Physics

What physics studies

Physics is the study of motion, force, and energy. It asks why a ball rolls to a stop, why a bus throws you forward when it brakes, and where the energy in a battery goes.

Speed

Speed is how far something travels in a certain amount of time:

speed = distance ÷ time

A car that travels 120 miles in 2 hours has an average speed of 120 ÷ 2 = 60 miles per hour. Acceleration is a change in speed or direction: speeding up, slowing down, or turning.

Three laws of motion

About 340 years ago, Isaac Newton described three rules of motion. They are still used every day by engineers.

The first law: an object keeps doing what it is doing, resting or moving in a straight line, until a force acts on it. This tendency is called inertia. When a bus brakes suddenly, your body keeps moving forward, because nothing has yet stopped it.

The second law: the more force you apply, the more an object accelerates; the more mass the object has, the less it accelerates. In a formula, force = mass × acceleration. An empty shopping cart is easy to get moving; a loaded truck is not.

The third law: for every action there is an equal and opposite reaction. When you push on a wall, the wall pushes back on you. A rocket rises because it pushes hot gas down, and the gas pushes the rocket up.

Gravity, mass, and weight

Gravity is the pull that every mass has on every other mass. The bigger the masses and the closer they are, the stronger the pull. Gravity makes a dropped object fall, and it keeps the Moon in orbit around the Earth.

Mass is the amount of matter in an object. Weight is the pull of gravity on that mass. An astronaut has the same mass on the Moon as on Earth, but weighs about one sixth as much, because the Moon’s gravity is weaker.

Energy

Energy is the ability to do work or cause change. It comes in many forms: heat, light, sound, electricity, the chemical energy stored in food and fuel, and the energy of motion.

Two forms come up often on the test. Kinetic energy is the energy of motion; the faster and heavier something is, the more it has. Potential energy is stored energy, such as the energy of a rock sitting at the top of a hill. As the rock rolls down, its potential energy turns into kinetic energy.

Energy is never created or destroyed; it only changes form. This rule is called the conservation of energy. In a car, the chemical energy in gasoline becomes the motion of the wheels, and when the car brakes, that motion becomes heat in the brakes. When a question asks where energy “went,” it went into another form.

Work and simple machines

In physics, work is done when a force moves an object:

work = force × distance

Simple machines, such as the lever, the ramp, and the pulley, make work easier. They let you use a smaller force over a longer distance. A ramp lets you push a heavy cart up to a loading dock instead of lifting it straight up.

Heat on the move

Heat always flows from a warmer object to a cooler one, in three ways. In conduction, heat passes through direct contact, as when a metal spoon heats up in hot soup. In convection, heat is carried by a moving liquid or gas, as when warm air rises from a radiator. In radiation, heat travels as waves through space, as the Sun’s warmth reaches the Earth.

Waves, electricity, and magnetism

A wave carries energy from place to place without carrying matter along with it. Sound is a wave that needs something to travel through, such as air or water; that is why there is no sound in empty space. Light is a wave that can travel through empty space. Radio waves, microwaves, visible light, and X-rays are all members of the same family, the electromagnetic spectrum.

Electricity is the flow of electric charge, usually through a wire. A circuit is the closed path it flows around; open the switch and the flow stops. Electricity and magnetism are closely linked: a current in a wire makes a magnetic field, and a moving magnet can make a current. Power plants and electric motors depend on that link.