Newton's Second Law

"Change of motion is proportional to the force applied, and take place along the straight line the force acts."

Newton's second law for the gravity force - weight - can be expressed as

F = m g         (1)
where
F = force (weight)
m = mass
g = acceleration of gravity

The force caused by gravity - g - is called weight. Note! Mass - m - is a property.
The acceleration of gravity can be observed by measuring the change of velocity of a free falling object:

g = dv / dt         (2)
where
dv = change in velocity
dt = change in time

A dropped object accelerate to a speed of 9.81 m/s or 32.174 ft/s in one second.

Acceleration of Gravity in SI Units

g = 9.81 m/s²

Acceleration of Gravity in Imperial Units

g = 32.174 ft/s²

Velocity and Distance Traveled of a Free Falling Object

The velocity of a free falling object can be expressed as:

v = g t         (3)
where
v = velocity

The distance traveled by a free falling object can be expressed as:

s = 1/2 g t²         (4)
where
s = distance traveled by the object

The velocity and distance traveled by a free falling object:

Time (s)	Velocity				Distance
	m/s	km/h	ft/s	mph	m	ft
1	9.8	35.3	32.2	21.9	4.9	16.1
2	19.6	70.6	64.3	43.8	19.6	64.3
3	29.4	106	96.5	65.8	44.1	144.8
4	39.2	141	128.7	87.7	78.5	257.4
5	49.1	177	160.9	110	122.6	402.2
6	58.9	212	193.0	132	176.6	579.1
7	68.7	247	225.2	154	240.3	788.3
8	78.5	283	257.4	176	313.9	1,029.6
9	88.3	318	289.6	198	397.3	1,303.0
10	98.1	353	321.7	219	490.5	1,608.7

Note! The velocity is achieved without any aero-dynamical resistance (vacuum). The air resistance will be significant for higher velocities or for object with large surface area to mass ratio - feathers or similar.

Newton's First Law

"Every body continues in a state of rest or in a uniform motion in a straight line, until it is compelled by a force to change its state of rest or motion."

Newton's Third Law

"To every action there is always an equal reaction - if a force acts to change the state of motion of a body, the body offers a resistance equal and directly opposite to the force."

Common Expressions

• superimposed loads: kN/m²
• mass loads: kg/m² or kg/m³ 
• stress: N/mm²
• bending moment: kNm
• shear: kN

1 N/mm = 1 kN/m

1 N/mm² = 10³ kN/m²

1 kNm = 10⁶ Nmm