Singapore H2 Physics Wiki

Force Types and Interactions

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Force Types and Interactions

Why It Matters

Force analysis begins by identifying real interactions acting on the chosen body. Most errors in force problems come from adding a non-existent “motion force” or omitting a real contact or field force.

Definition

A force is an interaction that can change motion or deform a body. In JC Physics, forces are usually classified as contact forces or field forces.

Key Representations

Near Earth’s surface, weight is:

For limiting static friction or kinetic friction:

For a charge in an electric field:

For a current-carrying conductor in a magnetic field:

For a moving charge in a magnetic field:

Contact Forces

Contact forces arise when bodies touch, or when a body interacts with a surrounding fluid.

Normal Contact Force

The normal contact force is exerted by a surface on a body in contact with it. It acts perpendicular to the surface at the point of contact. It is not always equal to ; its magnitude is determined by force balance perpendicular to the surface.

Friction

Friction acts along the surface of contact and opposes relative motion or the tendency of relative motion. Static friction adjusts up to a limiting value, while kinetic friction acts when surfaces slide relative to each other.

Tension

Tension is a pulling force exerted by a string, rope, or cable. It acts along the string and pulls away from the body. Equal tension throughout a string requires the ideal model of a light inextensible string over a smooth pulley.

Fluid Contact Forces

Drag and upthrust are also contact forces, but they are better treated together with pressure and fluid motion in Fluid Forces and Resistive Motion.

Field Forces

Field forces act without direct physical contact.

Gravitational Force

Gravitational force acts on mass. Near Earth’s surface it is called weight and acts in the direction of the local gravitational field.

Electric Force

Electric force acts on charge. A positive charge experiences force in the direction of the electric field, while a negative charge experiences force opposite to the electric field.

Magnetic Force

Magnetic force acts on moving charges and current-carrying conductors. It is perpendicular to the relevant velocity or current direction and the magnetic field direction.

Common Exam Points

  • A force must have a real source: another body, a surface, a string, a fluid, or a field.
  • The resultant force is not drawn as an extra force on a free-body diagram.
  • Friction does not always oppose motion; it opposes relative motion or impending relative motion at the contact.
  • Normal contact force is not automatically equal to weight.
  • Field forces are real forces and should be included when relevant.

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