The true weight of an object is the mass of that object measured at a specific time. For most everyday purposes, an object’s weight does not change significantly over time, so we can consider its weight constant. However, from a scientific perspective, an object’s weight does fluctuate slightly depending on various factors.
How is weight related to mass?
An object’s mass is the amount of matter it contains. This does not change unless matter is added or removed from the object. Weight is the force exerted on an object by gravity. Gravity causes objects with mass to be attracted to each other. The strength of this attraction depends on the masses of the objects and the distance between them.
On Earth, we experience the gravitational attraction between our bodies and the planet. More massive objects experience a stronger gravitational force, which we perceive as greater weight. So while mass stays constant, weight changes depending on gravity. If you weighed yourself on Earth vs. on the moon, your mass would stay the same but your weight would be different because of the lower gravity on the moon.
What causes weight fluctuations on Earth?
There are a few factors that can cause slight variations in weight here on Earth:
- Altitude: Gravity decreases slightly the farther you are from Earth’s surface. So an object will weigh slightly less at higher altitudes.
- Latitude: Gravity is weaker at the equator than at the poles due to the centrifugal force from Earth’s rotation. So weight decreases slightly closer to the equator.
- Time of day: Gravity fluctuates very slightly over the course of a day due to the movement of masses on Earth, like ocean tides.
However, these fluctuations are minute, on the order of a few ten-thousandths of a percent. For most practical purposes, weight is considered constant on Earth.
When does weight change more dramatically?
There are some instances where an object’s weight can change more noticeably here on Earth:
- Moving between locations with different elevations/latitudes
- Gaining or losing a significant amount of matter (for example, through eating and excretion)
- Measuring under different conditions like air vs water buoyancy
The most drastic weight changes occur when moving between celestial bodies. On the moon, for example, gravity is about 1/6th as strong as on Earth. An object would weigh roughly 1/6th its Earth weight on the moon. Similarly, on Jupiter gravity is over twice as strong and objects would weigh more than twice their Earth weight.
Conclusion
In summary, an object’s true mass stays constant, but its weight depends on gravity and can fluctuate slightly. Here on Earth, those fluctuations are usually miniscule. More noticeable weight changes happen when moving locations or gaining/losing matter. The time of day has a negligible effect on weight due to small changes in Earth’s gravity over a day. So for most purposes, the “true weight” of an object can be considered its mass multiplied by Earth’s standard gravity, regardless of short-term variations.