Two particles $P$ and $Q$ each of mass $3m$ lie at rest on the $X$-axis at points $(-a, 0)$ and $(+a, 0)$,respectively. $A$ third particle $R$ of mass $2m$ initially at the origin moves towards the particle $Q$ with velocity $v$. If all the collisions of the system of $3$ particles are elastic and head-on,the total number of collisions in the system is

The displacement $x$ of a particle moving in one dimension under the action of a constant force is related to the time $t$ by the equation $t = \sqrt{x} + 3$,where $x$ is in meters and $t$ is in seconds. The work done by the force in the first $6$ seconds is.....$J$

From a building, two balls $A$ and $B$ are thrown such that $A$ is thrown upwards and $B$ is thrown downwards (both vertically) with the same speed. If $v_A$ and $v_B$ are their respective velocities on reaching the ground, then:

Two bodies $A$ and $B$ of masses $m$ and $2m$ respectively are placed on a smooth floor. They are connected by a spring. $A$ third body $C$ of mass $m$ moves with velocity $V_0$ along the line joining $A$ and $B$ and collides elastically with $A$ as shown in the figure. At a certain instant of time $t_0$ after the collision,it is found that the instantaneous velocities of $A$ and $B$ are the same. Further,at this instant,the compression of the spring is found to be $x_0$. Determine the spring constant $k$.

State whether the following statements are true or false:
$(a)$ If $\overrightarrow P \cdot \overrightarrow Q = 0$,then the angle between $\overrightarrow P$ and $\overrightarrow Q$ is $0^o$.
$(b)$ If two bodies stick together after a collision,the collision is called an elastic collision.
$(c)$ When an equal force is applied to a heavy and a light object on the same surface,more work is done by the force on the heavy object.

$A$ body of mass $2 \; kg$ initially at rest moves under the action of an applied horizontal force of $7 \; N$ on a table with coefficient of kinetic friction $= 0.1$. Compute the
$(a)$ work done by the applied force in $10 \; s$,
$(b)$ work done by friction in $10 \; s$,
$(c)$ work done by the net force on the body in $10 \; s$,
$(d)$ change in kinetic energy of the body in $10 \; s$,
and interpret your results.