Newtons Laws of Motion

Sir Isaac Newton is definitely most famous because of his exploits with an apple tree, and apple and (probably) a sore head, which infamously enabled him to proclaim the existence of a Force of nature which he called "Gravity" but he did discover something else, something which is still used today, and in the exact same form as Newton first discovered. (Unlike Gravity, (which has been improved upon by the likes of Albert Einstein) His Laws of Motion are still used today to enable physicists, engineers, mathematicians, and many others to calculate the behaviour of any object that is moving.

Newton devised just 3 simple laws of motion, all of which are essential to anybody with an interest in physics or engineering.

The First law was worded thus: Every body continues in its state of rest or uniform (unaccelerated) motion in a straight line unless acted upon by an external force

Now, whilst this may seem fairly obvious to everybody reading this, as it simply means that (for example) your computer (which is presumably stationary as you read this) will remain stationary until you pick it up, or push it away or pull it towards you. But this law has one part that people at the time had trouble grasping: uniform motion without the application of a force. And, if you think about it, the notion of something moving without you putting any force into moving it does seem to contradict our understanding of "the way things are", however; Newton explained that forces of friction were always present, so the object was being acted upon by invisible forces. Only in special cases, such as upon a frozen lake, where friction is practically zero, can Newton’s first law of motion be easily observed.

Another point of the First law was a concept of inertia, which is defined at the reluctance of a body to move from a stationary position (such as pushing a heavy box) or to stop when it is already in motion ( for example a person falling over the handlebars of a bike when it crashes into a wall). The law shows that in cases of inertia, a force must be applied. The mass of a body is effectively a measure of its inertia. If an objects velocity only changes by a small amount when a force is applied, then we say it has a high inertia (and usually a high mass). Centripetal force is another concept derived from Newton’s first law, which states that an object must have a force applied if it is to move in a circular fashion.

Newton’s Second Law of Motion is the more famous of the three, and has the majority of the mathematical concept and formulae within it. It is worded as: The rate of change of momentum in a body is directly proportional to the external force acting on the body and takes place in the direction of the force

The formula for momentum can be written as p=mv as it is the product of an objects mass and its velocity, and the applied force, stated in Newton’s law has the formula F=ma and is the definition of the Newton (unit of force and weight) with one Newton being the force which produces an acceleration of 1ms^-1 in a mass of 1kg. This brings up the question of the difference between mass and weight, which are often incorrectly used interchangeably. Mass is a fundamental property of all objects and cannot change. The weight of that object is the force acting upon the object due to gravity.

Finally, Newton’s Third Law of Motion states that: If a body A exerts a force on a body B, then B exerts an equal and oppositely directed force on A.

This law suggests, therefore, that all forces act in pairs, for example, I am exerting a force upon the chair on which I am now sat, the force being equal to my own weight, and the chair is exerting an equal force upwards, against my weight, which is why I do not simply fall through the chair.

 So, that was a (fairly) brief explanation of the 3 Laws of Motion put forth by Sir Isaac Newton, I hope that you have found this useful, please feel free to comment with questions, and suggestions for further posts.

 Happy Stargazing folks!!