Mass is often defined as the amount of matter in an object. But physicists define mass as a measure of inertia. Inertia is a property of all matter. Due to inertia, a motionless object tends to remain motionless. A moving object tends to continue to move at a constant velocity. That means that the object moves at a constant speed and in the same direction.
The greater an object’s mass, the greater the force needed to change its velocity by a given quantity in a given time. An example of such a change would be an increase in velocity from 15 meters per second northward to 20 meters per second northward in 10 seconds.
Scientists refer to a change in velocity as an acceleration. The relationship between force, mass, and acceleration is described by Newton’s second law of motion. The English scientist Isaac Newton discovered the law in the 1660’s. It was published in 1687. This law can be expressed as F = ma. In that equation, F is force, m is mass, and a is acceleration.
The modern metric system is called the International System of Units (SI). In the system, force can be given in newtons. Mass is expressed in kilograms. Acceleration is stated in meters per second per second. Suppose we apply a force of 1 newton to an object with a mass of 1 kilogram under four conditions. The first condition is that the object is already moving northward at a rate of 15 meters per second. The second condition is that we apply the force in the direction of motion. The third condition is that no other force is being applied to the object. The fourth condition is that there is no frictional force resisting the applied force. Each second we apply the force, the velocity of the object will increase by 1 meter per second. Thus, in 5 seconds, the object will be moving northward at a rate of 20 meters per second.
But suppose the object has a mass of 2 kilograms and we still want to increase its velocity by 1 meter per second each second. We must apply a force of 2 newtons to achieve this acceleration.
In the inch-pound system of units customarily used in the United States, there are two units of mass. In commercial and everyday use, people use the pound. A pound equals 0.454 kilogram. Engineers prefer to use the slug for mass and the pound for force. A slug is equal to 14.594 kilograms. A force of 1 pound will accelerate an object with a mass of 1 slug by 1 foot per second each second.
Mass and weight.
In commercial and everyday use, weight is understood to mean mass. In this sense, to weigh means to determine the mass of. For example, a man weighs himself to determine the amount of matter that makes up his body. In a country using the metric system, the weighing scale would display the man’s weight in kilograms. In the United States, the display would be in pounds.
In science and technology, weight refers to the gravitational force on an object. Newton’s law of gravitation was also published in 1687. According to this law, weight is proportional to mass. This law is represented by W = mg. In that equation, W is weight, m is mass, and g is the acceleration due to gravity.
On the surface of Earth, g is about 9.8 meters per second per second in the SI. it is about 32 feet per second per second in the inch-pound system. Thus, the gravitational force on a 1-kilogram object is about 9.8 newtons. The force on a 1-slug object is about 32 pounds.
