Isaac Newton's First and Second Laws of Motion

Newton's First Law of Motion

Principle: A resultant force is required to change the motion of an object.
- Applied to stationary objects: If there is no resultant force, the object remains stationary.
- Applied to moving objects: If the resultant force is zero, the object continues moving at the same velocity.

Principle: If a non-zero resultant force acts on an object, it will cause the object to accelerate.
- Non-zero force: Any force other than zero causes acceleration.
- Example: A particle with unbalanced force arrows pointing to the right.
  - Rightward acceleration could result in:
    1. Stationary object starts moving to the right.
    2. Object already moving to the right speeds up.
    3. Object moving to the left slows down.
    4. Slow leftward-moving object stops moving completely.
    5. Object changes direction without changing speed (due to change in velocity).

Example: Orbit of the moon around the Earth.
- Speed remains constant, but direction changes, resulting in constant acceleration.
- Gravitational pull from Earth acts perpendicular to moon’s motion.
Result: Moon has a constantly changing velocity and constant speed.

Proportionality: Size of resultant force is directly proportional to the acceleration it causes.
- Equation: F = ma (Force = Mass x Acceleration).
- Example Calculation:
  - Mass of particle = 0.25 kg.
  - Force to the left = 30 N; Force to the right = 42 N.
  - Resultant force = 42 N - 30 N = 12 N to the right.
  - Acceleration = Resultant force / Mass = 12 N / 0.25 kg = 48 m/s^2.

Definition: The tendency for the motion of an object to remain unchanged.
- Objects at rest stay at rest; objects in motion stay in motion unless acted on by a resultant force.
Inertial Mass: Measure of how difficult it is to change an object’s velocity.
- Found by dividing force by acceleration (F / a), same as Newton’s second law.
- Large mass (e.g., the moon): High inertia and requires a large force to change velocity.