Final answer:
To calculate the reading on a spring balance with two masses, add their masses together. Bathroom scales work by measuring force and converting it to mass, which depends on Earth's gravity. Digital scales provide more precise measurements than mechanical balances.
Step-by-step explanation:
To calculate the reading on a spring balance that has two masses attached, you would add the mass of both objects together if they are on the same side of the balance. The spring balance works by measuring the weight (the force due to gravity) and then converting this into mass. This is because the spring stretches in proportion to the applied force according to Hooke's Law. The weight-force measured is converted into mass units (such as kilograms) using the acceleration due to gravity, which is approximately 9.80 meters per second squared on Earth.
Bathroom scales are similar to spring balances and measure weight, which can then be converted to mass. When you stand on a bathroom scale, the springs inside compress, and this compression is what the scale reads and converts to a mass unit. If you push down on a table while standing on the scale, the reading will go up because you increase the force on the springs, thus indicating a higher weight and therefore mass. However, this reading does not accurately represent your actual mass, as you are artificially altering the force. Lastly, a bathroom scale calibrated on Earth would not give the same mass reading on the Moon because the gravitational force is different.
For precise measurements of mass, digital scales are preferred over mechanical double-pan balances, as they can measure to the nearest thousandth of a gram, whereas mechanical balances may only be accurate to the nearest tenth of a gram.