Weight is generally defined as the force acting on an object due to the gravitational pull. It is a vector quantity and changes according to the situation. For example, in free fall, the weight of any object is zero, as the total force acting on the object in free fall is cancelled to zero.
Weight is measured in Newtons, which is the SI unit of weight. It is generally measured with the help of a spring scale. Since weight is calculated by multiplying the mass of an object with the acceleration due to the gravitational pull, the weight of an object having one kilogram of mass is 9.8 Newtons on the surface of the Earth. This is due to the fact that the value of gravitational acceleration is 9.8 m/s2.
Weight and force have the same SI units as both of them are, by definition, physical forces. Weight is used interchangeably with mass in everyday usage. However, both are distinct quantities. The mass of an object is an intrinsic property. It remains the same everywhere in the universe. While the weight of an object can vary according to the gravitational pull, it is experiencing. For example, the weight of any object on the moon is six times less than the weight it has on the Earth.
The earliest discussions on the weight of an object can be traced back to the era of the ancient Greeks. At that time, the weight of an object was considered an inherent property. Interestingly, the Greek philosophers described weight as the tendency of an object to seek ground. To them, it symbolised nature trying to restore order to the state of things. According to this view, the Earth was considered the heaviest object in the universe, while the fire was thought of as the lightest thing in the universe.
In the seventeenth century, Issac Newton gave his theory of gravitation and explained the mechanism by which objects are attracted to each other. This completely revolutionised the concept of weight, which had seen little advancement since the time of the greeks. Newton’s law made a fundamental contribution to the concept of weight by differentiating it from the mass. It was the first time mass and weight was defined separately. Mass was instead linked to the inertia of an object. The more mass an object has, the more difficult it is to move it from a state of rest. Newton was also the first to put forward the idea that the weight of an object is not fixed but changes according to the gravitational field around it.
Newton differentiated between apparent weight and the true weight of an object. Apparent weight is the result of imperfect or faulty measurements and is affected by natural phenomena like buoyancy. The true, on the other hand, the true weight of an object is solely defined by the gravitational pull it experiences due to earth.
In honour of Newton’s achievement in the field of classical mechanics, the SI unit of weight was termed ‘Newton’.
One newton is defined as the force that gives an acceleration of one meter per second squared to an object with a one-kilogram mass. Newton was first standardised in 1946 when the CGPM defined it using the MKS system. Two years later, CGPM again used the same definition of Newton in its resolution. The transition from the MKS system to the SI system happened because the MKS system later changed into the SI system we use widely today.
On earth, an object of one-kilogram mass exerts a force of 9.8 newtons. This is often rounded off to 10 Newtons, as the acceleration due to gravity is sometimes taken to be 10 m/s2. Similarly, the weight of an average adult human is approximately 602 Newtons. This is calculated by assuming the average mass of an adult as 62 kilograms.
Though weight and mass are taken to be the same and are used as the same by people all around the world, the scientific community draws a distinction between the two. This is done because the earth’s gravitational field is not uniform across its surface. It can vary up to 0.5% depending on the location where the measurement is being done. Therefore, even if it does not change the measurement of mass for everyday usages, like weighing one kilo of apples or two tonnes of cement, it can cause a significant error when scientific calculations are done. Examples of such calculations involve calculating the mass of one electron or the velocities of atomic particles travelling through space. In such situations, the force exerted on them due to gravity has to be determined accurately to avoid faulty results.