Classical mechanics
Classical mechanics (often called Newtonian mechanics after Newton, who made major fundamental contributions to the theory) is the physics of forces acting on bodies. It is subdivided into statics, which deals with objects in equilibirium, and dynamics, which deals with objects in disequilibrium. Classical mechanics applies in the domain of everyday experience; it is superseded by relativistic mechanics for systems moving at large velocities, quantum mechanics for systems at small distance scales, and quantum field theory for systems with both properties. Nevertheless, classical mechanics is still very useful, because (i) it is much simpler than these other theories, and thus easier to apply, and (ii) it has a very large range of approximate validity, successfully describing systems as diverse as tops, baseballs, rockets, planets, and even large organic molecules.
Classical mechanics is broadly compatible with other "classical" theories such as classical electrodynamics and thermodynamics.
Description of the theory
Position and its derivatives
Let the vector r denote the position of a body, with respect to an arbitrary fixed point in space. r is a function of t, the time elapsed since an arbitrary initial time. The velocity, or the rate of change of position with time, is
- v = dr/dt
The acceleration, or rate of change of velocity, is
- a = dv/dt
The acceleration vector can be changed by changing its magnitude, changing its direction, or both. If the magnitude of v decreases, this is sometimes referred to as deceleration; but generally any change in the velocity, including deceleration, is simply referred to as acceleration.
Forces
Newton's second law relates the mass and velocity of a body to a vector quantity known as the force. Suppose m is the mass of the body and F the vector sum of all applied forces (or net applied force.) Then
- F = d (m v) / dt
m is not necessarily independent of t. For example, a rocket ejects propellant, so its mass decreases with time. The quantity m v is called the momentum.
When m is independent of t (as is often the case), the above equation becomes
- F = m a
The exact form of F is obtained from independent considerations, and depends on the particular situation. Newton's third law gives a particular prescription for F: if a body A exerts a force F on another body B, then B exerts an equal and opposite reaction force, -F, on A.
Another example of a force is the force of friction, which is generally a function of the velocity of the particle. For example:
- Ffriction = - λ v
where λ is some constant positive factor. If we have an independent relation for F such as the one above, it can be substituted into Newton's second law to obtain an ordinary differential equation, called the equation of motion. If friction is the only force acting on an object, the equation of motion is
- - λ v = m a = m dv/dt
This can be to obtain
- v = v0 exp (- λ t / m)
where v0 is the initial velocity. This tells us that the velocity of this body decays exponentially to zero. This expression can be further integrated to obtain r.
Other important forces include the gravitational force and the Lorentz force for electromagnetism.
Energy
If a force F is applied to a body that has achieves a displacement δr, the work done by the force is the scalar quantity
- δW = F · δr
Suppose the mass of the body is constant, and δWtotal is the total work done on the body, obtained by summing the work done by each applied force. From Newton's second law, we can show that
- δWtotal = δT
where T is called the kinetic energy. For a point particle, it is defined as
- T = 1/2 m |v|2
For extended objects composed of many particles, the kinetic energy of the composite body is the sum of the individual particles' kinetic energies.
A particular class of forces, known as conservative forces, can be expressed as the gradient of a scalar function, known as the potential energy and denoted V:
- F = - ∇ V
Suppose all the forces acting on a body are conservative, and V is the total potential energy, obtained by summing the potential energies corresponding to each force. Then
- F · δr = - ∇ V · δr = - δ V
- ⇒ - δ V = δ T
- ⇒ δ (T + V) = 0
This is a frequently useful result, known as the conservation of energy: the total energy E = T + V is constant in time.
Further results
Newton's laws provide many important results for composite bodies. See angular momentum.
There are two important alternative formulations of classical mechanics: Lagrangian mechanics and Hamiltonian mechanics. They are equivalent to Newton's laws, but are often more useful for solving problems.
History
To Be Done
Recommended Reading
- Feynmann, R., Six Easy Pieces.
- ---, Six Not So Easy Pieces.
- ---, Lectures on Physics.
- Kleppner, D. and Kolenkow, R. J., An Introduction to Mechanics, McGraw-Hill (1973).