Basics Physics Pdf

[Ronald Frison]

Physicsis the science that quantifies reality. Its influence extends to all the natural sciences, including biophysics, astronomy, and chemistry. Physics classifies all interactions between matter and energy and tries to answer the most central questions of the universe. From Aristotle and Isaac Newton to Marie Curie, philosophers and scientists have been using physics to understand the world for at least 2,000 years.

In any field, a scientist needs a handle on the basics before finding answers to fundamental questions. In physics, different types of matter-energy interactions define the basic branches of the sciences. Energy takes the form of heat, light, radiation, sound, motion, and electricity. It can be stored in an object's position, chemical bonds, physical tension, and atomic nuclei. Matter refers to anything with mass, or anything made up of atoms, that takes up space. From the bonding of atoms to the combustion of an engine, matter and energy interact in all facets of life, defining the physical world.

As current and former students are aware, physics makes sense of the relationships between matter and energy through mathematics; although, an appreciation for how physics shapes the world doesn't require advanced computational skills. Stacker used a variety of scientific and educational resources to compile a list of basic physics concepts to help explain how the world works. From Newton's Laws of Motion to electric forces, these concepts explain why matter behaves the way it does.

Physicists commonly use velocity and acceleration to characterize motion. Velocity refers to motion in a specific direction, while acceleration measures how quickly or slowly velocity changes. For example, when driving somewhere, both a driver and a car have velocity, meaning they move in a specific direction at some speed. Said driver probably changes how fast they travel from time to time, alternately accelerating and decelerating.

Most famous as the force that makes things fall down, more fundamentally, gravity is a force of attraction. Not only does it attract things to Earth's surface, but it keeps planets orbiting stars. Gravity is also the reason things have weight. Everything has mass, a measure of the amount of matter in an object, but the force of Earth's gravitational pull is what creates weight.

The low-speed limits posted for on and off-ramps are there for a reason: centripetal force. When something accelerates along a circular path, centripetal force keeps it going in the circle. For curved exit ramps, the speed limits have been specially calculated to ensure that centripetal force keeps the car on its path.

Torque is the reason doors have knobs and hinges on opposite sides and is the force that causes an object to rotate or twist about an axis. It requires more force to rotate an object when pushing closest to the axis of rotation, which is why doorknobs are nearly as far as possible from the hinges.

Simple harmonic motion involves oscillations, like a block bouncing up and down on a spring, or a pendulum swinging left, right, and back again. With this kind of movement, an object passes through a central position to one side and then moves the same amount to the other side after each pass through the center so that maximum displacement is equal on both ends.

From river flow to wind patterns, fluid dynamics explains some of the most common forces of nature. Physicists and engineers study flow rates of fluids, type of flow (like smooth or turbulent), friction, pressure, fluid thickness, and more to understand liquids and gases. Anyone with air travel experience has benefitted from the study of fluid dynamics. The shape of airplane wings takes advantage of airflow, the curved top and flat bottom manipulating air pressure to lift the plane.

Thermodynamics regards different kinds of heat and energy transfer. Heat is a form of energy and can transfer from a hot object or area to a cooler one through radiation, physical contact, or the flow of heated particles known as convection. Heat represents energy transferred between systems because of a temperature difference, while temperature measures how fast atoms are moving.

Electricity exists thanks to positive and negative charges, largely carried by two subatomic particles: protons, which are positively charged, and electrons, which are negatively charged. Opposite charges attract each other, while like charges repel. Whenever one of these charged particles moves, it creates an electrical current.

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So you can see from the equation, as wavelength increases, frequency decreases (and vice versa). This is because Frequency is inversely related to wavelength. The SHORTER the wavelength the HIGHER the frequency and the LONGER the wavelength the LOWER the frequency.

The average speed/velocity of sound in all mediums is 1540 cm/s. However, depending on what medium the sound waves travel through, it can drastically change the propagation speed of sound as it passes through.

Two of the factors that affect the speed of sound are the stiffness and density of the material it is traveling through. The stiffer the medium, the faster the sound waves will travel and that is why sound waves travel faster in solids than in liquids or gases. So the ultrasound propagation speed from slowest to fastest is: Lung (air)

The importance of Impedance in ultrasound becomes apparent at the interface of two tissue types with significantly different impedance values. Ultrasound waves will reflect when this situation occurs. The proportion of ultrasound waves reflected back is proportional to the difference in impedance (or density) of two tissue types

We will go into more detail on the artifacts caused by reflection in the detailed Ultrasound Artifacts Section below but they include: reverberation artifact, mirror image artifact, comet tails, and ring down artifact.

So when ultrasound waves travel through tissue and meet another tissue with slightly different impedance values, the speed changes somewhat and cause the ultrasound waves to change in direction. This change in direction is called Refraction!

The degree of how much refraction occurs is dependent on what angle the ultrasound wave encounters the second medium and how much of a change in speed there is in the second medium. This is seen mostly in situations at the rounded interfaces between a fluid-filled circular structure and the adjacent soft tissue. This is what gives rise to the edge artifact seen in ultrasound with black lines arising from the edge of fluid-filled structures such as the gallbladder, cyst, vessels, and bladder.

Attenuation is a fairly easy concept to understand compared to impedance. It just describes how rapidly does a medium reduce the intensity of an ultrasound wave as it passes through it. The two mediums with the highest amounts of attenuation are actually AIR and BONE!

As you can see attenuation is not simply dependent on the density of the material like impedance is. Look at ultrasound physics table below to see the relationship between tissue density, impedance, and attenuation:

One of the most used modes with ultrasound is Doppler. Initially, Doppler may seem confusing with all of the different Doppler modes available to you (color Doppler, power Doppler, pulse wave Doppler, continuous wave Doppler, and tissue Doppler).

The Doppler Effect (or Doppler Shift) is used to evaluate movement either towards or away from the ultrasound probe/transducer. The most common Doppler ultrasound application we think of is detecting movement of blood, but we can also use Doppler on ultrasound to evaluate tissue and muscle movement.

Below is a figure detailing how the Doppler Shift is used and how the angle of insonation is extremely important in what the transducer will detect as the amount of flow/movement. For any type of Doppler you want the flow/movement to be going directly towards your probe (zero degrees) as you move more towards a 90 degree angle there will be no flow detected by the ultrasound machine.

So the most important thing you can do to improve your Doppler technique for any mode is to make sure that the movement of whatever you are measuring is parallel to your ultrasound probe as much as possible (zero degrees). Anything above 25-30 degrees will significantly underestimate your measurements. And if you are perpendicular, the cosine of 90 degrees = 0 and the ultrasound Doppler will read no flow or movement.

A common question that comes up with color Doppler is: What do the colors on ultrasound mean? The answer is: RED means there is flow TOWARDS the ultrasound probe and BLUE means that there is flow AWAY from the ultrasound probe. It is a misconception that red is arterial and blue is venous. It actually just depends on the direction blood is flowing relative to the angle of your ultrasound beam.

There is a mode similar to color Doppler that you may encounter called Power Doppler. This mode does not show up as red or blue on the screen but only uses a single yellow color signifying the amplitude of flow. It is more sensitive than color Doppler and is used to detect low flow states such as venous flow in the thyroid or testicles.

Going through each of these Doppler modes is beyond the scope of this post. However, if you want a good explanation of exactly how Pulse Wave Doppler and Tissue Doppler are used, check out the diastolic dysfunction post HERE.

Ultrasound artifacts are frequently encountered and can be a source of confusion for interpreting providers. Ultrasound artifacts can be understood with a basic understanding of the ultrasound physics we just discussed pertaining to reflection, refraction, and attenuation.

Edge artifact on ultrasound occurs because of refraction. Ultrasound waves are deflected from their original path when they encounter curved and smooth-walled structures. This will result in a shadow-like line that comes off of the edge of these structures. The most common times you will see this are: vessel walls, gallbladder, cystic structures, testicle, aorta.

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