When an object falls toward Earth, a lot of different things happen, ranging from energy transfers to air resistance to rising speed and momentum. Understanding all the factors at play prepares you for understanding a range of problems in classical physics, the meaning of terms such as momentum, and the nature of the conservation of energy. The short version is that when an object falls toward Earth, it gains speed and momentum, and its kinetic energy increases as its gravitational potential energy falls, but this explanation skips many important details.
TL;DR (Too Long; Didn't Read)
When an object falls toward Earth, it accelerates due to the force of gravity, gaining speed and momentum until the upward force of air resistance exactly balances the downward force due to the object’s weight under gravity – a point referred to as terminal velocity.
The gravitational potential energy an object has at the start of a fall is converted into kinetic energy as it falls, and this kinetic energy goes into producing sound, causing the object to bounce, and deforming or breaking the object as it strikes the ground.
Speed, Acceleration, Force and Momentum
Gravity causes objects to fall toward the Earth. Over the entire surface of the planet, gravity causes a constant acceleration of 9.8 m/s2, commonly given the symbol g. This varies ever so slightly depending on where you are (it’s about 9.78 m/s2 at the equator and 9.83 m/s2 at the poles), but it stays broadly the same across the surface. This acceleration causes the object to increase in speed by 9.8 meters per second every second it falls under gravity.
Momentum (p) is closely linked to speed (v) through the equation p = mv, so the object gains momentum throughout its fall. The mass of the object doesn’t affect how quickly it falls under gravity, but massive objects have more momentum at the same speed because of this relationship.
The force (F) acting on the object is demonstrated in Newton’s second law, which states F = ma, so the force = mass × acceleration. In this case, the acceleration is due to gravity, so a = g, which means that F = mg, the equation for weight.
Air Resistance and Terminal Velocity
The Earth’s atmosphere plays a role in the process. The air slows the object’s fall due to air resistance (essentially the force of all the air molecules hitting it as it falls), and this force increases the faster the object falls. This continues until it reaches a point called terminal velocity, where the downward force due to the object’s weight exactly matches the upward force due to air resistance. When this happens, the object can’t accelerate anymore and continues to fall at that speed until it hits the ground.
On a body like our moon, where there is no atmosphere, this process wouldn’t occur, and the object would continue to accelerate due to gravity until it hit the ground.
Energy Transfers on a Falling Object
An alternative way to think about what happens as an object falls toward Earth is in terms of energy. Before it falls – if we assume it’s stationary – the object possesses energy in the form of gravitational potential. This means it has the potential to pick up a lot of speed due to its position relative to the surface of the Earth. If it’s stationary, its kinetic energy is zero. When the object is released, the gravitational potential energy is gradually converted into kinetic energy as it picks up speed. In the absence of air resistance, which causes some energy to be lost, the kinetic energy just before the object strikes the ground would be the same as the gravitational potential energy it had at its highest point.
What Happens When an Object Hits the Ground?
When the object hits the ground, the kinetic energy has to go somewhere, because energy isn’t created or destroyed, only transferred. If the collision is elastic, meaning the object can bounce, much of the energy goes into making it bounce up again. In all real collisions, energy is lost when it hits the ground, some of it going into creating a sound and some going into deforming or even breaking the object apart. If the collision is completely inelastic, the object is squashed or smashed, and all of the energy goes into creating the sound and the effect on the object itself.