Motion

In physics, motion is the change in position of an object with respect to a reference point over a given time. It helps us understand how things move and behave in the world around us. Motion is described using terms like displacement, distance, velocity, acceleration, speed, and frame of reference.

When an object does not change its position relative to a reference point, it is said to be at rest or stationary. However, in modern physics, everything in the universe can be considered to be in motion because there is no single absolute frame of reference to judge motion against.

Motion applies to many different things, from everyday objects and matter particles to radiation, curvature, and even space-time. Understanding motion helps scientists and engineers design everything from cars to space rockets.

Equations of motion

The study of motion in physics can be split into two main areas. One area, called kinematics, looks at how objects move without thinking about why they move. The other area, called dynamics, studies how forces affect motion. These ideas help us understand everything from how a ball rolls to how planets orbit the sun.

Laws of motion

Classical mechanics is used to describe the motion of big objects moving slower than light, like cars, planets, and even parts of machines. It helps us understand how things move and is based on three important rules made by Sir Isaac Newton a long time ago.

These rules are:

1. An object will keep doing what it is doing — staying still or moving — unless something pushes or pulls it.
2. The force on an object changes how it moves, and this can be calculated using mass and how fast the object speeds up or slows down.
3. When one object pushes another, the second object pushes back with an equal force in the opposite direction.

These rules help us understand how things move in space and on Earth.

First law:	In an inertial reference frame, an object either remains at rest or continues to move in a straight line at a constant velocity, unless acted upon by a net force.
Second law:	In an inertial reference frame, the vector sum of the forces F on an object is equal to the mass m of that object multiplied by the acceleration a of the object: F → = m a → {\displaystyle {\vec {F}}=m{\vec {a}}} . If the resultant force F → {\displaystyle {\vec {F}}} acting on a body or an object is not equal to zero, the body will have an acceleration a {\displaystyle a} that is in the same direction as the resultant force.
Third law:	When one body exerts a force on a second body, the second body simultaneously exerts a force equal in magnitude and opposite in direction onto the first body.

Orders of magnitude

Humans, like everything else in the universe, are always moving^: 8–9 . We can easily see our body parts moving and walking, but there are many other ways we move that are harder to notice. These hidden movements need special tools to see them clearly. Two big reasons make these movements hard for us to feel: Newton's laws of motion stop us from feeling motion when we're attached to something, and we often lack a good way to see that we're moving. Even smaller movements are too tiny for our senses to detect normally.

The universe itself is always changing. Space, called spacetime, is expanding, which means everything in the universe is stretching out like a rubber band. This was shown by Edwin Hubble, who found that galaxies are moving away from Earth.

Our galaxy, the Milky Way, moves through space at about 600 kilometres per second. The Sun, which is part of the Milky Way, moves in a circle around the galaxy's center. The Earth spins on its axis, giving us day and night, and it also orbits the Sun, completing a full circle in one year. Inside our bodies, the heart pumps blood, and muscles help move food through our digestive system. Even our cells and the tiny particles inside them are always moving.

Light

Main article: Speed of light

Light travels at a speed of 299,792,458 meters per second in a vacuum. This speed is the same for all massless particles and fields, and it is the fastest speed at which energy, matter, information, or any effect can travel. No physical system can go faster than this speed.

The speed of light is always the same, no matter where you are or how fast you are moving. This makes it a key measurement for speed and a basic rule of nature. In 2019, the speed of light was officially defined as exactly 299,792,458 meters per second as part of updates to how we measure units around the world.

Superluminal motion

See also: Superluminal motion

Sometimes, things in space can seem to move faster than the speed of light. For example, bursts of energy from objects that may contain black holes can appear to move very quickly. This happens because of how we see distant objects. Light takes time to travel long distances, so when we look at something far away, we are seeing it as it was in the past. Because of this time delay, the movement can look faster than it really is. In reality, nothing can travel faster than the speed of light.

Types of motion

Motion can happen in many different ways. One common type is simple harmonic motion, where an object moves back and forth, like a pendulum. Linear motion is when something moves in a straight line, such as a car driving down a road. Other types include circular motion, like a Ferris wheel, and projectile motion, which is how things fly through the air when thrown.

There are also special kinds of motion such as Brownian motion, where tiny particles move randomly, and rolling motion, like a bicycle wheel moving forward. Objects can also have oscillatory or vibratory motion, swinging or shaking from side to side. Sometimes, objects can do more than one type of motion at the same time, combining different movements together.

Fundamental motions

Motion in physics is all about how things change their position over time. There are several basic types of motion, such as moving in a straight line, moving in a circle, swinging back and forth, and moving like waves.

Here are some key kinds of motion:

• Linear motion
• Circular motion
• Oscillation
• Wave
• Relative motion
• Rotary motion