The first force we will investigate is that due to gravity, and we'll call it the gravitational force. We know that the acceleration due to gravity (if on Earth) is approximately g = 9.8 m/s2. The force, by Newton's Second Law is 

Fg = m g

where g is the acceleration due to gravity and m is the mass of the carriage. Let's add this to our diagram. 

Note that the force vector, labelled Fg, points downward, as this is the direction in which the gravitation force acts. 

Note also that this force is commonly called “weight”. This 'weight' (mg) is different from our everyday use of the word 'weight' (which is known in physics as 'mass').


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