Motion

In physics, motion is when an object's position changes over time. It helps us understand how things move in the world. We describe motion using words like displacement, distance, velocity, acceleration, speed, and frame of reference.

If an object stays in the same place, we say it is at rest. But in modern physics, everything is moving because there is no one perfect point to measure motion from.

Motion is important for many things, from everyday objects and matter particles to radiation, curvature, and space-time. Learning about motion helps scientists and engineers make things like cars and space rockets.

Equations of motion

The study of motion in physics has two main parts. One part, called kinematics, looks at how objects move. It does this without thinking about why they move. The other part, called dynamics, studies how forces change motion. These ideas help us understand many things. For example, they help us understand how a ball rolls and how planets orbit the sun.

Laws of motion

Classical mechanics helps us understand how big objects move, like cars, planets, and parts of machines. It uses three important rules made by Sir Isaac Newton long 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. We can see our body parts moving and walking, but there are other ways we move that are harder to notice. These hidden movements need special tools to see them clearly.

The universe is always changing. Space, called spacetime, is expanding. This was shown by Edwin Hubble, who found that galaxies are moving away from Earth.

Our galaxy, the Milky Way, moves through space. The Sun moves in a circle around the galaxy's center. The Earth spins, giving us day and night, and it 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 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 is the fastest speed at which anything can go. Nothing can travel faster than light.

The speed of light is always the same, no matter where you are or how fast you are moving. It is a key measurement for speed and a basic rule of nature. In 2019, the speed of light was officially set at exactly 299,792,458 meters per second.

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 look like they are moving 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 about how things change where they are over time. There are several simple types of motion.

Here are some key kinds of motion:

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