What Is Physics?

Physics is the branch of science concerned with the properties of matter and energy. Physics includes various subjects like heat, light, mechanism, sound, electricity, magnetism and structure of atoms.

The purpose of physics is to find ways to explain everything – from the tiniest particles to the largest stars, galaxies and objects known to man. It is termed as it science because it attempts to do all that on the basis of experiments, measurements and mathematical models.  These laws are then used to predict the behaviour of all kinds of objects and to invent the technological devices that we take for granted today.

The human endeavour to understand and explain physical phenomena is ancient, with early philosophers like Archimedes speaking of phenomena like buoyancy and levers. Later, with the dawn of the 17th Century, Physicists like Galileo Galilei and Isaac Newton explained these phenomena through the universal language of Mathematics. We soon found everything from the behaviour of light to the orbits of planets around the Sun being explained through equations.

Faraday and Maxwell brought us  the laws of electricity, magnetism and electromechanical waves, thus paving the way for light bulbs and electricity supply to our homes. Many others contributed to explaining optics(the physics of light) and thermodynamics(the physics of heat).

A whole new chapter started around the beginning of 20^th century with the discovery of X Rays by Wilhelm Röntgen, observation of Radioactivity by Henri Becquerel and formulation of the Quantum Hypothesis by Max Planck.  Modern Physics was further augmented through Albert Einstein’s General and Special Relativity and Neil’s Bohr’s Atomic Models.

Heisenberg and Schrödinger ‘s Quantum mechanics gave scientists a better understanding of chemistry and solid-state physics, which in turn has led to new materials and better electronic and optical components. Nuclear Physics helps us produce electricity in a more efficient way than conventional production using fossil fuels. Elementary particle physics helps us study the intrinsic properties of the universe and is today being used to understand the very origin of the universe as well as the properties of massive objects like black holes.

Doppler Effect

Have you ever wondered why, when an ambulance or any car with a siren passes you, the sound of the siren seems to first get higher in tone as it approaches and lower once it moves away?

This is explained by the Doppler Effect:

The Doppler effect (or Doppler shift) is the change in frequency of a wave (or other periodic event) for an observer moving relative to its source. It is named after the Austrian physicist Christian Doppler, who proposed it in 1842 in Prague. It is commonly heard when a vehicle sounding a siren or horn approaches, passes, and recedes from an observer. Compared to the emitted frequency, the received frequency is higher during the approach, identical at the instant of passing by, and lower during the recession.

Let us take the example of the Doppler Effect on sound. We must first understand that sound travels in waves. It needs particles(atoms) to pass through the air. Sound is nothing but the passing on of vibration by air particles between a source and your ears. When your ear receives these vibrations, your brain interprets them as what we know as sound.

Figure 1: How sound waves travel through air (source is in the center)

Now, when the source, say a car, is moving, these waves are not as symmetrical as in figure 1. They get distorted because of the speed of the car, as illustrated in figure 2.

Figure 2: Distorted sound waves of moving source(the source is the orange dot in the middle of the waves, the arrow shows the direction of motion)

Now the high-ness or low-ness of these waves is defined by the frequency. Frequency, simply put, is the number of vibrations that happen in one second. In figure 2, the wavy lines moving up and down show the frequency: the closer packed they are, the higher the frequency, the further they are, the lower the frequency.

Now, for a moving source, as seen from figure 2, the frequency at the source will not be the same as:

1. the frequency according to someone who is standing in front of the source.
2. the frequency according to someone who is standing behind the source.

This effect can better be seen in figure 3.Figure 3(a) shows a source which is not moving while 3(b) shows a source which is moving.

Figure 3(a): The frequency is the same all around.

Figure 3(b): The frequency is different, according to whether you hear the sound at the source, in front of it or behind it.

Consider the following analogy: Someone throws one ball every second at a man. Assume that balls travel with constant velocity. If the thrower is stationary, the man will receive one ball every second. However, if the thrower is moving towards the man, he will receive balls more frequently because the balls will be less spaced out. The inverse is true if the thrower is moving away from the man. So the frequency is increased at the receiving end when thrower is moving towards the receiver because the balls have less distance to travel to reach the receiver. The same works the other way around: if the thrower is moving away, balls need to travel farther, and frequency as measured by receiver is reduced.

Similarly, the original question I asked can be answered by the same phenomenon. As the ambulance comes towards you with the siren switched on, the distance that the sound waves need to travel is progressively decreasing, but the waves travel at the same speed, the time taken is also decreasing. This results in a listener who is front of the ambulance hearing a sound with higher frequency than someone in the ambulance will hear. Conversely, for someone who is behind the ambulance, the sound will have lower frequency. This is because the sound waves still have the same speed, but the distance is increasing, and so the time taken by the sound to travel is increasing. So the listener behind the ambulance hears a lower sound than that heard by someone sitting inside the ambulance.

Figure 4: Doppler effect of car with siren

 

 

• Christian Andreas Doppler (29 November 1803 – 17 March 1853) was an Austrian mathematician and physicist. He is celebrated for his principle — known as the Doppler effect — that the observed frequency of a wave depends on the relative speed of the source and the observer.

 

Figures and definitions taken from:

• https://en.wikipedia.org/wiki/Sound
• https://en.wikipedia.org/wiki/Doppler_effect
• Doppler Effect first explained in 1842 by Christian Doppler: https://books.google.co.in/books?id=zl5RAAAAcAAJ&ots=uvGzPqcUkL&dq=c%20doppler&lr&pg=PA1#v=onepage&q=c%20doppler&f=false

Cross Roads Chicken

The chicken and the road joke is one of the oldest. However, very few people know that it actually dates back to the days of Isaac Newton, although it has changed somewhat over the years. Newton, in fact, used it on his colleagues at Cambridge University. Here is a reenactment of a conversation that took place in 1681:
Issac Newton: “Why did the chicken cross the road?”
John Flamsteed: “I don’t know.”
Issac Newton: “Chickens at rest tend to stay at rest, but chickens in motion tend to cross roads.”
John Flamsteed: “But what happen’s if it gets run over?”
Issac Newton: “That, my friend, is called an external force.”

This is a play on Newton’s First Law of Motion:

An object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force.

As an example: Let’s say that a ball is lying on level ground and there is no wind or anything else to make it move. The ball does not move unless someone comes and pushes it or picks it up. When the ball is lying on the ground, in its state of rest, or non-motion,it stays that way unless someone comes and moves it. Now let us consider another ball which is moving in a straight line on the floor. There being no wind or friction(the force which opposes the motion of objects along a surface) or any other influence to make it stop, change its speed, or its direction, the ball will keep rolling on the surface at the same speed.

Motion:  means that the ball/any other object is moving, which means that it does not stay in the same place.

Non-Motion: means that the object does not move. It stays in the same place. (like me as I write this post)

Friction: when two objects touch each other, there is a force which comes into action which opposes the sliding of the two objects against each other. This is called the force of friction. When you try to slide a heavy object along the floor, you find it difficult to move it not directly because of its weight, but because of the touching of the surfaces of the floor and the object.

 

To summarise:

• Newton’s First Law: Anything that is moving at constant speed in a certain direction(or is not moving at all) will maintain that speed and direction(or will remain stationary) unless something from the outside changes them.

Hide and Seek

One day, Einstein, Newton, Pascal, and Heisenberg met up to play a game of hide and seek. While Einstein counted, Pascal ran away and hid, but Newton stood right in front of Einstein and drew a one meter by one meter square on the floor around himself.

When Einstein opened his eyes, he immediately saw Newton and said “I found you Newton,” but Newton replied, “No, you found 1 Newton over a meter squared; you found Pascal!”

Meanwhile, Heisenberg ran around yelling exactly how fast he was going.

• Newton: newton is the SI unit of force. It is defined as the force used to change the speed of an object of mass 1 kilogram by 1 metre per second every second.
• Pascal: pascal is the SI unit of pressure. It is defined as the pressure caused by 1 newton force in a surface area of contact of 1 square metre.
  • Newton draws a square of side 1 metre around him. The area of a square is the square of the length of the side of the square. Hence when Einstein spots Newton standing over a square of side 1 metre, he has found 1 newton per square metre, i.e. Pascal.
• Heisenberg’s Uncertainty Principle: Werner Heisenberg said that in the study of subatomic particles, the position(where the particle is) and momentum(where and how fast it is going) cannot be known at the same time. Heisenberg yelled exactly how fast he was going, so nobody could know exactly where he was.For an analogy, I’ve always imagined taking a photograph of a ball thrown in the air, where you can change your shutter speed. With a very fast shutter speed there’s almost no blur, so you can tell very well exactly where the ball is. However, it’s just hanging there in the photo, so you can’t really tell how fast it’s moving or in what direction. On the other hand, if you take a longer shot, you’ll get a photo of a blur, letting you tell (if you backtrack from your shutter speed and the length of the blur) very much how fast the ball was moving, but not where it was at that instant.

To summarise:

• Newton= force that causes 1 metre per square second acceleration in an object of mass 1 kilogram.
• Pascal= pressure caused by 1 newton force acting on an area of 1 square metre
• Heisenberg: Both position and momentum cannot be known at the same time.