 0.5 m, 

(c) \(\frac{λ}{2}\), 

(d) \(\frac{3λ}{4}\)

A transverse harmonic wave on a string is described by

 y(x, t) = 3.0 sin (36 t + 0.018 x + \(\frac{π}{4}\)) 

where x and y are in cm and t in s. The positive direction of x is from left to right. 

(a) Is this a travelling wave or a stationary wave ? If it is travelling, what are the speed and direction of its propagation ? 

(b) What are its amplitude and frequency ? 

(c) What is the initial phase at the origin ?

A hospital uses an ultrasonic scanner to locate tumours in a tissue. What is the wavelength of sound in the tissue in which the speed of sound is 1.7 km s–1 ? The operating frequency of the scanner is 4.2 MHz
A bat emits ultrasonic sound of frequency 1000 kHz in air. If the sound meets a water surface, what is the wavelength of (a) the reflected sound, (b) the transmitted sound? Speed of sound in air is 340 m s –1 and in water 1486 m s–1 .

You have learnt that a travelling wave in one dimension is represented by a function y = f (x, t) where x and t must appear in the combination x – v t or x + v t, i.e. y = f (x ± v t). Is the converse true? Examine if the following functions for y can possibly represent a travelling wave : 

(a) (x – vt )2 

(b) log \([\frac{x + vt}{x_0} ] \)

(c) \(\frac{1}{(x + vt)}\)

Use the formula \(v=\sqrt\frac{γp}{ρ}\)  to explain why the speed of sound in air 

(a) is independent of pressure, 

(b) increases with temperature, 

(c) increases with humidity.

A constant force acting on a body of mass 3.0 \(\text {kg}\) changes its speed from 2.0 \(\text m \, \text s^{-1}\) to 3.5 \(\text m \, \text s^{-1}\) in 25 \(\text s\). The direction of the motion of the body remains unchanged. What is the magnitude and direction of the force ?
A constant retarding force of 50 \(\text N\) is applied to a body of mass 20 \(\text {kg}\) moving initially with a speed of 15 \(\text m \,\text s^{-1}\). How long does the body take to stop ?
One end of a string of length l is connected to a particle of mass m and the other to a small peg on a smooth horizontal table. If the particle moves in a circle with speed v the net force on the particle (directed towards the centre) is:
Give the magnitude and direction of the net force acting on a stone of mass 0.1 \(\text {kg}\),
  1. just after it is dropped from the window of a stationary train,
  2. just after it is dropped from the window of a train running at a constant velocity of 36 \(\text {km}/\text h\),
  3. just after it is dropped from the window of a train accelerating with 1 \(\text m \; \text s^{-2}\),
  4. lying on the floor of a train which is accelerating with 1 \(\text m \; \text s^{-2}\), the stone being at rest relative to the train.

Neglect air resistance throughout.

A pebble of mass 0.05 kg is thrown vertically upwards. Give the direction and magnitude of the net force on the pebble,
  1. during its upward motion,
  2. during its downward motion,
  3. at the highest point where it is momentarily at rest. Do your answers change if the pebble was thrown at an angle of 45° with the horizontal direction?

Ignore air resistance.

Give the magnitude and direction of the net force acting on
  1. a drop of rain falling down with a constant speed,
  2. a cork of mass 10 \(\text g\) floating on water,
  3. a kite skillfully held stationary in the sky,
  4. a car moving with a constant velocity of 30 \(\text {km} /\text h\) on a rough road,
  5. a high-speed electron in space far from all material objects, and free of electric and magnetic fields.
A body is moving unidirectionally under the influence of a source of constant power. Its displacement in time \(\text t\) is proportional to
A stone dropped from the top of a tower of height 300 m splashes into the water of a pond near the base of the tower. When is the splash heard at the top given that the speed of sound in air is 340 m s–1 ? (g = 9.8 m s–2)
A string of mass 2.50 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, how long does the disturbance take to reach the other end?