FORCES

The “Four Fundamental Forces” Strong Force Electromagnetic Force Weak Force Gravitational Force

The fundamental forces in nature are all field forces The fundamental forces in nature are all field forces. This means that an object produces a field, which is an invisible influence that extends through space. A second object, then, interacts with this field to produce a force on the object.

Remember: By Newton’s third law of motion, objects exert equal and opposite forces on each other. So the second object has a field that interacts with the first object to produce a force on it also.

Strong Force Strongest force In the nuclei of atoms Tight binding of quarks to form neutrons and protons Binding of protons and neutrons (“nucleons”) to form nuclei Very short-range (negligible beyond about 10-15 m) Carried (mediated) by gluons

Electromagnetic Force Often called “Coulomb force” 1/100 the strength of strong force Strength varies with inverse square of distance between charges May be attractive or repulsive Carried (mediated) by photons Long range force

Weak Nuclear Force Carried (mediated) by W+ and Z bosons Short range force in the nucleus One-millionth the strength of strong force Produces instability in some nuclei to produce beta decay

Gravitational Force By far the weakest force Strength varies as inverse square of distance Interaction among all objects in the Universe Called “weight” for an object in the gravitational field of another object: W=mg Carried (mediated) by gravitons (yet undetected) Long range

Physicists now believe that the weak and electromagnetic forces may be two manifestations of the same force, called the “electroweak force”.

Common Force Examples

Normal Force Occurs at surface of contact between two solid objects Outward from surface of contact, perpendicular to surface Type of electromagnetic or Coulomb force Repulsive forces among outer electrons that resist permeation of their electrostatic fields Essentially an electrostatic force

Friction Force Acts along surface of contact, parallel to surface Acts on an object or system in the direction opposite its relative motion to another object or system For solids, proportional to normal force and coefficient of friction For a solid moving in a fluid, proportional to relative velocity Essentially an electrostatic force

Tension Force Occurs in a cord, string, cable, rope, etc. Acts only away from the object to which it is attached (i.e., “you can’t push with a rope”) Essentially an electrostatic force

Spring (or Elastic) Force May push or pull on an object The force is in the opposite direction of the displacement of the spring and proportional to the amount of displacement F = -kx