— 1. One Particle Conservation Theorems —
— 1.1. Linear considerations —
Let be a constant vector such as . Then . Hence is constant.
The previous derivation shows that if is null along a given direction (), then the momentum component along that direction is a constant quantity.
— 1.2. Rotational considerations —
Definition 1 Given a reference point one can define the angular momentum of a particle relative to that point.

The angular momentum is a measure of the amount a rotation that a particle has relative to a given point. For example if a particle moves in a straight line relative to point its angular momentum is since is parallel to and the vector product of two parallel vectors is by definition (Do you see why? If not go to this post). see the definition of vector product and prove the previous statement.
Just like we had forces in rectilinear motion to account for the variations of momentum one has the torque in curvilinear motion to account for the variation of angular momentum
Definition 2 Given a reference point one can define the torque of a particle relative to that point.

Given the definitions of angular momentum it follows that
It is by definition.
Hence .
Thus if , and is constant in time.
— 1.3. Energetic considerations —
Let’s consider a particle moves under the action of a force and evolves from mechanical state to mechanical state .
Definition 3 The amount work done by a force against the inertial mass along the trajectory that leads from mechanical state to mechanical state is

It is
Hence, the integrand function for is a total differential (note that we assumed that doesn’t depend explicitly on time nor on the velocity). Hence
If depends uniquely on the mechanical states and and not on the trajectory that connects them one says that derives from a potential. In this case the force can be written as the negative gradient of a function that is said to be the potential energy function:
It follows
Let us define
Definition 4 The mechanical, , energy of a mechanic system is the sum of the kinetic energy and the potential energy

Now
and
For it is
Finally
If isn’t an explicit function of time it follows that and the mechanical system is said to be conservative.