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PHYSICS MCQs

Total Questions : 421 | Page 6 of 43 pages
Question 51.

A long spring , when stretched by `x` cm has a potential energy  `u` . On increasing the  length of spring by stretching to `nx` cm , the potential energy stored in the spring will be


  1.    `u/n`
  2.    n u
  3.    `n^2u`
  4.    `u/n^2`
 Discuss Question
Answer: Option C. -> `n^2u`

Initially potential energy  = `1/2 kx^2`

`rArr          u = 1/2 kx^2`

or ` 2u  = kx^2`

`rArr      k =  (2u)/(x^2)`

when it is stretched  to  `nx` cm

`:.`        `P.E. = 1/2  kx_1^2 = 1/2 xx (2u)/(x^2) xx n^2 x^2 = n^2 u`

`:. `         potential energy stored in the spring  = `n^2 u`


Question 52.

A balloon is rising from the surface of earth . Then its potential energy


  1.    increases
  2.    decreases
  3.    first increases hen decreases
  4.    remains constant
 Discuss Question
Answer: Option B. -> decreases

In the direction of effective gravitational a acceleration , potential energy of body decreases when balloon rises, buoyancy force is greater then weight of balloon . It indicates that effected gravitational  acceleration is in upward direction. Hence , (B) is correct.



Question 53.

If a compressed spring is dissolved in acid


  1.    the energy of the spring increases
  2.    the energy of acid decreases
  3.    the potential energy and kinetic energy of molecules of acid increases
  4.    the emperature of acid increases
 Discuss Question
Answer: Option C. -> the potential energy and kinetic energy of molecules of acid increases

The spring loses elastic potential energy which appears as energy of molecules of acid . Hence , (C) is correct


Question 54.

The potential energy of a particle of mass 0.1 jg moving along the `x`-axis is given by `U = 5x ( x - 4)J, where `x` is in metre . It can be concluded that


  1.    the particle is acted upon by a constant force
  2.    the speed of the particle is maximum at `x = 2 m`
  3.    the particle cannot execute simple harmonic motion
  4.    the period of oscillation of the particle is `pi/20 s`
 Discuss Question
Answer: Option B. -> the speed of the particle is maximum at `x = 2 m`


Question 55.

The potential energy is a function for the force between two atoms in a diatomic molecule is given by

`U(x)  = a/x^12 - b/x^6` where a and b are positive constants and x is the distance between the atoms . The position of stable equilibrium  for the system of the two atoms is given by


  1.    `x = a/b`
  2.    `x= sqrt(a/b)`
  3.    `x= sqrt(3a)/b`
  4.    `x=6sqrt((2a)/(b))`
 Discuss Question
Answer: Option D. -> `x=6sqrt((2a)/(b))`

`because       U(x) = a/x^12 - b/x^6`

At the stable equilibrium           ` (dU)/(Dx) = 0`

                                                   `- (12a)/(x^13) + (6b)/(x^7) = 0

                                          `x = ((2a)/(b))^(1/6)`


Question 56.

If a man of mass M jumps to the ground from a height h and his centre of mass moves a distance `x` in the time taken by him to hit the ground , the average force acting on hum is


  1.    `(Mgh)/(x)`
  2.    `(Mgx)/(h)`
  3.    `Mg (h/x)^2`
  4.    `Mg(x/h)^2`
 Discuss Question
Answer: Option A. -> `(Mgh)/(x)`

Here  `F = - (Delta U)/(Delta x)`

            = `- (-mgh)/(x)`

           =` (mgh)/(x)`


Question 57.

Which of the following is/are not conservative force ?


  1.    gravitattional force
  2.    electrostatic force in the coulomb field
  3.    frictional force
  4.    all of above
 Discuss Question
Answer: Option C. -> frictional force

The work done by force of friction in a closed loop  is not zero . Hence , frictional force is non-conservative in nature .


Question 58.

If a particle is compelled to move on a given smooth plane curve under the action of given force in the plane

`vec(F)` =` vec(XI)` +` vec (yj)`. Then


  1.    `vec(F) * vec(dr) = xdx + ydy`
  2.    `int vec(F) * vec(dr) ne 1/2 mv^2`
  3.    `vec (F) * vec(dr) ne xdx xx ydy`
  4.    `1/2 mv^2 ne int (xdx + ydy)`
 Discuss Question
Answer: Option A. -> `vec(F) * vec(dr) = xdx + ydy`

`vec (F) * d vec(r) = ( x hat(i) + y hat(j))  * ( dx hat(i) + dy hat(j))`

                                = `xdx + ydy`


Question 59.

In an elastic string whose natural length is equal to that of a uniform rod be attached to the rod at both ends and suspended by the middle point


  1.    the rod will sink until the total work done is non -zero
  2.    the rod will sink until the total work done is zero
  3.    sinking of rod is not determined on the basis of work done
  4.    sinking of rod is no possible.
 Discuss Question
Answer: Option B. -> the rod will sink until the total work done is zero


Question 60.

A particle moves along a curve of unknown shape but magnitude of force F is constant and always acts along to the curve Then.


  1.    `vec(F)` may be conservative
  2.    `vec(F)` must be conservative
  3.    `vec(F) `may be non-cinservatuve
  4.    `vec(F)` must be non -conservative
 Discuss Question
Answer: Option D. -> `vec(F)` must be non -conservative

Since , work done by this force depends upon path followed by the particle between two point . Hence , the force must be non-conservative.


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