CogKnoSys

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Physical

Motion

Motion is the change in an object's position over time, measured against a chosen reference frame.

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The idea

In physics, motion is the change in an object's position over time, measured against a reference frame, which is the background or viewpoint you measure from. If you sit on a moving train, you are at rest compared with your seat but moving quickly compared with the ground outside. An object that does not change position relative to a chosen frame is said to be at rest. Because modern physics holds that there is no absolute frame of reference, everything in the universe can be considered to be in motion.

Scientists describe motion with vector quantities, which have both a size and a direction. Displacement gives the distance and direction an object has moved, velocity gives its speed and direction, and acceleration describes how its velocity changes. The branch of physics that describes motion without asking what causes it is called kinematics, while the branch that studies forces and how they change motion is called dynamics.

The idea of motion reaches far beyond rolling balls and flying planes. Physicists also speak of the motion of waves, fields, radiation and even quantum particles, whose positions are described as probabilities. In general, motion means any continuous change in the position or arrangement of a physical system in space.

Check yourself

Question 1 of 2

What is the branch of physics that describes how things move without asking what causes the movement?

Pick one. The first pick counts.

Composition

what it's made of

  • Reference frame — The background or viewpoint against which position is measured.
  • Position — Where an object is, relative to the reference frame.
  • Time — Motion is a change in position over a given time.
  • Displacement — The distance and direction an object has moved.
  • Velocity — An object's speed together with its direction.
  • Acceleration — How an object's velocity changes over time.

Connection

how it relates

Under Physical

Related

  1. Muscle Contractionstrength 60 of 100

    Muscle contraction pulls on bones and makes the body move.

  2. Ocean Currentsstrength 50 of 100

    Ocean currents are continuous, directed movements of seawater.

  3. Thermal Energystrength 50 of 100

    Thermal energy comes from the random motion of the tiny particles that make up matter.

  4. Convection Currentsstrength 40 of 100

    In convection, warmer fluid rises and cooler fluid sinks, so heat is carried by the fluid's motion.

Change

how and why it evolves

Past
Isaac Newton described motion against a fixed, absolute background. Later physics concluded that no absolute frame of reference exists, so Newton's absolute motion cannot be determined.
Now
Physicists describe motion with vectors such as displacement, velocity and acceleration, and they apply the idea to waves, fields and quantum particles as well as to everyday objects.
Next
Physicists will likely keep refining how motion is described at very small scales and very high speeds, where quantum physics and relativity replace everyday rules.

Knowledge you can walk. Read the idea, then step up, across or down.

Motion

Motion is the change in an object's position over time, measured against a chosen reference frame.

Learn, discuss, create, buy or invest

The idea

In physics, motion is the change in an object's position over time, measured against a reference frame, which is the background or viewpoint you measure from. If you sit on a moving train, you are at rest compared with your seat but moving quickly compared with the ground outside. An object that does not change position relative to a chosen frame is said to be at rest. Because modern physics holds that there is no absolute frame of reference, everything in the universe can be considered to be in motion.

Scientists describe motion with vector quantities, which have both a size and a direction. Displacement gives the distance and direction an object has moved, velocity gives its speed and direction, and acceleration describes how its velocity changes. The branch of physics that describes motion without asking what causes it is called kinematics, while the branch that studies forces and how they change motion is called dynamics.

The idea of motion reaches far beyond rolling balls and flying planes. Physicists also speak of the motion of waves, fields, radiation and even quantum particles, whose positions are described as probabilities. In general, motion means any continuous change in the position or arrangement of a physical system in space.

Check yourself

2 quick questions, instant feedback.