Dynamic and Static Forces


Objects will interact by exerting forces on each other. Forces act as pushes or pulls on objects.

Forces fit into two broad categories:

  • Contact – Forces that act directly on an object only when it is in contact with another object.
  • Field – Forces that act on objects from a distance. Field forces are sometimes referred to as non-contact forces.

A range of forces are categorised into these two categories:

Contact Field
Frictional forces

Air resistance

Tension

Applied forces

Normal forces

Forces in springs

Gravitational

Electric

Magnetic

Description of Forces

Friction: The friction force is the force exerted by a surface as an object moves across it or makes an effort to move across it. The friction force often opposes the motion of an object. For example, if a book slides across the surface of a desk, then the desk exerts a friction force in the opposite direction of its motion. 

Air resistance: Air resistance is a special type of frictional force that acts upon objects as they travel through the air. The force of air resistance is often observed to oppose the motion of an object. This force will frequently be neglected due to its negligible magnitude (and due to the fact that it is mathematically difficult to predict its value). It is most noticeable for objects that travel at high speeds (e.g., a skydiver or a downhill skier) or for objects with large surface areas.

Tension: The tension force is the force that is transmitted through a string, rope, cable or wire when it is pulled tight by forces acting from opposite ends. The tension force is directed along the length of the wire and pulls equally on the objects on the opposite ends of the wire.

Applied forces: An applied force is a force that is applied to an object by a person or another object. If a person is pushing a desk across the room, then there is an applied force acting upon the object. The applied force is the force exerted on the desk by the person.

Normal forces: The normal force is the support force exerted upon an object that is in contact with another stable object. For example, if a book is resting upon a surface, then the surface is exerting an upward force upon the book in order to support the weight of the book. On occasions, a normal force is exerted horizontally between two objects that are in contact with each other. For instance, if a person leans against a wall, the wall pushes horizontally on the person.

Forces in springs: The spring force is the force exerted by a compressed or stretched spring upon any object that is attached to it. An object that compresses or stretches a spring is always acted upon by a force that restores the object to its rest or equilibrium position. For most springs (specifically, for those that are said to obey “Hooke’s Law”), the magnitude of the force is directly proportional to the amount of stretch or compression of the spring.

Gravitational force: The force of gravity is the force with which the earth, moon, or other massively large object attracts another object towards itself. By definition, this is the weight of the object. All objects upon earth experience a force of gravity that is directed “downward” towards the centre of the earth. The force of gravity on earth is always equal to the weight of the object.

Electric force: The electric force is the force caused by the interaction of electric charges. These forces can be attractive or repulsive depending on the type of charges involved in the interaction. Two positive or two negative charges will repel each other and a positive and a negative charge will be attracted to each other.

Magnetic force: A magnetic force is a force that is created by moving electric charges (electric currents) and magnetic dipoles, and exerts a force on other nearby moving charges and magnetic dipoles. At any given point, it has a direction and a magnitude (or strength). 

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