Definition:Ideal (Physics)

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This page is about Ideal in the context of Physics. For other uses, see Ideal.


An ideal (or idealized) object is one in which certain attributes are approximated to zero or infinity.



A particle is a representation of an object in the physical universe which is idealized as having no magnitude.

That is, it is modelled as being a mass concentrated at a single point.

Rigid Body

A rigid body is a body $B$ with the property that the distance between any two points of $B$ remains constant regardless of any external forces.


A chain is an inelastic thread whose stiffness and width are approximated to zero.

The mass of a chain is usually defined in terms of linear density.


A cord is an inelastic thread whose mass, stiffness and width are approximated to zero.

That is, it is a chain whose mass is zero.


A straightedge is an ideal tool for constructing straight lines.

A straightedge is of unlimited length, but has no markings on it, so it cannot be used for measurement.


A rod is a rigid straight body whose width and mass are approximated to zero.


A wire is a rigid body modelled by a specific curve whose width and mass are approximated to zero.


A bead is a particle whose motion is constrained by a wire, along which it can slide with zero friction.


A pivot is an immovable point in space around which a body located at that point may rotate with zero friction.


A cart is a body with a given mass which is free to move with zero friction in a single dimension, that is, back and forth.

It is usually imagined as having wheels on which it can roll.


A wall is an immovable vertical plane in space to which other physical objects may be attached.


An ideal spring obeys Hooke's Law:

$\mathbf F = -k \mathbf x$


$\mathbf F$ is the force caused by a displacement $\mathbf x$
$k$ is the constant of proportion.

The negative sign indicates that the force pulls in the opposite direction to that of the displacement imposed.


An immovable body is one whose position cannot be changed by applying a force to it.

It can be considered as having an infinite mass and zero momentum.