Newton's First Law

Newton's First Law, also known as the Law of Inertia, is a fundamental principle that governs how objects move. It's a simple but powerful concept: an object at rest stays at rest, and an object in motion stays in motion unless acted upon by an external force. This law underlies everyday experiences, from a passenger's lurch in a stopping car to a stationary book remaining unmoved without a push. 

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    Newton's First Law, although having no mathematical formula, sets the stage for understanding the relationship between force and motion. Grasping this law is the first step towards decoding the mysteries of physics and the world around us.

    Newton's first law: definition

    Newton's first law of motion states that a body will stay at rest or keep moving at a constant velocity unless acted upon by an unbalanced force. The first part seems to make sense since objects at rest typically do not just start moving. However, the second part appears to contradict our everyday experiences.

    When a ball is thrown, it does not keep moving forever. It eventually stops. For a car to move, for the ball to stop, or for a plane to stay in the air, a constant force must be applied. To understand Newton's first law of motion, you need to understand the forces that act on a body in motion.

    Newton's first law, An example showing a stationary body at rest next to another example with a body being moved by arrows to show velocity moving the body,StudySmarter

    Fig. 1 - Demonstration of Newton's first law by showing the forces that act on a body in motion.

    Newton's first law: examples

    Some examples of Newton's first law in effect are:

    1. When a car suddenly stops, passengers continue moving forward. This is because their bodies, in motion when the car was moving, want to stay in motion due to inertia.

    2. When you are standing on a bus, you tend to lean or fall backward when the bus starts moving forward. This is because your body was at rest and wants to stay at rest due to inertia.

    3. A book resting on a table will stay at rest unless an external force, such as a push or the wind, causes it to move.

    Newton's first law: formula

    Newton's first law doesn't have a formula in the traditional sense as it is a qualitative rather than a quantitative statement. However, it lays the foundation for the second law. If the net external force (F) on an object is zero, then its acceleration (a) is also zero. This can be expressed as:

    \[\text{If}\, F=0, \text{then}\, a=0\]

    Newton's first law in everyday life

    In everyday life, we (and other objects) experience resistive forces, whether they be gravity, friction, etc. These are what slow us down, requiring the application of constant force to keep moving.

    Think of something like an ice skating rink, which is a surface with very little friction. If you push a hockey puck, it will glide much farther than it would on a table (where there is more friction). However, there still is some friction present in its interaction with the ice, so it eventually stops. Newton's first law proposes that if you pushed this same hockey puck on a surface completely free of fiction, it would retain the exact velocity it had when it left the stick, and it would keep gliding forever unless some other forces were to act on it.

    Inertia

    Inertia is a quantity that relates to the willingness of objects to change the state of their motion.

    Inertia is the natural tendency of an object to remain at rest or in motion at a constant velocity along a straight line.

    Newton's First Law, A graph showing Inertia's affect on a moving object, StudySmarterFig. 2 Inertia is the tendency of an object to maintain its current state—either at rest or in motion—unless acted upon by an external force.

    An object will have inertia proportional to its mass, measured in kilograms. This means the more the mass of an object, the greater its inertia. In this way, mass is the quantitative measurement of an object's inertia, whether at rest or in motion.

    So Newton's first law can be summarised as the law of inertia. In the absence of a net force, an object will remain in its state of motion, whether at rest or moving with constant velocity. The sum of multiple forces acting on an object is called the net force. If the net force is zero, there will be no acceleration. This can be modelled mathematically as \(F = 0\).

    We are only going to deal with coordinate systems that do not accelerate. They are called inertial reference frames. They are either at rest or moving with constant velocity (accelerating reference frames). A good example of inertia at work is when passengers fall forward when a fast-moving bus stops suddenly.

    What is the importance of Newton's first law?

    As discussed earlier, Newton's first law can be considered as the law of inertia. This is very significant – it is what holds the universe together. If inertia were to be turned off, everything would fall apart.

    Remember that net force is the addition of all the forces that act on an object. All the forces considered here exist on Earth. It is quite different in space. Since there is almost no other force acting on bodies in space, they keep moving and could be moving forever. So Newton's first law gives us an understanding of the movements of bodies in space.

    This same law makes it possible for us to develop cars by knowing how much force it will take to get the car to move and how much it will need to stop it. The heavier the car, the more force it will require. This isn't just the case with cars but ships, bicycles, machines, rockets, etc.

    Newton's First Law - Key takeaways

    • Newton's first law of motion states that a body will stay at a constant velocity unless acted upon by an external force.
    • Intertia is the natural tendency of an object to remain at rest or in motion at a constant velocity along a straight line.
    • An object is compelled to stay in motion till it encounters a net force.
    • Newton's first law can be summarised as the law of inertia.
    • There will be no acceleration if the net force is 0.
    • An object's inertia is proportional to its mass.
    Frequently Asked Questions about Newton's First Law

    What is Newton's first law?

    Newton's first law of motion states that a body will stay at a constant velocity unless acted upon by an external force.

    How can you demonstrate Newton's first law?

    Driving a car and then braking suddenly will stop the car, but the people inside will keep moving until they are pulled back by their seatbelts.

    What is Newton's first second and third law of motion?

    Newton's second law of motion states that the time rate of change of the momentum of a body is equal in both magnitude and direction to the force imposed on it. 

    Newton's third law of motion states that when two bodies interact, they apply forces to one another that are equal in magnitude and opposite in direction.

    How does Newton's first law apply to a rocket launch?

    A rocket will remain motionless on a launchpad till a force is exerted for it to move upward.

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