1. FORCES

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

3. 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.

4. 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.

5. 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 m)
Carried (mediated) by gluons

6. 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

7. 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

8. 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

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

10. Common Force Examples

11. 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

12. 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

13. 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

14. 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