Newton's First and Second Laws of Physics

- SCIENCE 9

- TERM 1 – WEEK 1 & 2 - LESSON 1 & 2

Newton's First Law: Inertia

  • An object remains at rest unless acted upon by a force.

  • No Force = No Motion.

  • Force: A push or pull that can start motion, stop motion, or change direction.

Mass and Inertia Relationships

  • Inertia depends on mass.

  • Large mass = Greater inertia.

  • Small mass = Less inertia.

  • Example: A moving table or a table not moving unless pushed/moved.

Net Force and Balanced Forces

  • Net force (FnetF_{net}): The sum of all forces acting on an object; it decides acceleration and direction.

  • Note: The bigger force determines the direction.

  • Balanced forces: When forces are equal and opposite, they balance and keep things steady.

  • Unbalanced forces: When one force is stronger than the other, or forces acting on an object are not equal.

Newton's Second Law: Force, Mass, and Acceleration

  • This law relates how force, mass, and acceleration relate and affect motion.

  • Force makes objects accelerate.

  • More force = More acceleration.

  • More mass = Less acceleration.

Formulas and Units

  • Formulas derived from the pyramid (FF over m×am \times a):

    • F=m×aF = m \times a

    • M=FaM = \frac{F}{a}

    • A=FmA = \frac{F}{m}

  • Units:

    • Acceleration (AA): m/s2\text{m/s}^2 (meters per second squared).

    • Force (FF): N\text{N} (Newtons).

    • Mass (MM): kg\text{kg} (kilograms).

  • Solving Structure:

    1. Given

    2. Formula

    3. Substitute/solve

    4. Box the final answer

Net Force Calculation Rules

  • Same direction: Add (e.g., 5 N→+7 N→=12 N5\,\text{N} \rightarrow + 7\,\text{N} \rightarrow = 12\,\text{N}).

  • Opposite direction: Subtract (e.g., 10 N10\,\text{N} to the left and 12 N12\,\text{N} to the right = 2 N2\,\text{N}).

  • Equal and opposite: 0 N0\,\text{N} (e.g., 5 N+5 N=0 N5\,\text{N} + 5\,\text{N} = 0\,\text{N}).

  • Example Calculation: (+3)+(−2)=+1 N(+3) + (-2) = +1\,\text{N}.

  • Direction and Signs:

    • Bigger force wins in direction.

    • ++ = Right.

    • −- = Left.

  • Specific transcript examples: M=10 kgM = 10\,\text{kg}, F=30 NF = 30\,\text{N}, Fnet=4 NF_{net} = 4\,\text{N}, 5 N+6 N→=11 N5\,\text{N} + 6\,\text{N} \rightarrow = 11\,\text{N}.