In a closed system, the loss of momentum of one object the gain in momentum of another object.

In a closed system, the loss of momentum of one object is same as the gain in momentum of another object

according to law of conservation of momentum, total momentum before and after collision in a closed system in absence of any net external force, remains conserved . that is

total momentum before collision = total momentum after collision

P₁ + P₂ = P'₁ + P'₂

where P₁ and P₂ are momentum before collision for object 1 and object 2 respectively.

P'₁ - P₁  = - (P'₂ -  P₂)

so clearly gain in momentum of one object is same as the loss of momentum of other object

Sally

If Harry's negligent act injures Sally, and Susan, while attempting to come to Sally's aid breaks her arm in the process then, Harry is liable for the harm to Sally.

Explanation:

As a mathematical expression, we can say the action of Harry H, is directly proportional to the injury to Sally S; while the mistake of Susan N is directly proportional to injury to Sally S.

So, if H = S and N = S, it is safe to say

H = N = S.

Thus, Harry negligence create room for Susan mistake, which eventually harmed Sally.