Master rescue mechanics, including force, friction, and pulley systems like 3:1, 4:1, and 5:1 for efficient operations.
Physics is the study of the nature of matter and its properties. It involves accumulating extensive knowledge and emphasizing experiments to verify the properties of matter. By understanding the essence of matter, we can uncover the causes and principles behind various phenomena. This section focuses on the fundamentals of mechanics and motion within physics. Understanding these basics is crucial for handling equipment and rope rescue operations.
Force
A force is something that causes a change in the position or shape of an object. In mechanics, forces such as gravity, tension, and friction are represented visually using arrows. These arrows help us understand where, how much, and in what direction a force is acting. An arrow is also known as a “vector.”
Forces are expressed through the length and direction of arrows:
- Magnitude of the force: Represented by the length of the arrow.
- Direction of the force: Indicated by the direction of the arrow.
- Point of application: The starting point of the arrow.
These are known as the three elements of force.
Resultant Force and Component Forces
Forces can be combined or decomposed. As shown in the diagram, a force ( F ) can be decomposed into ( F_1 ) and ( F_2 ).
In this context:
- ( F ) is the “resultant force.”
- ( F_1 ) and ( F_2 ) are the “component forces.”
For example, pulling a load with a force ( F ) alone has the same effect as pulling with two smaller forces, each being less than ( F ).
The resultant force varies depending on the angle at which forces are applied. As the angle increases, the resultant force decreases.
- When two forces are in a straight line and in the same direction, the resultant force ( F = F_1 + F_2 ).
- When they are in opposite directions, the resultant force ( F = F_2 – F_1 ).
Magnitude of Force
The unit of force is the Newton (N). You might wonder why not kg or g? These are units of mass. A Newton is defined as mass times gravitational acceleration: ( 1.0 \text{ kg} \times 9.8 \text{ m/s}^2 = 9.8 \text{ N} ). On the Moon, where gravity is one-sixth that of Earth’s, the force in Newtons would be one-sixth of that on Earth. While Newtons vary with location, mass in grams remains constant.
Mechanical Advantage
Mechanical advantage is based on the effect of pulleys. A mechanical advantage system reduces the effort needed to lift a load.
Pulley Effect
Consider a load suspended by a rope passing through a fixed pulley. The load’s weight is evenly distributed between both sides of the rope. If the load is 50 kg, a person must hold the rope with a force equivalent to 50 kg on their side. The fixed pulley supports a total weight of 100 kg.
In a movable pulley system, the load is shared between two sections of the rope, effectively halving the force needed to lift the load. However, the distance you must pull the rope is doubled.
Note: This theory assumes 100% efficient pulleys, which do not exist in reality. Actual pulley efficiency ranges from 50% to 98%.
Load Changes Due to Angle
The table below shows how load changes with angle:
| Angle (°) | Load (%) |
|---|---|
| 0 | 50.0 |
| 30 | 51.8 |
| 60 | 57.7 |
| 90 | 70.7 |
| 120 | 100.0 |
As shown, during rescue operations, it is crucial to consider the resultant and component forces. Larger angles exert unnecessary loads, so aim for angles within 90 degrees.
Handy Protractor
When you spread your thumb and index finger wide, the angle between them is about 90 degrees, which is generally considered the maximum angle. If you spread all your fingers wide, the angle between your thumb and pinky finger is about 120 degrees, known as the “limit angle.”
Friction
Friction is the resistance to motion between two contacting surfaces. It helps control the descent during rescues by providing resistance through devices like brake bar racks. However, friction can also hinder by absorbing energy when lifting loads. For example, a rope that bends 90 degrees around a building edge doubles the effort needed to lift the load due to increased friction.
Friction converts energy into heat, absorbed by the contacting surfaces and then dispersed into the air. Smaller objects in contact generate higher temperatures due to limited mass and surface area to dissipate heat.
Typical Friction Rates
- Brake bar rack: 75% – 98%
- Carabiner (180° bend): 40% – 60%
- DCD: 90% – 95%
- Pulley (ball bearing): 5%
- Pulley (bronze bushing): 30%
- Rescue 8 DCD: 80% – 95%
Mechanical Advantage Systems
Mechanical advantage systems combine ropes and pulleys to move heavy objects with less effort.
1:1 System
Requires pulling with a force equal to the load’s weight. More effort is needed due to friction when ropes pass through pulleys or rough terrain.
2:1 System
Reduces the effort to half the load’s weight, but the pulling distance is doubled. It requires only one rope and one pulley.
3:1 System
Commonly used in rescue operations, requiring fewer materials and relatively simple to set up. Known as the “Z-rig.”
4:1 System
Uses two double pulleys, allowing one rescuer to lift with four times the force.
5:1 System
Similar to the 3:1 system and frequently used in rescues. It allows three rescuers to lift vertically.
Mastering the 3:1, 4:1, and 5:1 systems is essential for rescue operations.
Demonstration Videos
- 3:1 System
- 4:1 System
- 5:1 System
References National Fire Protection Association (NFPA)
