Newton’s first law of motion, also known as the law of inertia, states that a body at rest remains at rest or, if in motion, remains in motion at a constant velocity unless acted on by a net external force.
Experience suggests that an object at rest will remain at rest if left alone, and that an object in motion tends to slow down and stop unless some effort is made to keep it moving. What Newton’s first law of motion states, however, is the following:
NEWTON’S FIRST LAW OF MOTION
A body at rest remains at rest, or, if in motion, remains in motion at a constant velocity unless acted on by a net external force.
Note the repeated use of the verb “remains.” We can think of this law as preserving the status quo of motion.
Rather than contradicting our experience, Newton’s first law of motion states that there must be a cause [which is a net external force] – for there to be any change in velocity [either a change in magnitude or direction].
The idea of cause and effect is crucial in accurately describing what happens in various situations. For example, consider an air hockey table. When the air is turned off, the puck slides only a short distance before friction slows it to a stop. However, when the air is turned on, it creates a nearly frictionless surface, and the puck glides long distances without slowing down. Additionally, if we know enough about the friction, we can accurately predict how quickly the object will slow down. Friction is an external force.
The property of a body to remain at rest or to remain in motion with constant velocity is called inertia. Newton’s first law is often called the law of inertia. As we know from experience, some objects have more inertia than others. It is obviously more difficult to change the motion of a large boulder than that of a basketball, for example. The inertia of an object is measured by its mass. Roughly speaking, mass is a measure of the amount of “stuff” (or matter) in something. The quantity or amount of matter in an object is determined by the numbers of atoms and molecules of various types it contains. Unlike weight, mass does not vary with location. The mass of an object is the same on Earth, in orbit, or on the surface of the Moon. In practice, it is very difficult to count and identify all of the atoms and molecules in an object, so masses are not often determined in this manner. Operationally, the masses of objects are determined by comparison with the standard kilogram.
Practice Questions
Key Points
• Newton’s first law of motion states that a body at rest remains at rest, or, if in motion, remains in motion at a constant velocity unless acted on by a net external force. This is also known as the law of inertia.
• Inertia is the tendency of an object to remain at rest or remain in motion. Inertia is related to an object’s mass.
• Mass is the quantity of matter in a substance.
Key Terms
Inertia: the tendency of an object to remain at rest or remain in motion.
Mass: the quantity of matter in a substance; measured in kilograms.
Newton’s first law of motion/Law of Inertia: A body at rest remains at rest or, if in motion, remains in motion at a constant velocity unless acted on by a net external force; also known as the law of inertia.
Force: Any interaction that, when unopposed, will change the motion of an object.