CHAPTER 9: STABILITY

9.1    STABILITY

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.

 6.    The centre of gravity of an object is defined as the point where the whole weight of an object appears to act. Therefore, the centre of gravity of an object is also the point of equilibrium of that 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.

CHAPTER 10: SIMPLE MACHINE

10.1    LEVERS

1.    A machine is any device that helps us do work more easily. A machine is also sometimes called a force multiplier as it helps us to overcome a heavy load with less effort or force.

2.    Simple machines include:

 

3.    Complex machines such as bicycles, sewing machines, cars and cranes are made up of two or more simple machines.

4.    A lever is a simple machine which turns about a fixed point called the fulcrum (F) when a force called the effort (E) is applied to overcome a resisting force known as the load (L).

• A man is trying to lift a boulder with a long stick. By pressing on the long end of the stick, he can use a small effort to lift the large load. The longer the distance of the effort from the fulcrum, the less effort is required to lift the load.
• The long stick is called the lever.
• The boulder to be lifted is called the load.
• The force used by the man to lift the boulder is called the effort.
• The point at which the stick turns is called the fulcrum.

5.    Levers are classified into first-class levers, second-class levers and third-class levers based on the relative positions of the fulcrum, effort and load.

a.    First-class lever.

• Fulcrum between effort and load.
• The load and effort act in the same direction.
• Effort is further from the fulcrum than the load is.
• Effort moves through a longer distance than the load.
• A small effort (force) is used to move a large load.
• Claw hammer, pliers, scissors, crowbar.

 

b.    Second-class lever.

• Load between fulcrum and effort.
• The load and effort act in opposite directions.
• Effort is further from the fulcrum than the load is.
• Effort moves through a longer distance than the load.
• A small effort (force) is used to move a large load. 
• Wheelbarrow, bottle opener, paper cutter, nutcracker.

 c.    Third-class lever.

• Effort between fulcrum and load.
• The load and effort act in opposite directions.
• Load is further from the fulcrum than the effort is.
• Load moves a longer distance than the effort.
• A large effort (force) is used to move a small load. A small movement by the effort produces a large movement by the load. 
• Fishing rod, human arm, broom, ice tongs.

6.    A force can be used to produce a turning effect to accomplish a desired task.

• When we use a spanner to loosen a nut, we are applying a force that has a turning effect that causes the nut to loosen.

• When we pull open a door, we are applying a force that has a turning effect that causes the door to open.

7.    The turning effect of a force is called the moment of a force. The moment of a force can be clockwise or anticlockwise, depending on which way they turn.

8.    The moment of a force about a point is the product of the force and the perpendicular distance of the force to the point.

• The greater the force used, the greater is the moment of the force.
• The longer the distance is from the turning point, the greater is the moment of the force 

Moment about the pivot
 = Force x Perpendicular distance
 = 10 N x 5 m
 = 50 Nm






9.    In a lever, the two forces that act on it are the effort and the load. These two forces produce opposing moments, which is a pair of clockwise and anticlockwise moments.

10.   When a lever is balanced about the turning point, the total clockwise moment is equal to the total anticlockwise moment about that point. This is the principle of moments.

Example 1:
A plank is hinged to one end to the wall as shown below. A load of mass 4 kg is placed at the other end. How much force is needed to keep the plank horizontal?

Load x Distance of load = Effort x Distance of effort
         (4 x 10) N x 0.8 m = Effort x 0.2 m
                  40 N x 0.8 m = Effort x 0.2 m
                              Effort = (40 N x 0.8 m) / (0.2 m)
                                        = 160 N

Example 2:
A steel bar is supported at 10 cm from the right end as shown below. A 2 kg mass is hung at the end of the bar. If the reading on the spring balance is 5 N, what is the length of the steel bar? (Ignore the weight of the steel bar).

 

Let the distance from the fulcrum to the spring balance be d.
Load x Distance of load = Effort x Distance of effort
                           5 N x d = (2 x 10 N) N x 0.1 m
                                     d = (20 N x 0.1 m)/(5 N)
                                        = 0.4 m
Therefore, the length of the steel bar is (0.4 + 0.1) m
                                        = 0.5 m or 50 cm.




10.2    APPRECIATING THE INNOVATIVE EFFORTS IN THE DESIGN OF MACHINES TO SIMPLIFY WORK

1.    Machines are simple tools invented by humans to make work easier. We should be grateful to scientists and inventors who design and improvise machines to make our life easier and more comfortable.

2.    Devices such as bottle opener, stapler, scissors, pliers, paper cutters and so on are commonly used in our daily life.

3.    Most of the machines we use today are compound machines, created by combining several simple machines. We have built bullet trains, ships, submarines, supersonic planes and spaceships.