# Definition:Ideal (Physics)

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

## 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 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.

### 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 mass 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 an idealized straight body which has:

but whose width is approximated to zero.

A **rod** can be defined as rigid or flexible, according to the needs of the model to which it may contribute.

### Bar

A **bar** is a straight rigid body whose length in one dimension is such that its width in the other two dimensions is negligible.

### Wire

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

### Bead

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.

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