Which two forces are always equal and opposite for an object on a surface?

Suppose any object with mass in our planet.

The object will be affected by the gravitational force, that pulls the object down. Now, a table in your dining room is also affected by this force, but the table is in a surface (the ground) and it does not move.

We know that if an object does not move, then the net force acting on it is equal to zero, this means that there is a force equal and opposite ot the gravitational force.

This force is the normal force, that the ground applies on the table. This force comes as a "response" to the table pushing the ground (By 3rd Newton's law).

So two forces that are always equal and opposite are the forces caused by this law.

can be answered using momentum.

momentum = p

p1= mass1 * velocity1

p1 = p2

therefore

p1 = 6kg * 4m/s

p1 = 24kgm/s

p2=m2 * v2

24 kgm/s = 7kg * v2

v2 = 24 kgm/s / 7kg

v2= 3.42 m/s