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.
Lamina
A lamina is an ideal of a (rigid) body embedded in ordinary space such that one of its dimensions is infinitesimal.
That is, its thickness can be considered to be uniform and positive, but small enough to be ignored.
Such a lamina can be interpreted as a body which is all surface.
Thus it has area and (area mass) density but zero thickness.
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 is approximated to zero.
It may also be the case that the mass of the wire is also 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 (ideally) 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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