An apple falling to the ground is not an example of centripetal acceleration.
If there is no friction, the force that moves the box forward horizontally must be matched by the same force.
If there is friction, then the force moving it forward = frictional force + the additional force you need to add.
Answer:
The distance between the two objects must be squared.
Explanation:
Gravitational force always act between two objects that have mass. The gravitational force is a weak force and attractive in nature.
The force of pull depends on the masses of the two objects and the distance between them.
The formula to calculate gravitational force between two objects having masses 'm' and 'M' and separated by a distance 'd' is given as:

Where, 'G' is called the universal gravitational constant and its value is equal to
.
Now, from the above formula, it is clear that, the force of gravitation is inversely proportional to the square of the distance between the two objects.
Thus, the quantity that must be squared in the equation of gravitational force between two objects is the distance 'd'.
Since the new distance is 3 times the old distance,
the new force is (1/3²) = 1/9th of the old force.
That's kind-of Choice-D, but I really don't like the way choice-D is worded.
"9 times smaller" is really pretty meaningless.