Forces & Interactions
25 concepts in Physics
Forces and interactions describe how objects push, pull, and influence one another. This topic is grounded in Newton's three laws of motion, which explain why objects speed up, slow down, change direction, or remain at rest. Students learn to identify and represent forces such as gravity, friction, normal force, tension, and applied force using free-body diagrams. They study net force and equilibrium, discovering that an object's motion changes only when unbalanced forces act on it. The concepts extend to gravitational and electromagnetic interactions at a distance, as well as pressure and buoyancy in fluids. Understanding forces is essential for engineering, sports science, vehicle safety, and architecture. It also provides the conceptual foundation for more advanced physics topics like momentum, circular motion, and field theory.
Suggested learning path: Begin with identifying forces and drawing free-body diagrams, then study Newton's laws and their applications, and progress to friction, gravity, and interactions at a distance.
Force
A push or pull acting between two objects that can change an object's motion, speed, or shape.
Mass
The amount of matter in an object; a measure of how much it resists acceleration.
Weight
The gravitational force acting on an object due to its mass, directed toward the center of a massive body.
Newton's First Law
An object at rest stays at rest, and an object in motion stays in motion at constant velocity, unless acted on by a net force.
Newton's Second Law
The acceleration of an object is equal to the net force applied divided by its mass, in the direction of the force.
Newton's Third Law
For every action force, there is an equal in magnitude and opposite in direction reaction force.
Inertia
The intrinsic tendency of an object to resist any change in its state of motion, whether at rest or moving.
Friction
A contact force that opposes the relative motion or tendency of motion between two surfaces in contact.
Normal Force
The perpendicular contact force that a surface exerts on an object pressing against it, directed away from the surface.
Gravity
The universal attractive force between any two objects with mass, decreasing with the square of distance.
Net Force
The single resultant force obtained by adding all individual force vectors acting on an object, accounting for direction.
Equilibrium
A state in which all forces acting on an object balance so that the net force equals zero and there is no acceleration.
Tension
The pulling force transmitted through a rope, string, or cable when it is pulled taut at both ends.
Free Body Diagram
A diagram that represents a single object and shows all external forces acting on it as labeled arrows.
Momentum
The product of an object's mass and velocity, representing the quantity of motion it carries.
Impulse
The product of force and time interval, equal to the resulting change in an object's momentum.
Conservation of Momentum
In a closed system with no external forces, total momentum before = total momentum after.
Centripetal Force
The net force that keeps an object moving in a circle, directed toward the center.
Torque
The rotational equivalent of force; a measure of how much a force tends to cause an object to rotate about an axis.
Spring Force
The restoring force exerted by a spring, proportional to how much it's stretched or compressed.
Kinetic Friction
The friction force acting on an object that is already sliding across a surface.
Static Friction
The friction force that prevents a stationary object from beginning to slide when an external force is applied to it.
Elastic Collision
A collision in which both the total momentum and the total kinetic energy of the system are fully conserved after impact.
Inelastic Collision
A collision in which momentum is conserved but kinetic energy is not — some energy is lost to heat, sound, or deformation.
Angular Momentum
The rotational equivalent of linear momentum — a measure of how much rotational motion an object has.