A force is a push or pull arising from an interaction between objects. It has both magnitude and direction, and its SI unit is the newton (N). A spring balance measures force. Several forces may act on one object, but its acceleration depends on their combined effect, called the net force. Forces along the same direction add; opposite forces subtract, with the resultant directed towards the larger force. Balanced forces give zero net force, while unbalanced forces give a non-zero net force. Therefore, an applied force does not necessarily cause acceleration: another force may balance it.

Friction opposes relative sliding, or the tendency to slide, between surfaces in contact. It can slow a sliding object, but it can also push a walking person forwards. Newton’s first law states that zero net force leaves an object at rest or moving with unchanged velocity. This resistance to a change in motion is called inertia. Constant velocity means constant speed and direction, so acceleration is zero. Everyday moving objects often stop because friction acts on them, not because motion requires a continuous force. A driving force that balances resistance maintains steady motion rather than increasing speed.

Newton’s second law relates an object’s acceleration to the net force: F_net = ma, or a = F_net/m, for constant mass. Acceleration points in the direction of the net force, which need not be the direction of motion. For a fixed mass, doubling the net force doubles acceleration; for a fixed net force, doubling the mass halves acceleration. Mass is measured in kilograms and acceleration in m/s², so 1 N = 1 kg m/s². Near Earth’s surface, weight is the gravitational force mg, where g is approximately 9.8 m/s². Weight is a force, not another name for mass.

Newton’s third law states that when one object exerts a force on another, the second simultaneously exerts an equally strong force in the opposite direction on the first. These partners act on different objects, so they do not cancel when calculating either object’s net force. A paddle pushes water backwards, while water pushes the paddle forwards. A rocket similarly pushes exhaust gases backwards and receives a forward force; it does not need air to push against. Equal interaction forces need not produce equal accelerations because a = F_net/m also depends on mass and any other forces acting.

A system is a chosen group of objects studied together. Consider two boxes of masses m1 and m2 joined by a taut, light string on a frictionless horizontal surface. If an external horizontal force F pulls them, they share acceleration a = F/(m1 + m2). The string’s tension acts within the chosen system, so its opposing contributions cancel when calculating the system’s total net force. Weight and the surface’s supporting forces balance vertically. This model assumes negligible string mass and friction. Predictions apply under these assumptions; significant friction must be included as an external force rather than ignored.