An object is at rest on the ground. The object experiences a downward gravitational force from Earth. Which of the following predictions is correct about why the object does not accelerate downward? Select two answers. Justify your selections.A) The bonded molecules of the object are repelled upward by the bonded molecules of the ground with the same magnitude as the gravitational force downward on the object.B)The normal force is exerted upward on the object from the ground with the same magnitude as the gravitational force downward on the objectC) The bonded molecules of the object are attracted downward by the bonded molecules of the ground with the same magnitude as the gravitational force downward on the object.D) The force of friction is exerted upward on the object from the ground with the same magnitude as the gravitational force downward on the object.

As the object experiences a downward gravitational force from Earth, in order to be at rest, it must experience an upward force with the same magnitude as the gravitational force on the object.

This force is supplied by the normal force, which can adopt any value in order to meet the condition imposed by Newton´s 2nd Law, and is always perpendicular to the surface on which the object is placed (in this case, the ground).

At a molecular level, this normal force is supplied by the bonded molecules of the ground that behave like small springs being compressed by the molecules of the object, exerting an upward restoring force upward on them.

So, the statements A) and B) are true.

AFOKE88 AFOKE88 · Answer 2 · 2021-10-08T19:51:10+02:00

The predictions that are correct about why the object at rest on the ground does not accelerate downward are;

Option A; The bonded molecules of the object are repelled upward by the bonded molecules of the ground with the same magnitude as the gravitational force downward on the object.

Option B; The normal force is exerted upward on the object from the ground with the same magnitude as the gravitational force downward on the object

We are told that the object is at rest on the ground. This implies that there is no net force being exerted on it.

We are now told that the object experiences a downward gravitational force from the earth. As a result of the downward gravitational force experienced by the object from Earth, the condition for which it will remain at rest is that there must be an upward force acting on it that has the same magnitude as that of the downward gravitational force but acting perpendicular to the surface upon which the object is resting.

Since we are not given any force acting vertically as the upward force, the only logic is that the upward force comes from the molecules on the ground that bond with the object at rest. This means they act like resistive forces to the downward gravitational force.

Looking at the given options, only Options A and B are correct.

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