Everything around us can be explained through science, and oft the definition is far more complex than we might realise. We might jump up and down, or push a friend on the arm, and we know what is going to happen, but we may not understand what are the laws of physics, those complex and fundamental laws, that can explain how these simple actions operate in the world around us. We often enjoy the way things around us seem to work, whether in nature, or through human invention, and we do not give credit for how fantastic and complex the physics laws behind it might be. While the world is magical, and a lot of it remains unexplained, we will delve into some of the laws of physics that we have discovered and that we can explain.
Gravity
The law of universal gravitation was put together by our good friend Sir Isaac Newton that you may recall from a previous article, allegedly discovering gravity from observing an apple falling from a tree. This groundbreaking (pun intended) work in physics outlined gravity and motion and how they relate to each other. The physical law of gravity explains that an object will attract another object in direct proportion to their combined mass and inversely proportional to the square of the distance between them. This explains why the moon (which is smaller and less heavy than the Earth) is attracted by the Earth’s gravitational pull, which in turn is attracted into orbit around the Sun, which is larger and exerts a gravitational pull on the Earth (and the rest of the solar system).
The Laws of Motion
The law of motion, or specifically, the three laws of motion, can explain how an aeroplane can soar through the air even though it is a hugely heavy metal object. Newton developed the three laws of motion around 1686, and they can be found in “Principia Mathematica Philosophiae Naturalis”.
We will state the three laws and then break them down a little bit further for discussion and understanding.
The laws of motion are as follows:
Every object in a state of uniform motion (that is constant motion – or remaining still) will remain in that state until an external force acts on it.
Force is equal to the change in momentum per change in time, or explained differently, the mass of the unit multiplied by the force of the acceleration. As an equation: F = ma
Every action will have an equal and opposite reaction.
The first law of Newton states that every object is going to remain in its state of rest or motion in a straight line unless an external force compels it to change by acting on it. This is also referred to as the law of inertia, by Galileo Galilei, or simply referred to as the definition of inertia. The main takeaway is that if there is no “net force” (for example, if you and your friend were playing tug-of-war and both pulling on the rope with the same amount of strength, the rope will not move) acting on an object, it will remain in a constant velocity. In the example with the rope, the external forces have cancelled each other out, so the rope remains still. However, if someone else helps you, and you now have a greater force pulling the rope, then it will move towards you due to the changed level of force.
The second law states that the velocity of an object can change when it is acted upon by an external force. It also explains how we have come to define a force, which is equal to the change in momentum per change in time. We can explain Newton’s second law quantitatively using the equation F=ma (force equals mass times acceleration) where F (force) is equal to m (mass) multiplied by a (acceleration). It describes, through numbers, the change that a force can produce on the motion of an object. Further, the time-rate of change of the momentum of the object is equal in both direction and magnitude (as it is a vector quantity) to the force acting on it. The momentum (which is also a vector quantity – meaning it has magnitude and direction) is a product of the total force acting on a body.
The third law states that for every action, there will be an opposite and equal reaction. This means that when you exert a force on someone (for example you push your friend), then your friend also exerted an equal force on you (because if you were off-balance, you could push yourself over too). A more interesting explanation could be how a rocket ship blasts off into space, and how the force exerted downwards by the thrust-engines is equal to the force of the ground pushing up on the rocket, which causes it to fly upward.
All three of Newton’s laws can also help to explain the conservation of momentum.
Conservation of momentum
This fundamental law of physics states that the momentum of a system is constant if there are no external forces acting on the system. It is very similar to Newton’s first law and might be best explained through an example. Let’s consider two objects, Ball-A and Ball-B. If these balls interact with each other, the forces between them are equal and opposite when they are touching each other, even if they are different weights. This is due to Newton’s second law, where force is the time rate of change of the momentum. And for both the balls not to move, we can see that the rate of change of momentum of Ball-A is equal to the negative (or minus) the rate of change of momentum of Ball-B. We can then see mathematically that if the rate of change is always equal and opposite, the total change in the momentum of Ball-A is equal and opposite of the total change in the momentum of Ball-B, which means the sum of the two momentums is zero. Which is the conservation of momentum.
Laws of Conservation of Mass and Energy
There are two main components to the law of conservation of mass and energy. These were presented by Albert Einstein in a journal on electrodynamics. The first is the principle of relativity, where the laws of physics are the same for all inertial reference frames. This means that the laws of physics will apply to everything equally.
And the second principle regards the consistency of the speed of light. It explains that light always propagates through a vacuum at a definite velocity, which is independent of the state of motion of the emitting body. This one is a bit more complex and means that the speed of light is the same whether in a vacuum or not. And it is measured in the same way for all observers.
Laws of Thermodynamics
These are actually remarkably similar to the laws of conservation of mass-energy as explained by Albert Einstein’s equation E=Mc2, as explained above, but this time how to relates to thermodynamic processes. In thermodynamics, we consider pressure, temperature, and volume. The first law states the relationship between internal energy and external (added) heat to a system. The second law demonstrates the natural flow of heat within a closed system. And the third law states that it is impossible to create a thermodynamic process that is perfectly efficient.
The many laws of physics allow us to understand and describe the beauty of natural science that surrounds us each day. And with these understandings and predictions, we can continue to develop as a race.
