Phi (capital_, lowercase_) is the twenty-first Greek letter. Greek lower-case letters, the upper left corner of the bend is open; and when used as symbols, they usually write, changing the shape of a reduced capital_ (Unicode: U + 03D5). Capital_is used for magnetic flux in physics.
When the inclined plane is self-locking, the inclination angle of the inclined plane is less than a certain value (usually related to the friction coefficient of the inclined plane), the phenomenon of self-locking will occur. When self-locking, an object cannot be propelled even with infinite force.