Newtons third law (inertia) is to blame
1) The charge left on the fur is equal and opposite to the charge transferred to the rod:

In fact, when the rod is rubbed with the fur, a net charge of

has been transferred to the rod, leaving it negatively charged. If we assume the fur was initially neutral, this means that we have now an excess of positive charges on the fur, and the amount of this charge must be equal (in magnitude, but with opposite sign) to the charge transferred to the rod.
2) The mass transferred to the rod is equal to the total mass of the electrons transferred to the rod.
The charge transferred to the rod is

The charge of 1 electron is

So the number of electrons transferred is

The mass of 1 electron is

, therefore the total mass transferred to the rod is
Umm I think the ocean floor but I don't think so
Answer:
The speed of the sled is 9.2 m/s
The speed of the boulder is 0.82 m/s
Solution:
As per the question:
Mass of the boulder,
Mass of the sled,
Mass of the boy,
Initial Velocity, v = 10.0 m/s
Now,
To calculate the speed of both the sled and the boulder after the occurrence of the collision:
m = 
Initial velocity of the boulder, 
Since, the collision is elastic, both the energy and momentum rem,ain conserved.
Now,
Using the conservation of momentum:

where
v' = final velocity of the the system of boy and sled
= final velocity of the boulder

(1)
Now,
Using conservation of energy:


(2)
Now, from eqn (1) and (2):


Now,

