4.2 (Part 2): Drag, Thrust, Electric and Magnetic Forces

Drag

• Definition: A resistant force experienced by objects moving through fluids (gases like air, liquids like water).

• Symbol: \vec{D}

• Direction: Points opposite the direction of motion.

• Significance: Can be large for objects at high speeds or in dense fluids; dominant for small particles in water. Generally negligible in air for heavy, compact objects at moderate speeds, unless specified in problems.

Thrust

• Definition: A force that propels objects forward, occurring when an engine expels gas molecules at high speeds.

• Agent: Exhaust gas molecules pushing on the engine (a contact force).

• Direction: Opposite to the direction in which the exhaust gas is expelled.

• Symbol: No special symbol, can be denoted as \vec{F}_{thrust}. Used by rockets, jet engines, and some animals (e.g., squids).

Electric and Magnetic Forces

• Nature: Long-range forces, similar to gravity.

• Action: Act on charged particles.

• Relevance: Will be studied in detail later; generally not significant for dynamic problems in early chapters.