Simple Nature - Light and Matter

A moving object’s momentum equals the sum of the momenta of all its atoms. To avoid

having to carry out this sum, we can use the concept of thecenter of mass. The center of mass

can be defined as a kind of weighted average of the positions of all the atoms in the object,

xcm=

∑

mjxj

∑

mj

,

and although the definition does involve a sum, we can often locate the center of mass by

symmetry or by physically determining an object’s balance point. The total momentum of the

object is then given by

ptotal=mtotalvcm.

The rate of transfer of momentum is calledforce,F= dp/dt, and is measured in units of

newtons, 1 N = 1 kg·m/s^2. As a direct consequence of conservation of momentum, we have the

following statements, known asNewton’s laws of motion:

If the total force on an object is zero, it remains in the same state of motion.

F= dp/dt

Forces always come in pairs: if object A exerts a force on object B, then object B exerts

a force on object A which is the same strength, but in the opposite direction.

Although the fundamental forces at the atomic level are gravity, electromagnetism, and

nuclear forces, we use a different and more practical classification scheme in everyday situations.

In this scheme, the forces between solid objects are described as follows:

A normal force,Fn, is perpendicular to the surface of contact, and prevents

objects from passing through each other by becoming

as strong as necessary (up to the point where the ob-

jects break). “Normal” means perpendicular.

Static friction,Fs, is parallel to the surface of contact, and prevents the

surfaces from starting to slip by becoming as strong as

necessary, up to a maximum value ofFs,max. “Static”

means not moving, i.e. not slipping.

Kinetic friction,Fk, is parallel to the surface of contact, and tends to slow

down any slippage once it starts. “Kinetic” means

moving, i.e. slipping.

Workis defined as the transfer of energy by a force. (“By a force” is meant to exclude energy

transfer by heat conduction.) Thework theoremstates that when a force occurs at a single point

of contact, the amount of energy transferred by that force is given by dW=F·dx, where dxis

the distance traveled by the point of contact. Thekinetic energy theoremis dKcm=Ftotal·dxcm,

where dKcmis the change in the energy, (1/2)mvcm^2 , an object possesses due to the motion of

its center of mass,Ftotalis the total force acting on the object, and dxcmis the distance traveled

by the center of mass.

The relationship between force and interaction energy isU=−dF/dx. Any interaction can

be described either by giving the force as a function of distance or the interaction energy as a

function of distance; the other quantity can then be found by integration or differentiation.

An oscillator subject to friction will, if left to itself, suffer a gradual decrease in the amplitude

of its motion as mechanical energy is transformed into heat. Thequality factor,Q, is defined as

1074 Chapter Appendix 5: Summary