Newton’s Laws, Part 1

As my interest in maths grew out of seeing it applied, I thought I should start writing posts on its applications. Physics applications are almost always maths related and you can’t start a conversation about physics without starting with Newton’s Laws.

Sir Isaac Newton is famous for his work in physics and maths. I find it amazing that his accomplishments occurred in the 1600’s. One of his most famous works, some would say his most famous, was his Principia Mathematica Philosophiae Naturalis (The Mathematical Principles of Natural Philosophy). In his day, the term natural philosophy meant science. In this publication, Newton set out his three laws of motion:

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

The first law is sometimes called the law of inertia. You experience inertia every day when you try to push an object or stop an object from moving. You have to apply a force to start an object moving or stop one from moving. The second law explains why a greater force is needed to stop a moving car than a baby stroller. The third law explains why rockets work, what happens when you release a ballon before it’s tied, and why a gun or rifle has a recoil.

This series of posts will be mainly about Newton’s second law. This law, in equation form, is

F = ma , where F is force, m is mass, and a is acceleration.

Before we work with this equation using numbers, let’s see what this equation means.

If you try to stop a rolling car, you are trying to decrease its velocity from a certain value to zero. In other words, you are trying to decelerate it. Deceleration is negative acceleration, and according to Newtons second law, because the mass of the car is rather large, a large force is required to stop it from rolling. A baby stroller going the same speed requires less force because its mass, m, is much smaller than a car’s mass.

Even though this equation is very simple, there are entire books dedicated to this equation. The rest of my posts could easily be about this equation alone, but I will try to keep it down to just a few.

In my next post, I will talk about the units that we will use for the three things that make up this equation. I will also talk about the difference between mass and weight.