Since the block’s mass is 630 g, it’s mass must be 1053.5 / 630 = 1.67 m/s. To find the velocity of the center of mass of the system, you divide the total momentum by the total mass. The total mass of the system is 630 + 4.3 = 634.3 m/s. And its total momentum before and after the collision is 3083.1 g-m/s.

Prior to the collision, the center of mass of the two-block system has a speed of 3.00 m/s.

The speed of the centre of mass is the sum of each mass’s momentum divided by the total mass of the system.

In a collision, the velocity change is always computed by subtracting the initial velocity value from the final velocity value. If an object is moving in one direction before a collision and rebounds or somehow changes direction, then its velocity after the collision has the opposite direction as before.

In collisions between two isolated objects momentum is always conserved. Kinetic energy is only conserved in elastic collisions.

In inelastic collision between two bodies, total linear momentum is always conserved.

Kinetic energy is conserved in perfectly elastic collision only but some kinetic energy is lost in inelastic collisions. So, total kinetic energy is not conserved in inelastic collision.

Answer: As the direction of the radial velocity of the planet changes, The linear momentum of planets is not constant in the planetary system( refer figure below). Since there is no external force on the Sun+planet system, so the total linear momentum of the Sun+planet remains constant.

Momentum is not conserved if there is friction, gravity, or net force (net force just means the total amount of force). What it means is that if you act on an object, its momentum will change. This should be obvious, since you are adding to or taking away from the object’s velocity and therefore changing its momentum.