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Newtons Three Laws of Motion


As an engineer, Newtons three laws of motion are very important to us. I will briefly explain them here and discuss the significance of each one.

Law 1: Inertia


Commonly refered to as "inertia", this law simply states that any object will stay at a constant velocity unless acted upon by an unbalanced force. Inertia is a measure of the mass of an object, so an object with a larger mass will have a higher inertia than one with a lower mass.

Simply put: things don't just start or stop moving without external interaction.

Law 2: F = ma


a body that is subjected to a force will accelerate with an acceleration of F/m in the same direction of the applied force. This rule states that the acceleration of the object depends on the mass of that object and the net force applied to it.

this can be extended for the case of multiple forces acting on a single body to:

[math]\Sigma F = m\Sigma a[/math]

now, what if the body in question is in static equilibrium and therefore not accelerating (IE [math]\Sigma a = 0[/math]), then we have the equation:

[math]\Sigma F = 0[/math]

which is an equation of static equilibrium which you will become familiar with later on as they very important in mechanics and are heavily used to solve problems especially involving stationary objects.

Law 3: Every action has an opposite and equal reaction


The heading just about sums this one up and it is more easily described with examples:

1) if you push against a wall and it doesn't move, the wall is pushing against you exactly as hard as you push against it and in the opposite direction (you are in static equilibrium with the wall).
2) The Earth's gravity pulls just as hard on the Sun as the Sun's gravity pulls on Earth (equal and opposite reactions) however, because the Sun is so massive, it is barely accelerated at all by the Earth's gravitational field.
3) If you jump up, you are pushing the Earth down (a fraction of a millimeter) and the earth pushes back on you (several centimeters).

This law is important to keep in consideration as an engineer, you must realise that every action has an equal and opposite reaction when looking at a problem to ensure that you don't forget some forces into your calculations. It is especially important to consider this when drawing Free Body Diagrams.