Newton's Laws of Motion: First and Second Law

Newton’s Laws of Motion: First and Second Law

Newton’s First Law of Motion

  • Newton’s First Law of Motion, also known as the Law of Inertia, states that an object will remain at rest or move in a straight line at constant speed unless it is acted upon by a net external force.

  • Inertia is the tendency of an object to resist changes in its state of motion. The inertia of an object is directly proportional to its mass - the greater the mass, the greater the inertia.

  • This law explains that it is the nature of all objects to continue doing what they are already doing (whether that’s moving or being at rest) unless a force steps in to make them do otherwise.

  • This law helps us understand why seatbelts are important in cars. When a car stops suddenly, the body wants to keep moving due to inertia, but the seatbelt provides a force that stops the body.

  • Objects slide off a dashboard when a car stops quickly because those objects want to keep moving forward (again, due to inertia), and there is no force to stop them (unless they hit something, like the windscreen).

Newton’s Second Law of Motion

  • Newton’s Second Law of Motion describes the relationship between force, mass, and acceleration. It states that the acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass.

  • This law is often summarised by the equation: F = ma where:
    • F = Total force applied to the object,
    • m = Mass of the object,
    • a = Acceleration of the object.
  • If the mass is constant, the force applied to an object equals mass times acceleration. This means that for a given mass, the acceleration of the object is directly related to the force applied.

  • If the force being applied is held constant, then the acceleration is inversely proportional to the mass. So, the greater the mass of the object, the less it will accelerate under the action of the same force.

  • An illustration of this law is pushing different objects. If you push a full shopping trolley and an empty one with the same force, the full one (greater mass) will accelerate more slowly than the empty one (smaller mass).

  • In conclusion, an object’s acceleration depends on two things: how much force is applied to it and how much mass it has to be moved.

  • The greater the force applied to an object, the greater its acceleration will be. Conversely, the greater an object’s mass, the smaller its acceleration will be when a force is applied.