Physics Flashcards
Explain Newton’s three laws of motion
- “a body moving with a given speed will maintain that speed in a straight line unless a force acts upon it—no force means no acceleration.”
- “a force (F) will make a body accelerate by an amount (a) that’s inversely proportional to its mass (m): F=ma.”
- “whenever one body applies a force (the “action” force) to a second one, the second one simultaneously applies an equal and opposite “reaction” force on the first. Stepping off a boat onto a pier will make the boat move away, for instance.”
Newton showed that these laws neatly explain the orbits of the planets around the Sun when combined with Newtonian gravity.But they are not valid for objects moving at very high speeds or in very intense gravitational fields, when relativity theory is required.
Explain Motion in physics
- “the motion of an object is described in terms of quantities such as velocity, acceleration, and displacement, the distance of a moving object from its origin.”
- “Velocity is a vector quantity, specifying not only an object’s speed but also its direction, while force is a measure of the push or pull that an object needs to change its velocity, resulting in acceleration—the rate of change of velocity over time.”
Explain Centripetal and Centrifugal forces
- Centripetal force is one that makes a body move in a curved path. Gravity is an example of a centripetal force in Newtonian gravity making a planet orbit a star by continually accelerating the planet toward the star at the orbit’s center. “Without this centripetal force, the planet would fly off into space in a straight line.”
- When you whirl a tennis ball over your head on a string, the ball feels a centripetal “pull” force. The centripetal force is often confused with the centrifugal (outward) force, which can be a “fictitious” force. It accounts for the sense of being pushed outward when looping-the-loop on a roller coaster.
- The centrifugal force can also be a reaction force to a centripetal force, according to Newton’s third law of motion (see page 10). In the case of a tennis ball on a string, the whirling ball exerts an outward centrifugal force on the person spinning it.
Explain Newtonian gravity
- Isaac Newton’s law of universal gravitation, published in 1687, was the first clear mathematical description of how bodies such as planets and stars attract each other under their mutual gravitational pull.
- A falling apple accelerates toward the ground, so Newton reasoned from his laws of motion that there must be a force, which he called gravity, acting on the apple.
- This force might have a huge range and could also be responsible for the orbit of the Moon around the Earth, if the Moon had just the right speed to remain in orbit despite constantly “falling” toward the Earth.
- He went on to show that the gravitational force between two massive objects is directly proportional to the product of their masses and weakens with the square of the distance between them.
But troublingly, the theory didn’t explain why the force was transmitted across empty space. This problem is resolved in Einstein’s general relativity theory.
Explain Special relativity
- Special relativity is the theory of motion published by Albert Einstein in 1905. Einstein developed it from two basic principles: the laws of physics must be the same for any observer moving at a constant velocity, and the speed of light is always the same regardless of the speed of the light source.
- Relativity abandons the idea that it’s possible to have a universal standard of time and space. Instead, the length of an object or time interval depends on who is measuring it.
Take the case of a train moving at close to the speed of light relative to an observer. The observer would perceive the train to be shorter than the passengers on board would measure, while the observer would see a clock on the train run slow. (This is not just an illusion—measurements show that unstable particles moving fast through Earth’s atmosphere decay much more slowly than they do at rest in a laboratory) - Special relativity forbids massive objects from travelling as fast as the speed of light in a vacuum, which would require an infinite amount of energy.
Explain General relativity
- General relativity is Einstein’s theory of gravity, developed by 1915. Unlike Newtonian gravity, Einstein’s theory views gravity as a natural upshot of the geometry of curved space and ditches the notion that gravity is “action at a distance.”
- Large masses like planets move in response to the curvature of space–time, distorted by mass itself. Matter tells space how to curve; curved space tells matter how to move.
It’s difficult to visualize in three dimensions, but it helps to imagine a star’s mass making a depression in a two-dimensional sheet. A nearby planet would be forced to curve around it like a ball in a roulette wheel. - Some predictions of general relativity are different from those of Newtonian gravity. Although both theories predict that the Sun’s gravity bends light from background stars, which can be measured when sunlight is blocked during a solar eclipse, Einstein’s theory predicts twice as much deflection as Newton’s.
- Measurements show general relativity is correct on this, and it has passed all other tests so far without incident.
Explain Temperature and Pressure
- Temperature is a measure of how hot an object is, which reflects the amount of kinetic energy in its molecules.
- For common purposes, most countries measure temperatures using the Celsius scale of temperature, which sets water’s freezing point at zero degrees (0°C) and its boiling point at 100°C. The United States uses the Fahrenheit scale, in which water freezes at 32°F and boils at 212°F.
- Matter can be cooled by reducing the kinetic energy of its molecules, but the laws of thermodynamics predict that there is a minimum possible temperature, which turns out to be −459.67°F (−273.15°C). This is “absolute zero,” where particles would theoretically be motionless.
- Pressure is the force exerted by one substance on another substance, per unit area. The pressure of a gas is the force the gas exerts on the walls of its container. The standard unit of pressure is the pascal (1 newton of force per m2). The typical air pressure at sea level on Earth is about 100,000 pascals.
Explain Heat transfer
- Heat can be transmitted through matter in three ways: conduction, convection, and electromagnetic radiation.
- Unlike conduction and convection, radiation can transmit energy across empty space.
- Conduction is the mechanical transfer of heat through matter from a warm part to a cooler one without any bulk motion.
- In gases and liquids, heat conduction occurs due to collisions and diffusion of molecules during their random motion.
- In solids, molecules conduct heat by vibrating against each other or when free electrons carry kinetic energy from one atom to another. Metals are the best conductors of heat.
- Liquids and gases also transfer heat in convection currents, which involve bulk fluid motion. For instance, a hot bubble of gas in the Sun’s atmosphere can carry heat into a higher, cooler layer before cooling and sinking.
- Heat transfer can also occur when radiation carries energy between one object and another. For instance, sunlight heats the Earth by making molecules in the Earth’s atmosphere and surface vibrate.
Explain Brownian motion
- Brownian motion describes the jittery, random motion of relatively large particles suspended in a fluid or gas, such as smoke particles in air.
- It is named after Robert Brown, a Scottish doctor and botanist who studied it in detail in 1827.Brown noticed that pollen grains in water jiggled, following zigzag paths.
- Later, in 1905, Albert Einstein showed that this Brownian motion can be predicted mathematically by assuming these large, suspended particles are constantly bumped by smaller fluid molecules moving due to their own thermal energy. One prediction was that the displacement of a suspended particle from its origin over time should be proportional to the square root of the time elapsed.
- Experiments by French physicist Jean Perrin soon confirmed that Einstein’s predictions were correct, indirectly proving that molecules and atoms exist despite the fact that they were too small to be seen directly.
This might seem obvious now, but it was still common at the time to believe that matter was not grainy and could be divided indefinitely.
Explain Work and Energy
- Work refers to an activity involving a force and movement, while energy is the capacity for doing work—a bit like a “currency” that gets used up in the process.
- In the context of a moving object, the work done by a force equals the force multiplied by the distance moved.
- In the context of thermodynamics, work has a more complex definition. It refers to energy transferred to a gas, for instance, but only if that energy causes a macroscopic change to the gas, perhaps making it expand its volume against an external pressure. It doesn’t include the input of heat energy if the heat merely increases the microscopic thermal motions of particles.
- The work done to compress a gas in a container with a movable piston is approximately equal to the gas pressure multiplied by the volume change. The change in internal energy of a gas is equal to the heat added minus the work done by the gas, which is one way of stating the first law of thermodynamics.
Explain four Laws of thermodynamics
The four laws of thermodynamics define the relationships between quantities like temperature and work in “thermodynamic systems”—a loose term for any matter with thermal energy, such as gas molecules in a container.
“Thermal equilibrium” describes the state of two systems in contact with each other, which have no net exchange of energy because they’ve reached the same temperature.
4th law:
The “zeroth law” of thermodynamics says that two systems in thermal equilibrium with a third one must also be in thermal equilibrium with each other. Scientists felt the need to state the intuitively obvious zeroth law after they adopted the other three.
The first law:
says energy in an isolated system is conserved. Chemical energy might change into kinetic energy, but the total stays the same.
The second law:
says that because energy varies in its quality or ability to do useful work, the entropy of an isolated system—a measure of the energy input that doesn’t do mechanical work—always increases.
The third law:
says minimum entropy occurs at absolute zero.
Explain Phases of matter
- Classically, matter can exist in three phases: solid, liquid, and gas.
- Traditionally, solids are defined as matter with a fixed volume and shape, containing closely packed particles. Liquids keep the same volume but flow to fill the bottom of a container, while gases expand to occupy all available volume.
- Phase transitions can occur due to alterations in pressure or temperature. At normal atmospheric pressure, pure water melts from solid ice into a liquid above 32°F (0°C) and boils into water vapor at 212°F (100°C).
- The energies of individual water molecules in a boiling kettle are not identical but follow a bell curve, which means that liquid and gas phases can coexist. At the so-called triple point of a substance, all three phases can coexist. For instance, water ice, liquid, and vapor can mingle in a container at 32.02°F (0.01°C) at very low pressures.
- Plasma, a searingly hot ionized (electrically charged) gas, is often called a fourth state of matter. It streams out from stars like the Sun into interstellar space.
- More exotic states of matter include Bose–Einstein condensates.
Explain Surface tension
- Surface tension results from the inward pull of molecules on the surface of a liquid, making them adopt the smallest surface area possible.
- It effectively makes the surface stronger, allowing a small object such as a sewing needle to effectively float on water, even though the object may be much denser than the liquid.
- In the bulk of a liquid, molecules face a tug of war in which they’re pulled equally in all directions by neighboring molecules, so the forces on them cancel out. But surface molecules lack upward force, so they’re pulled together and down, making the surface contract to its minimum size.
- Surface tension holds water droplets together and would make them spherical in the absence of other forces such as gravity, because a sphere has the smallest surface to volume ratio.
- Many animals take advantage of surface tension on ponds. Common insects called water striders, or pond skaters, rely on it to walk on water and sense vibrations from nearby prey using sensitive hairs on their legs and bodies.
Explain Archimedes’s principle
- Archimedes’s principle states that the buoyant force on an object submerged in a fluid (liquid or gas) is equal to the weight of the fluid that the object has displaced.
- It implies that an object will sink in a fluid if its average density is greater than that of the fluid.
Archimedes was a Greek scientist and engineer who lived during the third century BC. Later historians suggest that he was tasked with determining whether a crown supposedly crafted from pure gold also contained some cheaper silver. While taking a bath, Archimedes noticed that the water level rose when he got in, and he realized that by placing the crown in water and measuring the displaced water volume, he could establish the volume of the crown and thus calculate its density and purity without damaging it.
Legend has it that Archimedes then ran down the street naked shouting “Eureka!,” Greek for “I have found it.” His principle explains why ships float and why hot-air balloons rise—warm air within a balloon is less dense than the cooler air outside.”
Explain Fluid dynamics, Reynolds number, Bernoulli effect
- Fluid dynamics is the science of how fluids (both liquids and gases) flow. It’s essential for many practical applications, including the design of efficient aircraft, ships, and oil pipelines as well as weather forecasting.
- A boat moving through water encounters two main kinds of resistance—inertial forces from the water (effectively the water’s resistance to motion) and viscosity or stickiness.
- In fluid dynamics, the “Reynolds number” expresses the relative importance of these factors in flows across a surface, such as a ship’s hull or a pipeline. A low Reynolds number gives smooth fluid motion, while turbulent flow with chaotic eddies and vortices occurs at high Reynolds numbers.
- the Bernoulli effect: the faster a fluid flows, the lower its pressure. The curved upper surfaces of aircraft wings are shaped to force air to follow a longer path over the top of the wing, speeding it up. This lowers pressure above the wing and creates a net upward force, or lift.
Explain Wave types
- A wave is a disturbance that propagates through empty space or a medium such as air or water, usually transporting energy as it travels.
- In “transverse waves,” the disturbance is at right angles to the wave’s direction of motion. Electromagnetic radiation, including visible light, is a form of transverse wave in which magnetic and electric fields oscillate at right angles to the wave’s direction of travel.
- In “longitudinal waves,” the disturbance is parallel to the wave’s direction. These include sound waves in gases and liquids.
- Water waves are an example of a wave that is both transverse and longitudinal—a floating cork will move in a circle as a wave washes past it.
- Waves are characterized by their wavelength (the distance between peaks or compressions), frequency (the rate at which waves pass a given point), and amplitude or intensity.
- Standing or stationary waves occur when waves are held in a fixed position—for instance, when a guitar string vibrates. Such waves always involve a whole or half-number of waves, and hence the length of the string determines the wavelengths it can maintain.
- Torsional wave: A wave motion in which the vibrations of the medium are periodic rotational motions around the direction of propagation.
Explain Sound waves
- Sound waves are pressure oscillations that propagate through a gas, liquid, or solid—sound can’t travel in empty space.
- In gases and liquids, sound is a longitudinal wave, but transverse sound waves can pass through solids.
- People hear sound because it makes our eardrums vibrate. These vibrations pass through the inner ear to nerve cells that send signals to the brain, which interprets them as sound.
- A higher wave frequency means that the air pressure fluctuation switches back and forth more quickly, and we hear this as a higher pitch.
- Human hearing is normally limited to frequencies between 20 and 20,000 hertz (repeating wave cycles per second) and the upper limit tends to drop with age.
- The speed of sound depends only on the transmitting medium.
- In air with a temperature of 68°F (20°C) at sea level, sound travels at about 767 mph (343 m/s).
- The intensity of sound is measured in decibels, with a typical conversation registering about 60 decibels and motorbike engines topping 100 decibels.
Explain Doppler effect
- The Doppler effect describes changes in the frequency of waves depending on how the wave source is moving relative to an observer.
- It explains why the siren of a fire engine sounds higher in pitch as it comes toward us, then sounds lower after the vehicle has driven past.
When a source of sound waves moves toward an observer, each successive wave emitted comes from a position closer to the observer, who hears it more quickly due to the shorter travel time. Effectively, the waves bunch together to create an increase in frequency. Conversely, when the wave source moves away, successive waves are emitted from greater distance. The waves stretch out, making their frequency drop. - The effect is named after Austrian physicist Christian Doppler, who described the effect for light waves in 1842.
- Frequency also determines the color of light, so the Doppler effect alters the color of a light source approaching an observer or receding at very high speed—a green light appears more blue when approaching and more red when receding.
Explain Electric charge
- Electric charge is a property of many standard model particles, including the electron. This property makes them feel a force from other charged particles.
- Electrical charge can be either negative or positive, with negatively charged particles attracting positively charged ones while repelling their own kind.
- The unit of electric charge is called the coulomb (C); 1 coulomb is the charge transported per second by an electric current of 1 ampere. The negative charge of an electron is −1.602 × 10−19C.
- For simplicity, the electron charge is often denoted as −1, while that
on a positively charged proton is +1. - Electric charge plays a pivotal role in our very existence, allowing solid structures like the Earth, buildings, and animals to exist. Atoms are mostly empty space, but they don’t fall through each other due to repulsion between electrons in neighboring atoms. Charged particles zinging around in the Sun’s atmosphere also play a crucial role by generating the radiation that keeps our planet’s surface warm and hospitable.