## Newton's Laws of Motion

Perhaps the most heavily used equation in physics is*Newton's second law of motion*:

In this formulation, the applied force is considered positive in the direction of positive mass-position . The force and acceleration are, in general,

*vectors*in three-dimensional space . In other words, force and acceleration are generally vector-valued functions of time . The mass is a scalar quantity, and can be considered a measure of the

*inertia*of the physical system (see §B.1.1 below).

### Newton's Three Laws of Motion

*Newton's three laws of motion*may be stated as follows:

- Every object in a state of uniform motion will remain in that state of motion unless an external force acts on it.
- Force equals mass times acceleration [ ].
- For every action there is an equal and opposite reaction.

*law of inertia*, was pioneered by Galileo. This was quite a conceptual leap because it was not possible in Galileo's time to observe a moving object without at least some frictional forces dragging against the motion. In fact, for over a thousand years before Galileo, educated individuals believed Aristotle's formulation that, wherever there is motion, there is an external force producing that motion. The second law, , actually implies the first law, since when (no applied force), the acceleration is zero, implying a constant velocity . (The velocity is simply the integral with respect to time of .) Newton's third law implies

*conservation of momentum*[137]. It can also be seen as following from the second law: When one object ``pushes'' a second object at some (massless) point of contact using an applied force, there must be an equal and opposite force from the second object that cancels the applied force. Otherwise, there would be a nonzero net force on a massless point which, by the second law, would accelerate the point of contact by an infinite amount. In summary, Newton's laws boil down to . An enormous quantity of physical science has been developed by applying this simple

^{B.1}mathematical law to different physical situations.

### Mass

*Mass*is an intrinsic property of matter. From Newton's second law, , we have that the amount of force required to accelerate an object, by a given amount, is proportional to its mass. Thus, the mass of an object quantifies its

*inertia*--its resistance to a change in velocity. We can measure the mass of an object by measuring the

*gravitational force*between it and another known mass, as described in the next section. This is a special case of measuring its acceleration in response to a known force. Whatever the force , the mass is given by divided by the resulting acceleration , again by Newton's second law . The usual mathematical model for an ideal mass is a dimensionless

*point*at some location in space. While no real objects are dimensionless, they can often be treated mathematically as dimensionless points located at their

*center of mass*, or

*centroid*(§B.4.1). The

*physical state*of a mass at time consists of its

*position*and

*velocity*in 3D space. The amount of mass itself, , is regarded as a fixed parameter that does not change. In other words, the

*state*of a physical system typically changes over time, while any

*parameters*of the system, such as mass , remain fixed over time (unless otherwise specified).

### Gravitational Force

We are all familiar with the*force of gravity*. It is a fundamental observed property of our universe that any two masses and experience an

*attracting force*given by the formula

where is the distance between the centroids of the masses and at time , and is the

*gravitation constant*.

^{B.2}The law of gravitation Eq.(B.2) can be accepted as an

*experimental fact*which defines the concept of a

*force*.

^{B.3}The giant conceptual leap taken by Newton was that the law of gravitation is

*universal*--applying to celestial bodies as well as objects on earth. When a mass is ``dropped'' and allowed to ``fall'' in a gravitational field, it is observed to experience a

*uniform acceleration*proportional to its mass. Newton's second law of motion (§B.1) quantifies this result.

### Hooke's Law

Consider an ideal spring suspending a mass from a rigid ceiling, as depicted in Fig.B.1. Assume the mass is at rest, and that its distance from the ceiling is fixed. If denotes the mass of the earth, and is the distance of mass 's center from the earth's center of mass, then the downward force on the mass is given by Eq.(B.2) as*acceleration due to gravity*. Changes in due to the motion of the mass are assumed negligible relative to the radius of the earth (about miles), and so is treated as a constant for most practical purposes near the earth's surface. We see that if we double the mass , we double the force pulling on the spring. It is an

*experimental fact*that typical springs exhibit a displacement that is approximately proportional to the applied force for a wide range of applied forces. This is

*Hooke's law*for ideal springs:

where is the

*displacement*of the spring from its natural length. We call the

*spring constant*, or

*stiffness*of the spring. In terms of our previous notation, we have

*spring force*exerted upward by the spring on the mass (which is not moving). We know that the spring force is equal and opposite to the gravitational force because the mass would otherwise be accelerated by the net force.

^{B.4}Therefore, like gravity, a displaced spring can be regarded as a definition of an applied force. That is, whenever you have to think of an applied force, you can always consider it as being delivered by the end of some ideal spring attached to some external physical system. Note, by the way, that normal interaction forces when objects touch arise from the

*Coulomb force*(electrostatic force, or repulsion of like charges) between electron orbitals. This electrostatic force obeys an ``inverse square law'' like gravity, and therefore also behaves like an ideal spring for small displacements.

^{B.5}The specific value of depends on the physical units adopted as well as the ``stiffness'' of the spring. What is most important in this definition of force is that a doubling of spring displacement doubles the force. That is, the spring force is a

*linear*function of spring displacement (compression or stretching).

### Applying Newton's Laws of Motion

As a simple example, consider a mass driven along a frictionless surface by an ideal spring , as shown in Fig.B.2. Assume that the mass position corresponds to the spring at rest,*i.e.*, not stretched or compressed. The force necessary to compress the spring by a distance is given by

*Hooke's law*(§B.1.3):

*i.e.*, yielding

^{B.6}

where we have defined as the initial displacement of the mass along . This is a

*differential equation*whose solution gives the equation of motion of the mass-spring junction for all time:

^{B.7}

where denotes the

*frequency of oscillation*in radians per second. More generally, the complete space of solutions to Eq.(B.4), corresponding to all possible initial displacements and initial velocities , is the set of all sinusoidal oscillations at frequency :

*initial conditions*,

*i.e.*, the initial position and initial velocity of the mass (its

*initial state*) when we ``let it go'' or ``push it off'' at time .

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