# Definition:Ideal (Physics)

## Definition

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

## Examples

### Particle

A particle is a representation of an object in the physical university 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.

### Chain

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.

### Cord

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.

### Straightedge

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.

### Rod

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

### Wire

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.

### Pivot

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

### Cart

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.

### Wall

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

### Spring

An ideal spring obeys Hooke's Law:

$\mathbf F = -k \mathbf x$

where:

$\mathbf F$ 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.

### Immovable

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.