What is force?

Force describes an interaction between objects that can change motion or shape. It is a vector, possessing direction as well as magnitude, and its SI unit is the newton. Several forces may act on one object; their vector sum, the net force, determines how its motion changes.

How do Newton’s laws describe force?

1. Inertia With zero net force, an object in an inertial reference frame remains at rest or continues in a straight line at constant velocity. Maintaining motion does not necessarily require a forward net force. Friction and air resistance commonly slow everyday objects. 2. Mass and acceleration For constant mass, net force equals mass multiplied by acceleration: F = m × a. A larger net force gives the same mass a larger acceleration. The same force accelerates a larger mass less. This relation applies to the total effect, not an arbitrarily selected individual force. 3. Action and reaction When one object exerts a force on another, the other exerts an equally large, opposite force on it. These act on different objects, so they do not cancel in the force sum for one object. During walking, a foot pushes the ground backward and the ground pushes the person forward.

How are forces analyzed?

4. Contact and field forces Friction, normal force and tension involve contact. Gravity and electrical interactions are described with fields. Normal force need not equal weight, as an inclined surface or accelerating elevator illustrates. 5. Mass and weight Mass is measured in kilograms. Weight is the force exerted by the local gravitational field and is measured in newtons. An object weighs differently on the Moon and Earth without its mass changing proportionally merely because of location. 6. Free-body diagrams Choose the object and draw only the external forces acting on it. Establish directions, units and an appropriate model before writing equations. Equilibrium means zero net force rather than absence of forces; rotational equilibrium also requires considering net torque.