Physics is 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.
Read on to see how physics allows engineers to develop life-saving technology like airbags, how it explains door knob placement, and why a person’s legs look so short when they’re standing in water.
One of the first lessons in a physics class is that of motion: how an object moves, how fast it moves, where it moves, and at what rate it speeds up and slows down.
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.
Newton’s first law
Nothing moves without a little push first. That’s essentially Isaac Newton’s first law of motion. If an object is moving at a constant speed (even if the speed is zero, and the object is stationary), it will stay that way unless a force, like the friction between a wheel and the ground, affects it. This concept is also called inertia. Newton’s first law explains why once a rocket is launched into the vacuum of space, without the resistance of air or other forces, it will keep traveling in a straight line at a constant speed indefinitely.
Newton’s second law
As alluded to in Newton’s first law, in order to move, an object needs a force. Generally, a force is a push or pull. For example, the front door needs a push before it can open. Newton’s second law asserts that a force depends on the mass of an object exerting that force and its acceleration. Quickly pushing a hand forward to open the door will create a much more forceful entrance than if the same person slowed their approach.
Newton’s third law
Forces don’t act in isolation; each is always accompanied by another force that pushes or pulls in the opposite direction. When pushing a chair across the floor, for example, not only does one exert a force that moves the chair, but the floor exerts another force—friction—opposing the push. Some examples of Newton’s third law in action include a car’s wheels push backward on the ground, making use of the road’s friction force and moving forward; or a bird’s wings push air down and back to generate lift and fly forward.
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.
Work and energy
Work happens whenever a force moves something. Whenever someone does work on another object, like moving a chair across the floor, they also transfer energy to that object. In this case, the person moving the chair gives it kinetic energy—the energy of motion.
This is part of the law of conservation of energy: Energy cannot be created nor destroyed but can be transferred to different objects and take different forms. This concept helps explain how fuel and engines work, and why car owners need to buy gasoline or charge their vehicles. When a driver starts up their car, the car doesn’t create kinetic energy to move; instead, the chemical or potential energy in the fuel of the car combusts in the engine to generate motion, converting potential energy into kinetic energy.
Some may think of momentum as that motivated, “on a roll” feeling that follows a series of successes. In physics, momentum is how much motion something has. It’s similar to the colloquial meaning, in that an object’s physical momentum determines how much force is needed to stop a moving object that’s “on a roll.” Impulse measures how much momentum changes over time.
These concepts help engineers design airbags, which increase the impulse—or time required to stop the momentum—of a driver during a crash. This means that the driver feels a smaller force from the crash, as they experience the change in momentum over a longer period of time.
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
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.
In the pendulum example, the pendulum swings just as far left as it does right. It’s called harmonic motion because musical sounds are combinations of simple harmonic waves, sound waves emitted by musical instruments.
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 benefited 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.
Thanks to thermodynamics, scientists and engineers have created air conditioning, central heating, and computers that don’t overheat. Biologists also benefit from this field: Thermodynamics governs how organisms receive, store, and expend energy. For example, plants take in heat energy in the form of the sun’s radiation and animals emit heat during energy metabolism.
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.
Every time someone turns on a light, electrons move from an area of negative charge through a wire toward an area of positive charge, generating a current to power the bulb. Electricity isn’t just useful for appliances, though, it also plays a fundamental role in biology, powering animals’ nervous systems. Neurons communicate with the help of electrically charged atoms, or ions, generating electrical impulses that power things like muscle movement.
The motion of electric charges creates current and generates an electromagnetic force, resulting in magnetism. Like charges, magnets consist of two opposite components. These components, called poles, are also similar to charges in that like poles repel while opposites attract. Each magnet has a north and south pole. Earth also has magnetic poles, though their location isn’t quite the same as the more popular geographic north and south poles. Scientists think that Earth’s swirling, metallic core creates the planet’s magnetic field, making Earth a giant magnet.
Eyeglasses, contact lenses, microscopes, movie projectors, cameras, and more all exist because of the physics of light, or optics. These innovations harness the principle of refraction or the angle at which light bends when entering a different material. For example, glass lenses—similar to the lens of an eye—use refraction to focus and magnify images. Refraction also creates the strange image of a disproportionately squat lower half when a person stands waist-deep in a pool. Light travels slower in water, so the human eye gazing at the pool from above perceives objects in water as closer than they actually are.