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Understanding Forces: Fundamentals, Convenience, and Vector Analysis

This article explores the concept of forces, both fundamental and convenient, and highlights the importance of vector analysis in understanding their magnitude and direction. It covers fundamental forces such as gravity and electromagnetism, as well as convenient forces like tension, spring force, normal force, friction, drag, and thrust.

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Understanding Forces: Fundamentals, Convenience, and Vector Analysis

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  1. Outline Force • Force as a Vector • Real Forces versus Convenience • The System

  2. Force as a Vector • Forces are vectors (magnitude and direction) • It is typical that multiple forces act on an object simultaneously • In such cases, use vector addition

  3. Fundamental forces • Gravity • - the force between masses • - holds planets and stars together, makes things fall, etc… • Electromagnetism • - the force between charges • - responsible for all familiar “everyday” forces (except gravity) • “Weak Nuclear Forces” • -changes one ‘flavour’ of quark into another, radioactive decay • “Strong Nuclear Forces” • - holds atomic nuclei together

  4. Chart: Fundamental forces Electromagnetic and weak are actually just the electroweak force (1980s) Weak force was discovered because of Beta decay

  5. Forces of Convenience • Tension Force – string, rope, wire… • Spring Force – elasticity returns an object to its original shape • Normal Force – not that we’re calling other forces abnormal • Friction – when one surface moves with respect to another • Drag – air resistance, fluid resistance • Thrust – what makes a rocket move? These forces aren’t fundamental, but they are a convenient way to visualise the world around us

  6. Tension Force • Elastic (or inelastic) deformation of molecular bonds • Really just the electromagnetic force • Can only pull Demo: pull something with a licorice string. Inelastic, but still pulls somewhat Demo: try to push something with a rope

  7. Spring Force • Can push or pull • Elastic (or inelastic) deformation of molecular bonds • Really just the electromagnetic force Demo: show elastic and inelastic deformation (plywood)

  8. Normal Force • Normal means orthogonal (orthogonal means at right angles) • Results from compression of molecular bonds • Can only push • Really just the electromagnetic force Demo: trampoline deformation

  9. If we look closely, the normal force arises from the table being bent : as the table tries to straighten, it pushes back. This is really an elastic force; the table behaves like a spring. At the atomic level, all contact forces are due to electromagnetic forces.

  10. Friction Force • Acts to reduce the relative motion of two surfaces • Results from compression of molecular bonds • Static and Kinetic forms, but still the same concept • Really just the electromagnetic force • Can push or pull

  11. Drag Force • The friction experienced when moving through a fluid • You are currently in a fluid Demo: rip a page out of a physics text book. Which will drop faster? Then put them one on top of the other and drop.

  12. Thrust Force • Caused by expelled exhaust gasses • Best understood after learning about momentum Demo: inflate a balloon and then let it go.

  13. Setting up Force Problems • Very important to correctly identify the system you are considering • Imagine a closed surface around that object • All forces in contact with that surface are forces that act on that object • Remember to count long-range forces as well

  14. QQ34: System Example: Identify all of the forces that are acting on a box moving over a rough surface. Demo: box of magnets gets stuck to a vertical surface

  15. Do for next class: • Read: Sections 4.4, 4.5 • Suggested problems: 4.6, 4.8, 4.9, 4.10

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