1. The stability of an object refers to its ability to maintain its original position when tilted and released.
- A stable object is difficult to topple over and will return to its original position when tilted and released.
- An unstable object topples over easily and will continue to move further from its original position when tilted and released.
2. An object can be balanced at its point of equilibrium. The whole weight of the object seems to act at this point.
3. The point of equilibrium of a flat object can be determined by using the plumb line whereby the point of intersection of the lines is the point of equilibrium of the object.
4. Depending on the shape of the object, the position of the point of equilibrium may vary.
- Regular shapes – point of equilibrium lies in their geometrical centre, which can be determined easily by measurement.
- Irregular shapes – point of equilibrium may not lie in their geometrical centre and must be determined using the plumb line.
- The point of equilibrium can also lie outside the material making up the object.
5. The weight of an object is due to the gravitational force acting on it. Although the gravitational force acts on all parts of the object, there is one point where the object’s whole weight can be considered to concentrate. This point is called the centre of gravity of the object.
- An object is stable when it is supported at its centre of gravity.
- An object becomes unstable when it is supported away from its centre of gravity.
7. The stability of an object depends on:
a. The position of the centre of gravity.
- The lower the centre of gravity, the more stable the object.
- When the plank is tilted, model R falls first, followed by model Q and lastly, model P. Model P is the most stable because its centre of gravity is the lowest.
b. The base area.
- The wider the base area, the more stable the object.
- When the plank is tilted, model X falls first, followed by model Y and lastly, model Z. Model Z is the most stable because its base area is the largest.
9.2 IMPORTANCE OF STABILITY
1. Stability is important in our lives, as without it, there is a danger of toppling over that may lead to accidents, great financial loss and lives. Stability also enables us to live comfortably and safely.
2. A stable object has a low centre of gravity and a large base area. Therefore, the stability of an object can be improved by:
a. Lowering its centre of gravity.
- Decreasing the height of an object.
- Adding heavy base to an object.
b. Increasing its base area.
3. If an object is tilted, it will topple over if a vertical line from its centre of gravity falls outside its base as in (c).
4. The applications of the principles of stability can be seen on devices and natural phenomena in daily life such as follow:
- Crocodiles, lizards, tortoises and rhinoceroses are very stable because they have a low centre of gravity (shorter legs) and a large supporting base.
- A giraffe has a long neck to reach for leaves on the trees. Therefore, it has a high centre of gravity. It stands with its feet apart to lower its centre of gravity and increase its base area when it bends its head to drink water.
- A baby first learns to crawl before he tries to stand up. This is because the baby is more stable in a crawling position which has a lower centre of gravity and wider base area than in a standing postion.
- An old person will bend forward and use a walking stick to make himself more stable while walking.
- Boxers and people who practice martial arts always stand with their feet wide apart and their body low when fighting.
- A weightlifter spreads out his feet when lifting a heavy load to widen the base for stability.
- The passengers of a double-decker bus are not allowed to stand on the upper deck and to fill up the seats in the lower deck first to lower the centre of gravity of the bus.
- Passengers in a moving boat are not allowed to stand up suddenly.
- A tightrope walker uses a long pole or an umbrella to adjust his centre of gravity so that it is always directly above the rope. When in danger of falling off, he would crouch low on the tightrope to lower his centre of gravity to restore balance.
- A person climbing a hill bends his body forwards. He bends his body backwards when going downhill.
- Tall buildings, houses, schools and factories are usually built on heavy concrete foundations to lower the centre of gravity of the buildings.
- Most vehicles are designed with large base areas. The heavy engines are also positioned at the bottom of the vehicles to lower their centre of gravity.
- The racing car is built with a low centre of gravity so that it remains stable when driven at high speeds. Its tyres are wide and set wide apart to give the car a large base area.
- Children’s bicycles are fixed with two additional wheels so that the base area is larger, making them more stable.
- A lorry transporting goods has the heavy goods loaded first with the lighter ones placed on top of them to lower its centre of gravity.
- Furniture such as tables and chairs are designed with widely-spaced legs so that the base areas are large.
- When arranging items in a tall shelf, we arrange the heavier items at the bottom of the shelves.
- Laboratory apparatus usually have large base areas to make them more stable. Bunsen burners and retort stands have broad bases and are made heavy to lower their centres of gravity.
- The bottom part of a glass is made out of thicker glass to lower its centre of gravity so that it is more stable.
- Many electrical appliances have a large base area to increase stability. Standing fans and refrigerators are designed with heavy bases to make them stable.
- A broader heel gives the shoe a large base area and thus provides better stability.
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