In statics, the sum of the forces is equal to zero

Lesson: In Statics the Sum of the Forces is Equal to Zero.

The term statics refers to an object or system in static equilibrium. The object or system is motionless because all of the forces balance to zero.

Newton's Second Law:

ΣF = Σ(ma) = m₁a₁ + m₂a₂ + m₃a₃ + • • • + m_na_n

The sum of the forces is equal to the mass times the acceleration. The 2nd law tells us that if the object or system is motionless, the acceleration is equal to zero. Therefore the sum of the vector forces must be equal to zero.

Example: Consider a table with four legs and a 200 kg object resting motionless in the center of the table:

    Object      Mass of object = 200 kg
______________
  |        |    Mass of table  = 100 kg
  |        |
  |        |    What forces act upon the
                 bottom of each table leg?

The force upward on the legs is found by balancing these forces in the equation given by Newton's Law:

ΣF = ma = 0 = - (200 kg X 9.81 m/s²) - (100 kg X 9.81 m/s²) + (Force_leg X 4)

Gravity is acting downward on the 200 kg object and the 100 kg table. Therefore, we may substitute the acceleration due to gravity (on Earth), g (9.81 m/s²), for a.

Force_leg = ((200 kg X 9.81 m/s²) + (100 kg X 9.81 m/s²)) / 4

The unit kg•m/s² is equivalent to the unit of force called a Newton (N). Thus when multiplying through, the kilograms cancel out.

Force_leg = (1962 N + 981 N) / 4

Force_leg = 735.75 N

This means that each leg exerts a downward force of 735.75 Newtons on the floor, and the floor simultaneously exerts the same force upward on each table leg.

Notice that it is critical that a consistent sign convention be followed throughout the entire analysis effort. The sign convention is typically chosen in more complex problems to ease the total amount of algebra necessary to analyze the equations in the x, y, and z axis. We chose positive values to mean upward forces and negative values to mean downward forces, but we could have also used the opposite, provided we were consistent.

Corrollary for torque

Lesson: In Statics the Sum of the Torques is Equal to Zero.

The sum of all rotational forces, or torques, denoted by the capital Greek letter tau ($\mathrm {T}$), is also zero. Commonly used units for torque are foot•pounds (ft•lb) and Newton•meters (N•m).

Newton's Second Law (applied to torques):

Σ$\mathrm {T}$ = Σ(ωI) = ω₁I₁ + ω₂I₂ + ω₃I₃ + • • • + ω_nI_n

The sum of the torques is equal to the rotational mass (or moment of inertia, I) times the angular acceleration (denoted by the lower case Greek letter omega, ω). The 2nd law tells us that if the object or system is motionless, the angular acceleration is equal to zero. Therefore the sum of the vector torques must be equal to zero.

Torques may also be calculated as forces times distances:

Σ$\mathrm {T}$ = Σ(Fd) = F₁d₁ + F₂d₂ + F₃d₃ + • • • + F_nd_n

Example: Consider a massless lever with two weights attached and a single massless support:

 |          +   Weight of object 1    = 10 lb
 |         /|   Distance from fulcrum = 10 ft
 |        / |   
 |      R/  |   Distance of support
W|     E/|S w1   from fulcrum         =  7 ft
A|    V/ |U    
L|   E/  |P     Weight of object 2    = 80 lb
L|  L/|  |P     Distance from fulcrum =  4 ft    
 |  / w2 |O    
 | /     |R     What forces act upon the 
 |/      |T      massless lever ? 
   FLOOR

First let's write the static torque equation for the system:

Σ$\mathrm {T}$ = Σ(Fd) = F₁d₁ + F_Sd_S + F₂d₂ = 0

Forces 1 and 2 are actually the weights of the two objects:

Σ$\mathrm {T}$ = Σ(Fd) = w₁d₁ + F_Sd_S + w₂d₂ = 0

Substitute in known values:

Σ$\mathrm {T}$ = Σ(Fd) = (-10 lb)(10 ft) + F_S(7 ft) + (-80 lb)(4 ft) = 0

Solve for F_S:

Σ$\mathrm {T}$ = Σ(Fd) = (-100 ft•lb) + F_S(7 ft) + (-320 ft•lb) = 0

F_S(7 ft) = (420 ft•lb)

F_S = (420 ft•lb)/(7 ft) = 60 lb

Thus, a force of 60 pounds acts upward on the lever from the support in order to balance the torques on the lever. Since the total weight on the lever is 90 pounds and 60 of those pounds are countered by the support, the remaining 30 pounds must act upward on the lever at the fulcrum in the lower, left corner.

Note that we didn't need to use the fulcrum as the point from which all distances are measured, we could have chosen any point along the lever. However, as with forces, a consistent sign convention must be used. In this case, positive may be used for clockwise (CW) torques measured from one point of view, with negative torques used for counterclockwise (CCW) torques measured from the same POV. The opposite system could also be used, so long as we were consistent.

Also note that the table example used previously stated that the object was in the center of the table. If not, then balancing the torques (in two directions) would result in more force supported by some table legs and less by others. If the object was directly over one of the legs, for example, then it's entire weight would be supported by that leg, in addition to one-fourth of the table's weight. The other legs would then only support one-fourth of the table's weight each.