Equal and opposite forces cancel each other out, resulting in no net force acting on the object. This concept is essential for understanding equilibrium in physics.

Gravitational force is the attractive interaction between masses. It is one of the fundamental forces in nature and acts on every object with mass.

A free-body diagram illustrates all external forces acting on an individual object. This visual tool helps in analyzing the net force and predicting the object's motion.

Friction is the resistive force that opposes the relative motion between surfaces in contact. It is a key concept in understanding how objects start or stop moving.

Gravity is the force that pulls objects toward the center of the Earth, keeping us grounded. This force is one of the most fundamental concepts in physics.

An object in equilibrium has all the forces balanced, which means the net force acting on it is zero. This is a direct application of Newton's First Law.

Gravitational force is directly proportional to the product of the masses involved. When mass increases while distance remains constant, the gravitational force increases accordingly.

Newton's Second Law establishes the relationship between net force, mass, and acceleration (F = m * a). This law is fundamental in linking how forces affect motion.

The arrow length in a free-body diagram reflects the magnitude of the corresponding force acting on the object. Longer arrows denote larger forces.

In basic models of friction, the frictional force is determined by the normal force and the coefficient of friction, not by the area of contact between surfaces. This counterintuitive result highlights how friction is modeled in introductory physics.

Kinetic friction acts on objects that are already in motion, opposing the sliding motion. It differs from static friction, which prevents motion from commencing.

Newton's Third Law explains that forces always occur in pairs. When one object exerts a force on another, the second object exerts an equal and opposite force back.

Constant velocity indicates that there is no acceleration, which according to Newton's First Law means the net force is zero. This serves as a key indicator of balanced forces.

The frictional force is directly proportional to the normal force (f = μN). Therefore, an increase in the normal force results in a corresponding increase in friction.

The system's total mass is 3 kg + 6 kg = 9 kg, so its acceleration is F/9. The tension is the force required to accelerate the 6 kg block, which is 6*(F/9) or (6/9)*F.

Subtracting the frictional force from the applied force gives a net force of 5 N (20 N - 15 N). Dividing this net force by the mass (5 kg) using F = m*a gives an acceleration of 1 m/s².

The total mass of the system is 9 kg, which gives an acceleration of 9 N / 9 kg = 1 m/s². The tension required to accelerate the 6 kg block is 6 kg * 1 m/s², resulting in 6 N.

On an inclined plane, gravity can be decomposed into two components: one parallel and one perpendicular to the surface. Static friction directly opposes the parallel component that would cause the block to slide.

Static friction is a self-adjusting force that matches the applied force up to a limit. It prevents motion until the force exceeds the maximum static friction value.

Centripetal force is the net inward force that constantly changes the direction of the moving car, keeping it on a circular path. Even though the car's speed remains constant, this inward force is essential for circular motion.