In fact, the force here should be decomposed into the direction of motion of the object and the direction perpendicular to the object.

Work is only done in the direction of the object's motion (it is easy to understand if LZ learns vectors, is W the vector inner product of FS? )

This requires some knowledge of mathematics. I don't know if you have learned derivative or definite integral (some cities seem to want to learn).

Remember that the component of F (resultant force) in the direction of motion of an object is F.

By momentum theorem (the following d is a differential symbol)

Fdt=mdv, so f=mdv/dt=dp/dt.

Yuan Gong DW = FDL = DPDL/DT = VDP = VD (MV) = D (MV 2/2).

In this way, the expression of kinetic energy is obtained (the above formula is the differential form of kinetic energy theorem, that is to say, kinetic energy is a state quantity)

E=mv^2/2

This is a relatively standard derivation.

Of course, there are more stringent deductions. LZ, read college physics books and learn more.