Equilibrium, in physics, the condition of a system when neither its state of motion nor its internal energy state tends to change with time. A simple mechanical body is said to be in equilibrium if it experiences neither linear acceleration nor angular acceleration; unless it is disturbed by an outside force, it will continue in that condition indefinitely. For a single particle, equilibrium arises if the vector sum of all forces acting upon the particle is zero. A rigid body (by definition distinguished from a particle in having the property of extension) is considered to be in equilibrium if, in addition to the states listed for the particle above, the vector sum of all torques acting on the body equals zero so that its state of rotational motion remains constant. An equilibrium is said to be stable if small, externally induced displacements from that state produce forces that tend to oppose the displacement and return the body or particle to the equilibrium state. Examples include a weight suspended by a spring or a brick lying on a level surface. An equilibrium is unstable if the least departure produces forces that tend to increase the displacement. An example is a ball bearing balanced on the edge of a razor blade.

When you moved your book across the desk, the book experienced a type offriction that acts on moving objects. This force is known as kinetic friction force. It is exerted on one surface by another when the two surfaces rub against each other because one or both surfaces are moving.

Just as the magnitude of an earthquake tells you how big the earthquake is, the magnitude of a mathematical term tells you how big that term is. In math, this means how far away the math term is from zero.

For numbers such as 1, 2, 3, and so on, the magnitude is simply the number itself. If the number is negative, the magnitude becomes the absolute value of the number. For example, the magnitude of 10 is 10. The magnitude of -10 becomes the absolute value of -10, which is 10. In both cases, the magnitude is the distance the mathematical term is from zero. Both 10 and -10 are a distance of 10 away from zero.

Motion is a change in position of an object over time. Motion is described in terms of displacement, distance, velocity, acceleration, time and speed. Motion of a body is observed by attaching a frame of reference to an observer and measuring the change in position of the body relative to that frame.