Forces & Interactions
29 concepts · ordered by prerequisite depth
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 order: 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.
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Force
A push or pull interaction between two objects that can cause a change in an object's velocity (speed or direction), described as a vector quantity.
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Newton's Second Law
The acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass, with the acceleration pointing in the same direction as the net force.
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Free Body Diagram
A simplified diagram that isolates a single object and represents all external forces acting on it as labelled arrows originating from the object's centre of mass.
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Continue from here · 26 concepts
Mass
The amount of matter in an object and a fundamental measure of how much it resists changes to its state of motion (inertia).
Inertia
The intrinsic tendency of an object to resist any change in its state of motion, whether at rest or moving.
Momentum
The product of an object's mass and velocity, representing the quantity of motion it carries.
Centripetal Force
The net inward force required to keep an object moving along a circular path, directed toward the centre of the circle, equal to $mv^2/r$ where $m$ is the object's mass, $v$ its speed, and $r$ the radius of the circle.
Gravity
The universal attractive force between any two objects with mass, decreasing with the square of distance.
Impulse
The product of force and time interval, equal to the resulting change in an object's momentum.
Net Force
The single resultant force obtained by vector addition of all individual forces acting on an object, which alone determines the object's acceleration according to Newton's second law, $\vec{F}_{net} = m\vec{a}$.
Newton's First Law
An object at rest stays at rest, and an object in motion continues moving at constant velocity in a straight line, unless acted upon by a net external force.
Spring Force
The restoring force exerted by a spring, proportional to how much it's stretched or compressed.
Tension
The pulling force transmitted through a rope, string, or cable when it is pulled taut at both ends.
Torque
The rotational equivalent of force; a measure of how much a force tends to cause an object to rotate about an axis.
Weight
The gravitational force acting on an object due to its mass, directed toward the center of a massive body.
Angular Momentum
The rotational equivalent of linear momentum, measuring the quantity of rotational motion in a spinning or orbiting object.
Conservation of Momentum
In a closed system with no net external force, the total momentum of all objects remains constant before and after any interaction — momentum is conserved.
Equilibrium
A state in which all forces acting on an object balance so that the net force equals zero and there is no acceleration.
Newton's Third Law
For every action force, there is an equal in magnitude and opposite in direction reaction force.
Normal Force
The perpendicular contact force that a surface exerts on an object pressing against it, directed away from the surface.
Collisions
A collision is an interaction in which objects exert large forces on each other for a short time, changing their momenta.
Elastic Collision
A collision in which both the total momentum and the total kinetic energy of the system are fully conserved after impact.
Friction
A contact force that opposes the relative motion or tendency of motion between two surfaces in contact.
Inelastic Collision
A collision in which the total momentum of the system is conserved but the total kinetic energy is not — some kinetic energy is converted.
Pulley Systems
Pulley systems are arrangements of ropes and wheels used to change the direction of a force or to gain mechanical advantage.
Rotational Equilibrium
Rotational equilibrium is the condition in which the net torque on an object is zero, so its angular velocity does not change.
Statics
Statics is the study of objects in equilibrium, where the net force and net torque are both zero.
Kinetic Friction
The friction force that opposes sliding motion between two surfaces in contact; in the simple dry-friction model its magnitude is $f_k = \mu_k N$, directed opposite to the motion.
Static Friction
The friction force that prevents a stationary object from beginning to slide when an external force is applied, adjusting in magnitude up to a maximum.