In physics, motion is a change in position of an object with respect to time. Motion is typically described in terms of displacement, distance.

Scalar, velocity, acceleration, time and speed. Motion of a body is observed by attaching a frame of reference to an observer and measuring the change in position of the body relative to that frame.

If the position of a body is not changing with the time with respect to a given frame of reference the body is said to be at rest, motionless, immobile, stationary, or to have constant (time-invariant) position. An object's motion cannot change unless it is acted upon by a force, as described by Newton first law. 

 

Momentum is a quantity which is used for measuring motion of an object. An object's momentum is directly related to the object's mass and velocity, and the total momentum of all objects in an isolated system (one not affected by external forces) does not change with time, as described by the law of conservation of momentum.

As there is no absolute frame of reference, absolute motion cannot be determined. Thus, everything in the universe can be considered to be moving.

More generally, motion is a concept that applies to objects, bodies, and matter particles, to radiation, radiation fields and radiation particles, and to space, its curvature and space-time. One also speak of motion of shapes and boundaries.

Thus, the term motion in general signifies a continuous change in the configuration of a physical system. For example, one can talk about motion of a wave or about motion of a quantum particle, where the configuration consists of probabilities of occupying specific positions.

Main types of motion

Linear motion  is the most basic of all motions. Linear motion is the type of motion in which all parts of an object move in the same direction and each part moves an equal distance. Linear motion is measured by speed and direction. Distance travelled by an object per unit of time is called velocity. Example: a moving car.

Rotary motion is motion in a circle. This type of motion is the starting point of many mechanisms. Example: a spinning wheel.

Reciprocating motion is back and forth motion. Example: the up and down motion of a yo-yo.

Irregular motion is motion which has no obvious pattern to its movement. Example: a flying bee.Uniform motion is motion at a constant speed in a straight line. Example: a rolling ball.Very often, objects move by complicated motion. Complicated motion can be broken down into simpler types of motion. An example of complicated motion is a flying Frisbee. The movement of a Frisbee consists of a linear motion and a rotary motion.

Frictional and gravitational forces affect the motion of objects. Gravitational and frictional forces slow down the motion. The frictional force which slows a body in motion is called kinetic friction. When two objects slide past one another, for example, when you push a box across the floor, part of the energy of your pushing moves the box, and part of the energy is lost to kinetic friction. The less textured the surface, the farther the object will move. That happens because smooth surfaces have less frictional force. Friction is affected by the texture of both the surface and the moving object.

Distance and displacementDistance and Displacement are two quantities that may seem to mean the same thing yet have distinctly different definitions and meanings.Distance is a scalar quantity that refers to "how much ground an object has covered" during its motion.Displacement is a vector quantity that refers to "how far out of place an object is"; it is the object's overall change in position.To test your understanding of this distinction, consider the motion depicted in the diagram below. A physics teacher walks 4 meters East, 2 meters South, 4 meters West, and finally 2 meters North.

Speed and velocitySpeed and velocityJust as distance and displacement have distinctly different meanings (despite their similarities), so do speed and velocity. Speed is a scalar quantity that refers to "how fast an object is moving." Speed can be thought of as the rate at which an object covers distance. A fast-moving object has a high speed and covers a relatively large distance in a short amount of time. Contrast this to a slow-moving object that has a low speed; it covers a relatively small amount of distance in the same amount of time. An object with no movement at all has a zero speed.

Velocity as a vector quantityVelocity is a vector quantity that refers to "the rate at which an object changes its position." Imagine a person moving rapidly - one step forward and one step back - always returning to the original starting position. While this might result in a frenzy of activity, it would result in a zero velocity. Because the person always returns to the original position, the motion would never result in a change in position.

Since velocity is defined as the rate at which the position changes, this motion results in zero velocity. If a person in motion wishes to maximize their velocity, then that person must make every effort to maximize the amount that they are displaced from their original position. Every step must go into moving that person further from where he or she started. For certain, the person should never change directions and begin to return to the starting position

Velocity is a vector quantity. As such, velocity is direction aware. When evaluating the velocity of an object, one must keep track of direction. It would not be enough to say that an object has a velocity of 55 mi/hr. One must include direction information in order to fully describe the velocity of the object. For instance, you must describe an object's velocity as being 55 mi/hr, east. This is one of the essential differences between speed and velocity. Speed is a scalar quantity and does not keep track of direction; velocity is a vector quantity and is direction aware.

The average speed during the course of a motion is often computed using the following formula:

In contrast, the average velocity is often computed using this formula.

Acceleration

Acceleration, in physics, is the rate of change of velocity of an object. An object's acceleration is the net result of any and all forces acting on the object, as described by Newton's Second Law.  The SI unit for acceleration is the meter per second squared(m/s2). Accelerations are vector quantities (they have magnitude and direction) and add according to the parallelogram law. As a vector, the calculated net force is equal to the product of the object's mass (a scalar quantity) and the acceleration.

For example, when a car starts from a standstill (zero relative velocity) and travels in a straight line at increasing speeds, it is accelerating in the direction of travel. If the car turns there is an acceleration toward the new direction. For this example, we can call the accelerating of the car forward a "linear acceleration", which passengers in the car might experience as force pushing them back into their seats. When changing directions, we might call this "non-linear acceleration", which passengers might experience as a sideways force.

If the speed of the car decreases, this is an acceleration in the opposite direction of the direction of the vehicle, sometimes called deceleration. Passengers may experience deceleration as a force lifting them away from their seats. Mathematically, there is no separate formula for deceleration, as both are changes in velocity. Each of these accelerations (linear, non-linear, deceleration) might be felt by passengers until their velocity and direction match that of the car.

An object's average acceleration over a period of time is its change in velocity  divided by the duration of the period . Mathematically,

Example of daily life (motion)

Made by the student of

the Doon Global school:

Aditi SharmaClass:9”a”

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