Question

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

Explanation:

When an object is dropped, in the absence of significant air resistance (which is a reasonable approximation for a penny dropped from 20 meters in many cases), the force of gravity acts on it. According to Newton's second law \(F = ma\), where \(F\) is the force (here, the force of gravity \(F = mg\), \(m\) is the mass of the penny, and \(g\approx9.8\ m/s^{2}\) is the acceleration due to gravity). So, \(mg=ma\), which simplifies to \(a = g\). This means the penny will accelerate as it falls.

• An object falls at a constant velocity (terminal velocity) when the force of air resistance equals the force of gravity. But for a short - distance fall like 20 meters, air resistance is not enough to balance gravity for a penny, so it does not fall at a constant velocity.
• Mass is an intrinsic property of an object and does not change with time during free - fall.
• There is no physical basis for an object changing direction every 9.8 meters during free - fall. Gravity acts in a constant downward direction.

Answer:

The force of gravity causes it to accelerate.