[Physics Class Notes] on Conservation of Mechanical Energy Pdf for Exam

Mechanical Energy

In the science of physical streams, mechanical energy is the sum of potential energy and kinetic energy. It is the macroscopic energy associated with a system. The conservation principle of mechanical energy states that if an isolated subject of a system is only to conservative forces then the mechanical energy is constant. 

If an object really moves in the direction which is opposite of a conservative net force, the energy which is potential will increase and if the speed of the object changes. The kinetic energy of the object also changes. 

Conservation of Mechanical Energy 

In all systems which are real, the force which is nonconservative. For example, frictional forces will be present but if they are of negligible magnitude the mechanical energy changes a little and its conservation is a useful approximation.

In the elastic collisions, the kinetic energy is conserved. but in the inelastic collisions, some mechanical energy may be converted into thermal energy. 

Many devices are used to convert mechanical energy to or from other forms of energy. An electric motor converts electrical energy to mechanical energy while an electric generator converts mechanical energy into electrical energy and a heat engine converts heat energy to mechanical energy.

 

Law of Conservation of Mechanical Energy

If we see that according to the principle of conservation of mechanical energy, in an isolated system the mechanical energy remains constant in time, provided the system is free of friction and other forces which are non-conservative. In any situation which is real and frictional forces and other non-conservative forces are present, the energy though cannot be destroyed or created in an isolated system, it can be converted to another energy form.

In a swinging pendulum which is subjected to the conservative gravitational force and frictional forces like air drag and friction, at the pivot are energies which are negligible. The passed energy is back and forth between kinetic and potential energy but never leaves the system. The pendulum reaches the greatest kinetic energy and least potential energy when in a vertical position. This is owing to the greatest speed and be the direction which is nearest to the Earth at this point. 

On the other hand, we can see that it will have its least kinetic and greatest potential energy at the extreme positions of its swing because it has zero speed and is at the farthest from the earth at these points. However, taking the frictional force into account,  there is a loss of mechanical energy with each swing because of the work which is negatively done on the pendulum by these forces which are non-conservative.

Principle 

Mechanical energy is the sum of the kinetic and potential energies in a system. The principle of the conservation of energy states that the total mechanical energy in a system is the sum of the potential and the kinetic energies which remain constant as long as the forces which are acting are conservative forces. 

We could easily use a circular definition and that says that a conservative force doesn’t change the total energy. Fiction, on the other hand, a force which is non-conservative because it acts to reduce the mechanical energy in a system. Note here that forces which are non-conservative do not always reduce the mechanical energy.