The new acceleration is c) 18m/s/s
Explanation:
Net force, mass and acceleration of an object are related by Newton's second law of motion:

where
F is the net force on the object
m is its mass
a is its acceleration
In the first trial of this problem, a net force of
is applied to the object, causing an acceleration of

Calling the mass of the object 'm', this means that
[N] (1)
In the second trial, the force applied is
, so we have

where a' is the new acceleration. Substituting (1) into the second equation, we find:

This is because the acceleration is directly proportional to the force applied: therefore, if the force applied doubled, the acceleration doubles as well.
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The Resistance needs to be the same to allow the current to double with the voltage.
Answer:
The distance from the pivot point that the small child will sit in order to maintain the balance is 1.8 m
Explanation:
Given;
mass of the bigger child, M = 30 kg
mass of the smaller child, m = 20 kg
distance between the two children, d = 3 m
This information can be represented diagrammatically;
3m
|<------------------------------------------------>|
----------------------------------------------------------------------------
↓ x Δ 3-x ↓
20kg 30kg
x is the distance from the pivot point that the small child will sit in order to maintain the balance
Take moment about the pivot;
Clockwise moment = anticlockwise moment
30(3-x) = 20x
90 -30x = 20x
90 = 20x + 30x
90 = 50x
x = 90 / 50
x = 1.8 m
Therefore, the distance from the pivot point that the small child will sit in order to maintain the balance is 1.8 m