Gravity

Gravity is a natural force which pulls objects of mass toward each other. The greater the mass is, the stronger the pull is. We know gravity best as the force which holds us to the earth. Gravity gives an object weight, and creates the pressure in air and water. Weight is directly related to mass and density. An object's inertia depends on mass, and centrifugal force depends on inertia. Pressure in air and water drive suction, flotation, and other forces which create movement. Consequently, gravity is present in the workings of many machines.

Heat

Heat can increase pressure, decrease density, ignite chemical reactions. Heat is the driving force behind steam power and the combustion engine, two of the most significant advances which ushered in the industrial age.

Magnetism

Magnetism is a force which can attract or repulse objects within its field. Unlike gravity, its effect is only...