# perpendicular to the direction of motion

#### VicNicSor

##### Banned
pitch
a swaying or oscillation of a ship, aircraft, or vehicle around a horizontal axis perpendicular to the direction of motion
oxford dictionaries

The horizontal axis here is the same as the direction of motion, am I right?
Thank you.

• #### Biffo

##### Senior Member
No. It cannot be both perpendicular and in the same direction. That is a contradiction.

Perpendicular to means at right-angles to.

#### VicNicSor

##### Banned
No. It cannot be both perpendicular and in the same direction. That is a contradiction.
I don't understand

Imagine, the axis H is the the direction of motion of a ship. The ship lies at anchor, swaying. So it moves perpendicularly to the axis H at any angle shown on the purple circle on the axis H (like the ship's snout slides on the circle's surface). Am I right?

#### amatriciana

##### Senior Member
VikNikSor, there is one horizontal axis which is given by the direction of motion of the ship, then there is the vertical axis which is perpendicular to this and goes upwards, and there is precisely one line (axis) that is perpendicular to both of these axes -- namely the horizontal axis that is at a right angle to the direction of motion. This is the horizontal axis used to define pitch.

Not to criticize the Oxford dictionary's definers too much, I personally think that the definition would have been clearer if it said "...around the horizontal axis perpendicular to the direction of motion", since this axis is uniquely determined.

EDIT: In the diagram you linked to, yes the horizontal axis used to define pitch is H, and the axis corresponding to the direction of the ship is the sight axis S. Pitch describes the movement of the ship if you think of it like a see-saw.

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#### entangledbank

##### Senior Member
The direction of motion is one horizontal axis, and there is another one perpendicular to it, making a cross in the horizontal plane. The third axis is vertical. All three axes are perpendicular to each other. The three forms of motion around these axes are called pitch, yaw, and roll, though I can never remember which is which (and have to make embarrassing hand gestures to try and visualize it).

cross-posted

#### VicNicSor

##### Banned
Ah, I thought perpendicular to the direction of motion refered to the movements of a ship(swaying or oscillation), not to the axis which lies along the direction of motion of the ship.
Thank you, all.

#### Andygc

##### Senior Member
Not to criticize the Oxford dictionary's definers too much, I personally think that the definition would have been clearer if it said "...around the horizontal axis perpendicular to the direction of motion", since this axis is uniquely determined.
In this case, criticism is not merited. The use of the indefinite article is plainly wrong at first glance, but in fact there is an infinite number of horizontal axes perpendicular to the direction of travel.

The three forms of motion around these axes are called pitch, yaw, and roll, though I can never remember which is which
Pitch: bow goes up and down.
Yaw: bow swings port and starboard.
Roll: tables tip from side to side and drinks and food end up in the scuppers.

All three together in synchrony but out of phase: Dutch roll ---> projectile vomiting.

#### RM1(SS)

##### Senior Member
The direction of motion is one horizontal axis, and there is another one perpendicular to it, making a cross in the horizontal plane. The third axis is vertical. All three axes are perpendicular to each other. The three forms of motion around these axes are called pitch, yaw, and roll, though I can never remember which is which (and have to make embarrassing hand gestures to try and visualize it).
Roll - movement around the direction of motion.
Pitch - movement around the other horizontal axis.
Yaw - movement around the vertical axis.

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