Belajar Fisika - Gaya Bollard

Bollards ini biasa terjadi pada dermaga dan digunakan ketika mooring kapal dan perahu. Bollards is common on quays and are used when mooring ships and boats.

Kekuatan usaha dalam tali dapat dihitung

The effort force in a rope can be calculated

S = F e^-μα         (1)
where
S = effort force in the rope (N)
F = load (N)
e = 2.718..
μ = friction coefficient (approximately 0.3 - 0.5 is common for a rope around a steel or cast iron bollard) 
α = angle where the rope is in contact with the bollard (radians)

Angle - turns, degrees and radians

• 1/4 turn : 90 degrees => α = 1/2 π
• 1/2 turn : 180 degrees => α = π
• 1 turn : 360 degrees => α = 2π
• 2 turns : 720 degrees => α = 4π

Example - A rope with one turn around the bollard 

Contoh - Sebuah tali dengan satu putaran sekitar tonggak

With a friction coefficient of 0.5 the effort force in the rope can be calculated as 

Dengan koefisien gesekan sebesar 0,5 upaya paksa dalam tali dapat dihitung sebagai

S = F e^{-0.5 2π}
    = 0.043 F (N)

As we can see - one turn around the bollard reduces the required effort force to less than 5% of the load.

Example - Shoring a ship

The retardation (negative acceleration) of a ship arriving at quay with velocity 0.05 m/s and stopped within 2 seconds, can be calculated as

a = dv/dt         (2)

= (0.05 m/s) / (2 s)
= 0.025 (m/s²)

With a mass of 20000 kg the required force F in the rope from the ship can be calculated as

F = m a         (3)

= (20000 kg) (0.025 m/s²)
= 500 N

The required effort force with a half turn around a bollard with a friction coefficient of 0.4 can be calculated as

S = F e^{-0.5 2π}

= (500 N) e^{-0.4 π}
= 142 (N)  

The Effort Force - Load Force ratio for various rope angles are indicate in the chart below:

• friction coefficient 0.5