Definition: Non-contact forces act over a distance without the objects needing to touch. These forces are mediated by fields, which are regions of space where an object experiences a force.
Gravitational Force: This is an attractive force that acts between any two objects with mass. On a planetary scale, this is experienced as weight, pulling objects toward the center of the planet.
Magnetic Force: This force acts between magnetic poles or moving charges. It can be either attractive (opposite poles) or repulsive (like poles) and is responsible for the behavior of compasses and electric motors.
Electrostatic Force: This is the force between stationary electrically charged objects. Like charges repel each other, while opposite charges attract, following the principle that electric fields exert forces on charges within them.
Mass vs. Weight: Mass is a scalar quantity representing the amount of matter in an object (measured in kg) and remains constant regardless of Weight is a vector quantity representing the force of gravity on that mass (measured in N) and changes depending on the local gravitational field.
The Weight Formula: The weight of an object is directly proportional to its mass and the local gravitational field strength (). This is expressed by the equation: where is weight in Newtons, is mass in kilograms, and is the gravitational field strength in N/kg.
Centre of Mass: For the purpose of force analysis, the weight of an object is considered to act through a single point called the centre of mass. In uniform, symmetrical objects, this point is located at the geometric center.
Resultant Force: The resultant (or net) force is the single overall force that represents the combined effect of all individual forces acting on an object. It is calculated by adding forces in the same direction and subtracting those in opposite directions.
Balanced Forces (Equilibrium): When the resultant force is zero, the forces are said to be balanced. An object in this state will either remain stationary or continue moving at a constant velocity, as there is no net force to change its motion.
Unbalanced Forces: If the forces do not cancel out, a non-zero resultant force exists. This unbalanced force causes the object to accelerate, decelerate, or change direction in the direction of the resultant vector.
| Feature | Contact Force | Non-Contact Force |
|---|---|---|
| Physical Contact | Required | Not required (acts through a field) |
| Examples | Friction, Tension, Normal Force | Gravity, Magnetism, Electrostatic |
| Medium | Direct physical interface | Gravitational, Magnetic, or Electric fields |
| Distance | Zero distance required | Can act over very large distances |
Vector Direction: Always specify the direction of a force in your answers (e.g., '5 N to the left'). Forgetting the direction for a vector quantity is a common way to lose marks.
Unit Conversion: Ensure mass is in kilograms (kg) before using the weight formula. If a mass is given in grams, divide by 1000 first ().
Free Body Diagrams: When drawing these, represent the object as a simple dot or box and draw all force arrows starting from the centre of mass, pointing away from the object.
Terminology Precision: Avoid using the word 'gravity' when you mean 'weight' or 'gravitational field strength'. 'Gravity' is a general phenomenon, while 'Weight' is the specific force you are usually calculating.
