"Add or subtract" means positive or negative, and symbols can be used to represent the positive or negative of rational numbers, or to add and subtract logarithms in four operations. Symbol is not a single concept in middle school physics. Some symbols are equivalent to rational numbers in mathematics, while others are used to express the nature and direction of physical quantities. The situation is more complicated.

In mathematics, if |a|=2 (absolute value), the actual value of a is 2. Numbers greater than 0 are called positive numbers. Positive numbers are usually preceded by a "+"symbol, which can usually be omitted. There are countless kinds of positive numbers, including positive integers, positive fractions and positive irrational numbers. Numbers less than 0 are called negative numbers, and negative numbers and positive numbers represent quantities with opposite meanings. Negative numbers are marked with a negative sign "-"and a positive number.

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Besides mathematics and physics, when studying physical problems, symbols often encounter opposite physical phenomena, properties and processes. In order to reflect their differences, these quantities are distinguished by adding a plus sign and a minus sign in front of them, where the plus sign does not mean direction or size.

It means the opposite effect, such as the work is scalar, and the work of the force on the object is positive and negative, where positive and negative mean neither direction nor too small, which means the effect of the force. Force does positive work on an object, that is to say, force pushes the object to move, that is to say, force is power for the object; When a force does negative work on an object, it means that the force hinders the movement of the object, that is, the force is resistance to the object, and the energy is input or output.

Extended source: Baidu Encyclopedia-plus sign or minus sign