The four stroke engine is an internal or heat combustion engine commonly used to power transport vehicles. The complete cycle requires four successive phases. During the intake stroke, a fuel-air mixture is drawn into the cylinder. It is then compressed by the upward movement of a piston. The mixture is ignited by a spark plug to initiate the combustion stroke, the source of power, and finally the spent gases are forced out during the exhaust stroke. The cycle repeats itself over and over again.

Et represents total mechanical energy, expressed in Joules.

This activity enables us to approach the concept of Gravitational Potential Energy (PE).

In our illustration, the original Potential Energy is arbitrarily chosen to be at the lowest point. (PE = 0 when h = 0). Potential Energy, expressed in Joules, is proportional to the mass (expressed in kilograms) and the altitude (expressed in meters) according to the equation:

PE = mgh

Only mechanical energy is represented. If one realizes  that Et is conserved, this is because the Potential Energy is transformed into Kinetic Energy (not shown in the animation) according to the unalterable relatioinship: Et = KE + PE.

A rigid body is in mechanical equilibrium when the sum of the forces, and torque, experienced by the system is zero.
Equilibria can be stable or unstable:

• Stable equilibrium: The response to a small perturbation in forces and torque that tends to restore the equilibrium.

• Unstable equilibrium: The response to a small perturbation in forces and torque that tends to move the system even farther away from its original point of equilibrium (actually, the system often returns to a new stable equilibrium after the disturbance!).

This animation permits the illustration of the forces involved. The position of the center of gravity (or center of mass) of the system, in relation to its support (balancing point) is the important element  here.

The four stroke engine is an internal or heat combustion engine commonly used to power transport vehicles. The complete cycle requires four successive phases. During the intake stroke, a fuel-air mixture is drawn into the cylinder. It is then compressed by the upward movement of a piston. The mixture is ignited by a spark plug to initiate the combustion stroke, the source of power, and finally the spent gases are forced out during the exhaust stroke. The cycle repeats itself over and over again.

We compute the work done by a constant force when a 500 Kg mass moves along different paths.
Energy (work !) is a scalar quantity. It can be positive, negative or zero.  The convention adopted here describes driving work when its value is positive and resistant work when its value is negative.
You will also see that the work done by a constant force is the same between two points, whatever the path you move along.

The total energy EM is not constant. The frictional damping force acts as a dissipative force, giving the movements characteristic of a damped oscillator. Click and drag the mass to set new initial conditions.

The total energy Em=Ec+Ep remains constant. It is shared between potential energy and kinetic energy in accordance with the energy conservation laws.
Click on the mass to modify its initial conditions.

Locks are required to allow ships to navigate on man-made waterways by regulating water levels. A compartment known as the flooding chamber is closed at each end by a mobile watertight gate. According to the laws of communicating vessels, valves are operated to balance the water levels inside the chamber and outside it in the upper or lower water course.