Tag: physics

Questions Related to physics

Statement 1: When we lean behind over the hind legs of the chair, the chair falls back after a certain angle.

Statement 2: Centre of mass lying outside the system makes the system unstable.

physics turning effects of forces stability and centre of mass center of mass centre of mass

  1. Statement 1 is false, Statement 2 is true

  2. Statement 1 is true, Statement 2 is true; Statement 2 is a correct explanation for Statement 1

  3. Statement 1 is true, Statement 2 is true; Statement 2 is not a correct explanation for Statement 1

  4. Statement 1 is true, Statement 2 is false

Show answer
Correct Option: D
Explanation:
The chair falls down after a certain angle because it becomes unstable as the normal reaction by ground on chair becomes zero.
There are several mechanical structure whose COM lies outside its body but they are quite stable.

So option D is correct.

On a stationary sail boat, air is blown at the sails from a fan attached to the boat. The boat will

physics turning effects of forces stability and centre of mass center of mass centre of mass

  1. remain at rest

  2. spin round

  3. move in the direction in which air is blown

  4. move in a direction opposite to that in which air is blown

Show answer
Correct Option: A
Explanation:

the boat will remain at the rest  because the thrust or recoil force of fan at boat and force of air on the sails will cancel each other and boat will not at all move .

so the answer is A.

The passengers in a boat are not allowed to stand because: 

physics turning effects of forces stability and centre of mass center of mass centre of mass

  1. This will raise the centre of gravity and the boat will be rocked

  2. This will lower centre of gravity and the boat will rocked

  3. The effective weight of system increases

  4. Of surface tension effects

Show answer
Correct Option: A
Explanation:

when passengers stands up on the boat from the sitting situation , the centre of gravity moves upwards and it increases the chances of toppling of boat .

so the answer is A.

State whether true or false.
The centre of gravity of regular shaped objects is at their geometric centre.
physics turning effects of forces stability and centre of mass center of mass centre of mass

  1. True

  2. False

Show answer
Correct Option: A
Explanation:

The given statement is true that centre of gravity of rectangular shaped objects is at their geometric centre.

The point through which the total weight of an object appears to act for any orientation of the object is ______.

physics turning effects of forces stability and centre of mass center of mass centre of mass

  1. Centre of buoyancy

  2. Centre of gravity

  3. Centre of curvature

  4. Median of a triangle

Show answer
Correct Option: B
Explanation:

The point through which the total weight of an object appears to act for any orientation of the object is centre of gravity.

State whether true or false.
The position of centre of gravity of an object depends on the acceleration due to gravity at the given place.
physics turning effects of forces stability and centre of mass center of mass centre of mass

  1. True

  2. False

Show answer
Correct Option: B
Explanation:
The center of gravity is a geometric property of any object. The center of gravity is the average location of the weight of an object with mass.
Therefore the position of centre of mass has nothing to do with the acceleration due to gravity.
So, the statement is FALSE

The centre of mass of a body:

physics turning effects of forces stability and centre of mass center of mass centre of mass

  1. Lies always at the geometrical center

  2. Lies always inside the body

  3. Lies always outside the body

  4. Lies within or outside the body

Show answer
Correct Option: D
Explanation:

The centre of mass of a body can lie within or outside the body.

For example, centre of mass of a uniform rod lies at its geometrical centre which lies within the rod whereas centre of mass of a uniform ring lies at its geometrical centre which lies outside the ring.

Two unequal masses are tied together with a cord with a compressed spring in between.
Which of the following energies is conserved for the system?

physics turning effects of forces stability and centre of mass center of mass centre of mass

  1. Kinetic energy

  2. Potential energy

  3. Mechanical energy

  4. None of these

Show answer
Correct Option: C
Explanation:
Both KE and PE are unserversed.
Hence, the answer is mechanical energy.


Two unequal masses are tied together with a cord with a compressed spring in between.
Which one is correct?

physics turning effects of forces stability and centre of mass center of mass centre of mass

  1. Both masses will have equal KE.

  2. Lighter block will have greater KE.

  3. Heavier block will have greater KE.

  4. None of above answers is correct.

Show answer
Correct Option: B
Explanation:
Lighter block will have greater kinetic energy to lighter block will have higher velocity mass so Heavier block, hence by equation $\dfrac{1}{2}mv^2,$ the lighter will have greater KE.
Hence, the answer is Lighter block will have greater KE.

A string is wrapped around a cylinder of mass $M$ and radius $R$. The string is pulled vertically upwards to prevent the centre of mass from falling as the cylinder unwinds the string, The work done on the cylinder for reaching an angular speed $\omega$ is:

physics turning effects of forces stability and centre of mass center of mass centre of mass

  1. $\cfrac { 2M{ R }^{ 2 }{ \omega }^{ 2 } }{ 3 } $

  2. $\cfrac { M{ R }^{ 2 }{ \omega }^{ 2 } }{ 3 } $

  3. $\cfrac { M{ R }^{ 2 }{ \omega }^{ 2 } }{ 2 } $

  4. $\cfrac { M{ R }^{ 2 }{ \omega }^{ 2 } }{ 4 } $

Show answer
Correct Option: D
Explanation:
Work done is the rotational KE acquired be cylinder,
$=\dfrac{1}{2} I\omega ^2$
$=\dfrac{1}{2}\dfrac{MR^2}{2}\omega ^2$
$=\dfrac{MR^2}{4}\omega ^2.$
Hence, the answer is $\dfrac{MR^2}{4}\omega ^2.$