Question

what happens when a penny is dropped from a height of 20 meters?
its mass doubles for every second it falls.
it falls at a constant velocity.
the force of gravity causes it to accelerate.
it changes direction every 9.8 meters.

Explanation:

Step1: Analyze mass property

Mass is an inherent property of an object and does not change with time of fall. So, "Its mass doubles for every second it falls" is wrong.

Step2: Analyze velocity

When an object is dropped (assuming no air - resistance for simplicity, or in a vacuum), it is in free - fall. In free - fall, the velocity is given by \(v = v_0+gt\) (\(v_0 = 0\) for a dropped object), and \(g=9.8\ m/s^{2}\). The velocity is not constant. So, "It falls at a constant velocity" is wrong.

Step3: Analyze acceleration

According to Newton's second law \(F = ma\), for an object in free - fall near the Earth's surface (neglecting air resistance), the force \(F = mg\). Then \(a=\frac{F}{m}=g\). The force of gravity causes an acceleration \(g = 9.8\ m/s^{2}\). So, "The force of gravity causes it to accelerate" is correct.

Step4: Analyze direction

In free - fall (assuming no obstacles and motion near the Earth's surface in a straight - line path), the direction of motion is always towards the center of the Earth (downward in the common sense) and does not change direction every \(9.8\) meters. So, "It changes direction every 9.8 meters" is wrong.

Answer:

C. The force of gravity causes it to accelerate