Center Of Mass MCQ - Practice Questions & Answers

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Quick Facts

Center of mass is considered one the most difficult concept.
15 Questions around this concept.

Solve by difficulty

Consider a two-particle system with particles having masses m₁ and m_{2 .} If the first particle is pushed towards the centre of mass through a distance d, by what distance should the second particle be moved, so as to keep the centre of mass at the same position?

$d$

$\frac{m_{2}}{m_{1}}d$

$\frac{m_{1}}{m_{1}+m_{2}}d$

$\frac{m_{1}}{m_{2}}d$

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A T shaped object with dimensions shown in the figure , is lying on a smooth floor. A force $\vec{F}$ is applied at the point P parallel to AB such that the object has only the translational motion without rotation. Find the location of P with respect to C

$\frac{4}{3}l$

$l$

$\frac{3}{4}l$

$\frac{3}{2}l$

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Concepts Covered - 1

Center of mass

Definition-

Centre of mass of a body is defined as a single point at which the whole mass of the body or system is imagined to be concentrated and all external forces are applied there.
It is the point where if a force is applied it moves in the direction of the force without rotating.

x, y, & z coordinates of centre of mass

For a system of N discrete particles

$x_{cm}=\frac{m_{1}x_{1}+m_{2}x_{2}.........}{m_{1}+m_{2}.......}$

$y_{cm}=\frac{m_{1}y_{1}+m_{2}y_{2}+m_{3}y_{3}.........}{m_{1}+m_{2}+m_{3}.......}$

$z_{cm}=\frac{m_{1}z_{1}+m_{2}z_{2}+m_{3}z_{3}.........}{m_{1}+m_{2}+m_{3}.......}$

Where $m_1$ , $m_2$ ........... are mass of each particle $x_1$ , $x_2$ .......... $y_1$ , $y_2$ ............ $z_1$ , $z_2$ are respectively x, y, & z coordinates of particles.

It is the unique point where the weighted relative position of the distributed mass sums to zero.

Centre of Mass of a continuous Distribution

$x_{cm}=\frac {\int xdm}{\int dm}, \; \;y_{cm}=\frac{\int ydm}{\int dm}, \;z_{cm}=\frac{\int zdm}{\int dm}$

Where dm is mass of small element. x, y, z are the coordinates of dm part.

Important points about position of centre of mass

Its position is independent of the coordinate system chosen.
Its position depends upon the shape of the body and distribution of mass.

And depending on this it may lies inside of the body as well as outside the body.

For symmetrical bodies having the homogenous distribution of mass ,the centre of mass coincides with the geometrical centre of the body.
It changes its position only under the translatory motion whereas there is no effect on its position because of rotatory motion of the body.

Centre of gravity-

Centre of gravity of a body is a point, through which the resultant of all the forces experienced by various particles of the body due to the attraction of earth, passes irrespective of the orientation of the body.
If the body is located in a uniform gravitational field,then the centre of mass coincides with the centre of gravity of body, and if not then its centre of mass and centre of gravity will be at two different locations.