How to Represent Equation with Variables in Relational Database
Here's one suggestion. Since you used the term component, I'll stick with that, but maybe theres a better term?

CREATE TABLE COMPONENTS
( COMPONENT_ID INT NOT NULL PRIMARY KEY
, ABBREVIATION CHAR(1) NOT NULL -- Example 'E'
, DESCRIPTION VARCHAR(50) NOT NULL -- Example 'Energy'
);

CREATE TABLE EQUATION_COMPONENT
( EQUATION_ID ...
, COMPONENT_ID ...
,    PRIMARY KEY ( EQUATION_ID, COMPONENT_ID )
,    FOREIGN KEY ( EQUATION_ID ) REFERENCES EQUATIONS ( EQUATION_ID )
,    FOREIGN KEY ( COMPONENT_ID ) REFERENCESCOMPONENTS ( COMPONENT_ID )
);


If you, for example, want to find all equations concerning both Mass and Energy:

SELECT EQUATION_ID
FROM EQUATION_COMPONENT
WHERE COMPONENT_ID IN ( id:s for Mass and Energy )
GROUP BY EQUATION_ID
HAVING COUNT(COMPONENT_ID) = 2