According to the principle of conservation of momentum, a. the amount of momentum of all the objects in the universe is constant. b. in an open system, the total amount of momentum of the objects is conserved. c. in a closed system, the speed of any two colliding objects will remain constant. d. in a closed system, an object's momentum before a collision will equal its momentum after the collision.

D. in a closed system, an object's momentum before a collision will equal its momentum after the collision.

Explanation:

As we know by newton's II law we have

so we will have

F = net force on the system

= change in momentum

so we can say if the momentum is conserved or the initial momentum of the system will be equal to the final momentum of the system

then we will say that

so net force on the system must be ZERO

so in such type of questions the system must be isolated and there is no external force on it

so correct answer will be

D. in a closed system, an object's momentum before a collision will equal its momentum after the collision.

air exerts forces on falling objects near earth’s surface.

explanation:

the term 'free falls' refer to object that are falling towards the earth, in a situation in which gravity is the only force acting on the object.

for an object in free fall, the acceleration is constant (g, the gravitational acceleration), and so the velocity of the object constantly increases.

for objects near earth's surface, there is always another force acting on the object: the resistance due to the air. this force is opposed to the direction of fall of the object, and it is proportional to the velocity of the object, so as the object falls down, this force increases up to a point when it balances the force of gravity, and the object continues its motion at constant velocity (called terminal velocity). since in this case gravity is not the only force acting on the object, we are not in a situation of free fall.