The speed of an object is the magnitude of the change of its position.
Speed of an object is the magnitude of the change of its position.
The second sentence seems wrong to me; I think since the "speed" in the second sentence is a property that belongs to a specific object, it is definite and should be used with the definite article. Hence, it follows that the following sentences without "the" are all incorrect.
The length of an object is how long the object is.
Length of an object is how long the object is.
The demand of a product is the number of consumers that are willing to purchase the product.
Demand of a product is the number of consumers that are willing to purchase the product.
However, I read a sentence on Wikipedia
Demand for a specific item is a function of an item's perceived necessity, price, perceived quality, convenience, available alternatives, purchasers' disposable income and tastes, and many other factors.
Is this sentence grammatically incorrect? If so, why? If not, what is the difference in meaning of the two types of sentences? What would an additional "the" do?