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.