An hourglass is inverted and placed on a scale. Compare the scale readings (a) before sand begins to hit the bottom; (b) while sand is hitting the bottom; and (c) when all the sand is on the bottom.

Often encapsulated in the phrase 'for every action, there is an equal and opposite reaction,' Newton's Third Law of Motion is essential for understanding how forces work in pairs. Consider the hourglass on the scale. When the sand grains fall and strike the bottom, they exert a force (the action) on the glass. Consequently, the glass must exert an equal and opposite force (the reaction) on the sand grains. This reaction force is what the scale detects, causing a temporary spike in the reading. It's vital to recognize that these forces are always equal and occur simultaneously.

In the context of our hourglass, the additional force can be likened to a slight 'jump' of the sand upon impact, illustrating Newton's principle that the forces between two interacting objects are equal in magnitude but opposite in direction. Understanding this law helps explain why the scale reading changes during the transfer of sand from the top to the bottom of the hourglass.

A fundamental aspect of physics is the relationship between force and motion. A force is any interaction that, when unopposed, will change the motion of an object. In our exercise, the moving sand particles enact a force upon impact with the glass, which is an example of how motion can create force. This impact force can cause the motion of the glass, even if ever so slightly, confirming the intrinsic tie between force and an object's movement.

To illustrate this, as each grain of sand hits the bottom of the hourglass, it is in essence pushing down on the scale for a brief moment, resulting in a higher scale reading. This outcome demonstrates how an object in motion (sand grains falling) directly influences the resultant force (the temporary increase in the scale's reading).

Measuring weight on a scale is more than just reading a number; it involves understanding the scale weight measurement process. A scale translates the force exerted on its surface into a weight reading. It's crucial to comprehend that scales measure the force due to gravity on an object's mass, which is perceived as weight.

For the hourglass, the scale reflects the combined force of gravity on the glass and sand when at rest. But when the sand falls and hits the bottom, there is a momentary exertion of additional force, thus leading to a temporary fluctuation in weight measurement. Knowing this, it's important to remember that any changes in the weight recorded by the scale directly correspond to changes in the forces being applied to the surface of the scale.

The concept of momentum transfer is closely related to Newton's third law and plays a crucial role in understanding why the scale reading changes. Momentum, a product of mass and velocity, is always conserved in an isolated system. When sand grains fall, they carry momentum and, upon collision with the glass bottom, transfer some of that momentum to the hourglass system.

This transfer is what creates the force that leads to the temporary spike in the scale reading. Recognizing the conservation of momentum aids in comprehending that the impacts are not just singular events but part of a continuous process that affects the forces and, subsequently, the scale reading. The calibration of the scale back to the original reading once the sand settles confirms that momentum has been transferred from the falling sand to the hourglass and, by extension, to the scale.