Newton’s first law of motion states that an object at rest will stay at rest, and an object in motion will stay in motion at the same speed and direction unless acted on by an unbalanced force. This is often stated as “an object in motion tends to stay in motion, and an object at rest tends to stay at rest.”
The law formally stated
The formal statement of Newton’s first law is:
An object at rest will remain at rest unless acted on by an unbalanced force. An object in motion continues in motion with the same speed and in the same direction unless acted upon by an unbalanced force.
This law is sometimes called the law of inertia. Inertia is the tendency of an object to resist any change in its motion. In the absence of an unbalanced force, an object in motion will maintain its state of motion.
What the law means for objects at rest
For objects at rest, Newton’s first law says that the object will remain at rest unless acted on by an unbalanced external force. “At rest” means the object is not moving from the perspective of an outside observer. The object will maintain this state of rest unless it is pushed, pulled, or otherwise compelled to change its motion by interacting with another object or force.
Some examples of objects at rest include:
- A book sitting on a table
- A car stopped at a traffic light
- A ball lying on the ground
These objects will remain at rest indefinitely until acted on by an outside force. For the book, that force could be someone picking it up. For the car, it could be the driver pressing the accelerator. And for the ball, it could be a person kicking it.
Why objects resist changes in motion
Newton’s first law is essentially a statement about inertia. Inertia is the tendency of an object to continue doing what it’s doing – either staying still or moving at a constant speed in a straight line.
But what causes inertia in the first place? Why do objects resist changes in motion?
The inertia of an object comes from its mass. The more mass an object has, the more inertia it has. Mass is a measure of how much matter an object contains. Matter is made up of atoms and molecules that are intrinsically resistant to changes in their state of motion. The more atoms and molecules in an object, the more inertia it will have.
Inertia depends on frame of reference
Whether an object is considered at rest or in motion depends on your frame of reference. A frame of reference is the perspective from which motion is measured. For example, a car sitting still in a parking lot is at rest from the perspective of an observer standing next to it on the pavement. But to an observer on the sidewalk, the parked car is moving past at the same speed that the Earth rotates.
So an object may be considered at rest or in motion, depending on the frame of reference. Newton’s first law applies regardless of the frame of reference – an object at rest will remain at rest, and an object in motion will continue moving at a constant velocity, unless acted on by a net external force.
External forces can change motion
According to Newton’s first law, the only way to change an object’s state of motion is by applying an external force. A force is a push or pull on an object. Net external forces cause acceleration – either speeding up or slowing down.
For example, kicking a ball at rest applies a force that accelerates the ball from rest to motion. Hitting the brakes in a moving car applies an external force that decelerates it.
Newton’s second law describes exactly how forces affect motion – force is equal to mass times acceleration. The acceleration is proportional to the strength of the net force and inversely proportional to the object’s mass.
Mass versus weight
Mass and weight are often used interchangeably in everyday language, but they are distinct concepts in physics. Mass is a measure of how much matter is contained in an object. The more mass an object has, the more inertia it exhibits.
Weight is the force exerted on an object by gravity. An object’s weight depends on its mass and the strength of the gravitational pull. Weight varies with location, but mass stays constant.
For example, you have the same mass on Earth as you would on the moon. But because the moon’s gravity is weaker, you would weigh about one-sixth as much there.
Examples of Newton’s first law
Newton’s first law explains many everyday phenomena. Here are some examples:
- A car’s momentum: A car in motion continues traveling at the same speed and direction unless the brakes are applied or it interacts with another object.
- A hockey puck: A puck at rest on the ice remains there until the player hits it with the stick. Once moving, it continues gliding across the ice at constant velocity.
- A spinning top: The spinning motion persists as the top’s inertia keeps it spinning until friction slows it down.
- A rocket launch: The rocket continues accelerating upward during the powered phase of flight due to the unbalanced upward force from the engines.
Objects in space
Newton’s first law is especially evident in the motion of objects in outer space. With minimal interference from friction or gravity, objects in the vacuum of space maintain their velocity unless acted on by a force.
For example, the Voyager 1 spacecraft launched in 1977 is still traveling away from the Sun at over 38,000 mph after decades. Its inertia keeps it moving unless it interacts with a planet or other object.
Astronaut propulsion
Astronauts take advantage of Newton’s first law to propel themselves while on spacewalks. If the astronaut throws an object in one direction, they will drift in the opposite direction due to the equal and opposite reaction force. Their inertia maintains this constant velocity unless another force is applied.
Common misconceptions
There are a few common misconceptions about Newton’s first law:
- Objects at rest have no forces acting on them. In fact, balanced forces are required to keep an object at rest – like the force of gravity and the support of the surface below an object.
- Motion requires a force. Continuous forces are not needed to keep an object moving at constant velocity. Only when acceleration occurs is there an unbalanced force.
- Friction brings moving objects to rest. Friction is actually a force that slows objects down, causing them to accelerate to zero velocity.
The importance of Newton’s first law
Newton’s first law was revolutionary when he published it in 1687. Before then, the common assumption was that a constant force was required to keep an object moving. Newton’s insight about inertia was crucial in the development of classical physics.
The first law lays the groundwork for dynamics systems analysis using Newton’s second and third laws. Together, they enable modeling and predicting how objects and systems interact and behave. This led to great advances in mechanics, astronomy, and engineering.
The law provides a precise, quantitative description of how velocity and acceleration relate to force and mass. It gives fundamental insight into inertia that is essential for understanding not just everyday objects on Earth but also motion in the heavens from planets to galaxies.
Limits and context of the first law
While Newton’s first law is tremendously powerful, it does have limits. It is an approximation that applies best to everyday objects moving at everyday speeds. Some contexts where it does not perfectly hold:
- Quantum mechanics – At subatomic scales, Newtonian mechanics breaks down and quantum effects dominate.
- Extremely high speeds – As objects approach the speed of light, relativistic effects modify the laws of motion.
- Gravitational fields – Strong gravitational fields like those near black holes can distort spacetime in ways that influence inertia.
However, for the macroscopic objects and motions we commonly encounter, Newton’s first law provides an accurate, elegant description of how objects persist in their state of motion or rest. It succeeds precisely because of its simplicity and general nature.
Conclusion
Newton’s first law of motion states that an object at rest will remain at rest, and an object in motion will continue moving at a constant velocity, unless acted on by an external unbalanced force. This law formalizes the concept of inertia – the tendency of objects to resist changes in motion.
Inertia comes from an object’s mass. The more mass, the more inertia it exhibits. Inertia depends on frame of reference – an object may be at rest or in motion depending on the perspective.
According to the first law, only external forces can change an object’s motion by causing acceleration. Friction, gravity, thrust, and other forces can overcome inertia. Newton’s first law successfully describes motion and rest in the everyday physical world.