Final answer:
The body's resistance to acceleration by an external force is its inertial mass. Mass is a measure of inertia, and the greater the mass, the greater the inertia and resistance to changes in motion. The kilogram is the SI unit of mass.
Step-by-step explanation:
The measure of the body's resistance to acceleration by some external force is its inertial mass. Inertia is the property of a body to remain at rest or to continue in motion with constant velocity, and this property is directly related to the mass of the body. An object with a larger mass exhibits greater inertia, meaning it resists changes in motion more than an object with a smaller mass. The SI unit of mass is the kilogram (kg).
It is important to note that while mass is related to weight, they are not the same. Weight is the force exerted by gravity on an object and can change depending on the location. For instance, an object's mass is the same whether on Earth, in orbit, or on the surface of the Moon. This inertial mass is a fundamental concept in Newton's first law, often referred to as the law of inertia.