Physics 2- Chapter21

Forces in Nature

• Definition of forces in nature:

  • Forces are interactions that cause changes in motion.

Types of Forces

• Gravitational Force

  • Defined by Newton's Law of Gravitation.

  • Any two masses experience a force of attraction.

  • Expressed mathematically as:

  • $F = G \frac{m<em>1 m</em>2}{r^2}$

    • where:

    • $F$ = gravitational force

    • $G$ = gravitational constant

    • $m<em>1, m</em>2$ = masses

    • $r$ = distance between masses

• Mass vs Weight:

  • Mass: A scalar quantity, measured in kilograms (kg).

  • Weight: A vector quantity (force), measured in Newtons (N), and defined as the force exerted by gravity on a mass.

  • The distinction:

  • Weight changes with gravitational acceleration (different on Earth vs Mars), but mass remains constant.

• Electrical Force:

  • Interactions between charged particles (can be attractive or repulsive).

  • Identified through phenomena such as static electricity (e.g., clothes sticking together after dryer).

  • Not a gravitational force because the direction of the force is not always downward.

Fundamental Forces in Physics

• There are four fundamental forces:

  1. Gravitational Force

  2. Electrical Force

  3. Strong Nuclear Force

  4. Weak Nuclear Force (e.g., beta decay)

Historical Perspective on Electrical Forces

• Benjamin Franklin:

  • First to study electric forces: conducted experiments identifying positive and negative charges using charged objects (e.g., testing glass and plastic rods).

  • Observed interactions of charges: like charges repel, unlike charges attract.

Atomic Structure and Charge

• Atoms:

  • Basic units of matter.

  • Composed of:

  • Nucleus: Contains positive charges called protons.

  • Electrons: Negatively charged particles that orbit the nucleus.

• Charge Values:

  • Proton: approximately $1.6 \times 10^{-19}$ Coulombs (C)

  • Electron: approximately $-1.6 \times 10^{-19}$ Coulombs (C)

Coulomb's Law

• Governs electrical forces between charged bodies:

  • Expressed as:

  • $F = k \frac{q<em>1 q</em>2}{r^2}$

    • where:

    • $F$ = electrical force

    • $k$ = electrostatic constant (approximately $8.99 \times 10^9 \, N \, m^2/C^2$)

    • $q<em>1, q</em>2$ = magnitudes of the charges

    • $r$ = distance between the charges

Concepts of Electricity

• Electricity:

  • Defined as the movement of charged particles (usually electrons).

  • Common sources: batteries, solar winds, and lightning.

• Conductors and Insulators:

  • Conductors: Materials that allow the flow of electrical current (e.g., metals).

  • Insulators: Materials that restrict the movement of electrical charges (e.g., rubber, glass, pure water).

• Solar Wind:

  • Flare of charged particles emitted from the sun, creating auroras on Earth.

Example Problems and Applications

1. Example of calculating electric forces using Coulomb's law with protons and electrons.

   • Given two protons and an electron separated by a certain distance: calculate the forces acting on them, their magnitude, and direction.

     • Direction Determination:

     • Attraction between opposite charges, and repulsion between like charges.

     • Net force calculations require considering vector components (x and y directions).

2. Discussing gravitational vs electrostatic forces:

   • Understanding the differences in their behaviors in various scenarios (i.e., two masses on Earth vs. other celestial bodies, or static charges in typical environments).

3. Calculation of net forces with resultant vectors:

   • Given charges at different angles, resolving into x and y components:

     • $F<em>{net} = \sqrt{(F</em>{x})^2 + (F_{y})^2}$

   • Finding angles via tangent ratios:

     • $\theta = \tan^{-1}\left(\frac{F<em>y}{F</em>x}\right)$

Closing Remarks

• Homework Tasks:

  • Assigned to practice concepts learned in this session:

  • Problems from the textbook on forces, Coulomb's law, and electrical effects to reinforce understanding.