Newton’s Second Law
One way to study the dynamics of a system in which there is motion is to measure the kinematics of the system a
see if there is any acceleration.
If there is acceleration, this implies that there is a net (unbalanced) force on some
part of the system.
If there is no acceleration, then this means that either there are no forces on the system, or that
the forces within the system are balanced.
Sir Isaac Newton was not the first person to study and measure the motion of objects.
As we stated in previous
activities, there have been others like Aristotle and Galileo who did this before him. Nor was he the first person
who tried to model motion with equations that would allow for predictive behavior.
However, he was the first
person to clearly state the basic laws of motion that allow us to analyze all systems.
These three laws, known as
Newton’s Laws of Motion, are
Fig. 1: Newton (
An object at rest or in a state of constant motion will remain in that state until acted upon by an unbalanced force.
The net force on an object is proportional to its acceleration, with the proportionality constant being called the mass, i.e.
For every force on an object, there is an equal and opposite force produced by the object but acting on the object that has originated
the first force.
The first of these laws is a restatement of one of Galileo’s discoveries.
While seemingly obvious, especially to us now, it is a very
It says that an object that is noticed to be accelerating must have a net force acting upon it, even if the manner of the
force is not readily noticeable.
For instance, if one were to swing a rock on the end of a string in a circle, it would be obvious that the
acceleration of the rock was due to the force exerted on it by the string, which is being accelerated by the hand holding it.
of an electron that is above the Earth’s atmosphere that also moves in a circular orbit?
Since it is accelerating, we know that there must be
some net force on it, even though there is nothing in contact with it (it turns out that the force operating on it is electromagnetism).
of a distant star that is going around in an elliptical orbit, seemingly by itself?
We know that some other object must be applying a force on
it, and by studying its motion, we might find its “neighbor” that is causing it to move thusly, even if its “neighbor” is a black hole that
cannot be observed directly.