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For the class of linkages, see straight line mechanism.

Classical mechanics |
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Core topics |

**Linear motion** (also called rectilinear motion^{[1]}) is motion along a straight line, and can therefore be described mathematically using only one spatial dimension. The linear motion can be of two types: uniform linear motion with constant velocity or zero acceleration; non uniform linear motion with variable velocity or non-zero acceleration. The motion of a particle (a point-like object) along a line can be described by its position , which varies with (time). An example of linear motion is an athlete running 100m along a straight track.^{[2]}

Linear motion is the most basic of all motion. According to Newton's first law of motion, objects that do not experience any net force will continue to move in a straight line with a constant velocity until they are subjected to a net force. Under everyday circumstances, external forces such as gravity and friction can cause an object to change the direction of its motion, so that its motion cannot be described as linear.^{[3]}

One may compare linear motion to general motion. In general motion, a particle's position and velocity are described by vectors, which have a magnitude and direction. In linear motion, the directions of all the vectors describing the system are equal and constant which means the objects move along the same axis and do not change direction. The analysis of such systems may therefore be simplified by neglecting the direction components of the vectors involved and dealing only with the magnitude.^{[4]}

Neglecting the rotation and other motions of the Earth, an example of linear motion is the ball thrown straight up and falling back straight down.

Main article: Displacement (vector)

The motion in which all the particles of a body move through the same distance in the same time is called translatory motion. There are two types of translatory motions: rectilinear motion; curvilinear motion. Since linear motion is a motion in a single dimension, the distance traveled by an object in particular direction is the same as displacement.^{[5]} The SI unit of displacement is the metre.^{[6]}^{[7]} If is the initial position of an object and is the final position, then mathematically the displacement is given by:

The equivalent of displacement in rotational motion is the angular displacement measured in radian. The displacement of an object cannot be greater than the distance. Consider a person travelling to work daily. Overall displacement when he returns home is zero, since the person ends up back where he started, but the distance travelled is clearly non zero.

Main article: velocity

Velocity is defined as the rate of change of displacement with respect to time.^{[8]} The SI unit of velocity is or metre per second.^{[9]}

The average velocity is the ratio of total displacement taken over time interval . Mathematically, it is given by:^{[10]}^{[11]}

where:

is the time at which the object was at position

is the time at which the object was at position

The instantaneous velocity can be found by differentiating the displacement with respect to time.

Main article: speed

Speed is the absolute value of velocity i.e. speed is always positive. The unit of speed is metre per second.^{[12]} If is the speed then,

The magnitude of the instantaneous velocity is the instantaneous speed.

Main article: acceleration

Acceleration is defined as the rate of change of velocity with respect to time. Acceleration is the second derivative of displacement i.e. acceleration can be found by differentiating position with respect to time twice or differentiating velocity with respect to time once.^{[13]} The SI unit of acceleration is or metre per second squared.^{[14]}

If is the average acceleration and is the average velocity over the time interval then mathematically,

The instantaneous acceleration is the limit of the ratio and as approaches zero i.e.,

Main article: jerk (physics)

The rate of change of acceleration, the third derivative of displacement is known as jerk.^{[15]}The SI unit of jerk is . In the UK jerk is also called as jolt.

Main article: jounce

The rate of change of jerk, the fourth derivative of displacement is known as jounce.^{[16]}The SI unit of jounce is which can be pronounced as *metres per quartic second*.

Main article: Equations of motion

In case of constant acceleration, the four physical quantities acceleration, velocity, time and displacement can be related by using the Equations of motion^{[17]}^{[18]}^{[19]}

here,

is the initial velocity

is the final velocity

is the acceleration

is the displacement

is the time

These relationships can be demonstrated graphically. The gradient of a line on a displacement time graph represents the velocity. The gradient of the velocity time graph gives the acceleration while the area under the velocity time graph gives the displacement. The area under an acceleration time graph gives the change in velocity.

Main article: Linear-rotational analogs

Linear motion | Rotational motion |
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Displacement = | Angular displacement = |

Velocity = | Angular velocity = |

Acceleration = | Angular acceleration = |

Mass = | Moment of Inertia = |

Force = | Torque = |

- Linear actuator
- Reciprocating motion
- Centripetal force
- Uniformly accelerated linear motion
- Inertial frame of reference
- Linear bearing
- Mechanics of planar particle motion
- Motion graphs and derivatives
- Rectilinear propagation

**^**Resnick, Robert and Halliday, David (1966),*Physics*, Section 3-4**^**"Basic principles for understanding sport mechanics".**^**"Motion Control Resource Info Center". Retrieved 19 January 2011.**^**"Basic principles for understanding sport mechanics".**^**"Distance and Displacement".**^**"SI Units".**^**"SI Units".**^**"Speed & Velocity".**^**"SI Units".**^**"Average speed and average velocity".**^**"Average Velocity, Straight Line".**^**"Speed and velocity".**^**"Acceleration".**^**"SI Units".**^**"What is the term used for the third derivative of position?".**^**"What is the term used for the third derivative of position?".**^**"Equations of motion".**^**"Description of Motion in One Dimension".**^**"What is derivatives of displacement?".**^**"Linear Motion vs Rotational motion".

- Resnick, Robert and Halliday, David (1966),
*Physics*, Chapter 3 (Vol I and II, Combined edition), Wiley International Edition, Library of Congress Catalog Card No. 66-11527 - Tipler P.A., Mosca G., "Physics for Scientists and Engineers", Chapter 2 (5th edition), W. H. Freeman and company: New York and Basing stoke, 2003.