All the different forces observed in nature can be explained in terms of four basic interactions that occur between elementary particles:
1. The gravitational force
2. The electromagnetic force
3. The strong nuclear force
4. The weak nuclear force
The everyday forces that we observe between macroscopic objects are due to either the gravitational force or the electromagnetic force.
Forces may be placed into two broad categories, based on whether the force resulted from the contact or non-contact of the two interacting objects.
Action at a distance:
The fundamental forces of gravity and electromagnetism act between particles that are separated in space. This creates a philosophical problem referred to as action at a distance.
Contact forces:
Many forces we encounter are exerted by objects in direct contact. These forces are electromagnetic in origin and are exerted between the molecules of each object.
Normal force:
Consider a book on a table. The weight of the book pulls it downward, pressing it against the molecules in the table’s surface, which resist compression and exert a force upward on the book. Such a force, perpendicular to the surface, is called a normal force.
Frictional force:
Objects in contact can also exert forces on each other that are parallel to the surfaces in contact. The parallel component of a contact force is called a frictional force.
Static friction:
Friction is a complicated, incompletely understood phenomenon that arises due to the bonding of molecules between two surfaces that are in close contact. This bonding is the same as the molecular bonding that holds an object together. When you apply a small horizontal force to a large box resting on the floor, the box may not move because of the force of static friction,
exerted by the floor on the box, balances the force you are applying. The force of static friction, which opposes the applied force, can adjust from zero to some maximum force f s, max depending on how hard you push. You might expect f s, max to be proportional to the area of contact between the two surfaces, but this is not the case. To a good approximation, f s, max is independent of the area of contact and is simply proportional to the normal force exerted by one surface on the other:
exerted by the floor on the box, balances the force you are applying. The force of static friction, which opposes the applied force, can adjust from zero to some maximum force f s, max depending on how hard you push. You might expect f s, max to be proportional to the area of contact between the two surfaces, but this is not the case. To a good approximation, f s, max is independent of the area of contact and is simply proportional to the normal force exerted by one surface on the other:f s, max = m s Fn
where, ms is called the coefficient of static friction, a dimensionless quantity that depends on the nature of the surfaces in contact. If you exert a horizontal force smaller than f s, max on the box, the frictional force will just balance this horizontal force. In general, we can write
f s £ m s Fn
Kinetic friction:
If you push the box hard enough, it will slide across the floor. When the box is sliding, molecular bonds are continually being formed and ruptured, and small pieces of the surfaces are being broken off. The result is a force of kinetic friction,
that opposes the motion. To keep the box sliding with constant velocity, you must exert a force on the box that is equal in magnitude and opposite in direction to the force of kinetic friction exerted by the floor.
that opposes the motion. To keep the box sliding with constant velocity, you must exert a force on the box that is equal in magnitude and opposite in direction to the force of kinetic friction exerted by the floor.The coefficient of kinetic friction m k is defined as the ratio of magnitudes of the kinetic frictional force f k and the normal force Fn:
f k =m k Fn
where m k depends on the nature of the surfaces in contact. Experimentally, it is found that m k is less than m s and is approximately constant for speeds ranging from about 1 cm/s to several meters per second.
The plot of the frictional force vs. the applied force illustrates some of the features of the frictional force. Note that the frictional force equals the applied force (in magnitude) until it reaches the maximum possible value µsN. Then the object begins to move as the applied force exceeds the maximum frictional force. When the object is moving the frictional force is kinetic and roughly constant at the value µkN which is below the maximum static friction force.Examples of kinetic friction:
¨ Sliding friction is when two objects are rubbing against each other. Putting a book flat on a desk and moving it around is an example of sliding friction.
¨ Rolling friction occurs when the two objects are moving relative to each other and one "rolls" on the other (like a car's wheels on the ground). The coefficient of rolling friction is typically denoted as μ r.
¨ Fluid friction is the friction between a solid object as it moves through a liquid or a gas. The drag of air on an airplane or of water on a swimmer are two examples of fluid friction.
| Contact Forces | Action-at-a-Distance Forces |
| Frictional Force | Gravitational Force |
| Tensional Force | Electrical Force |
| Normal Force | Magnetic Force |
| Air Resistance Force | |
| Applied Force | |
| Spring Force | |
| |