Why do objects in free fall have constant acceleration?

Objects in free fall have constant acceleration due to the constant force of gravity acting upon them.

In more detail, when an object is in free fall, the only significant force acting upon it is the force of gravity. This force is constant and acts in one direction - downwards towards the centre of the Earth. The force of gravity on Earth is approximately 9.8 m/s², which means that for every second an object is in free fall, its speed increases by about 9.8 metres per second.

This constant force results in a constant acceleration. Acceleration is defined as the rate of change of velocity per unit of time. Since the force (and therefore the acceleration) is constant, the object's velocity will increase at a steady rate. This is why objects in free fall have a constant acceleration.

It's important to note that this is an idealised situation. In reality, other forces such as air resistance can also act on the object, which can reduce the acceleration. However, for small objects falling from low heights, or in a vacuum where there is no air resistance, the acceleration due to gravity can be considered constant.

This principle is a fundamental part of physics and is encapsulated in Newton's second law of motion, which states that the acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass. In the case of free fall, the net force is the force of gravity, which is constant. Therefore, the acceleration is also constant.