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 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.
The first law, also called the 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 . 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.
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).
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.
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
Note that the force on the spring in Fig.B.1 is gravitational force. Equal and opposite to the force of gravity is the 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).
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):
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 :
Work = Force times Distance = Energy