SIMPLE MACHINE

SIMPLE MACHINE

SIMPLE MACHINE

MACHINES

A machine is any device which is used to simplify work.

• In a machine, a force is applied at one convenient point to overcome force acting at another point.

Simple Machine and complex Machine
A simple Machine is the one which involve one movement example; inclined plane

A Complex Machine is a combination of more than one simple machine. Example Bicycle

Six types of Simple Machine are;

1. Levers
2. Pulley
3. Inclined Plane
4. The Screw Jeck
5. Wheel and axle
6. Hydraulic Press

shifts the stone by one crowing its weight. The down ward force applied on the crowbar is called the effort and the weight of the stone is the load.

Effort - is defined as the force used to operate a machine.

Load - is the resistance which a machine overcomes.

Mechanical Advantage

In general a machine is designed in such a way that the applied force effort is less than the load. The action of the load to the applied effort is a measure of usefulness of mechanical advantage (M.A) of a machine.

Since mechanical Advantage is a ratio of two forces it has no unit.

Example

1. A certain machine is designed in such a way that a force of 150N is used to lift load of 600N. What is the mechanical advantage.

Solution

Effort = 150N

Load = 600N

Example:

1. In a certain machine a force of 10N moves down a distance of 2cm in order to raise a load of 10N through a height of 0.5cm. Calculate the velocity ratio of the machine.

Efficiency
is the ratio of work output to work input usually expressed in percentage.

efficiency is always less than 100%

This is due to energy loses due to friction.

Example

1. A simple machine was used to raise a load of weight 3920N through a height of 3.5m by applying an effort of 980N, if the distance moved by the effort was found to be 20m, find.

1. The mechanical advantages
2. The velocity ratio
3. The efficiency of the machine

Solution

          Load = 3920N

          Effort = 980N

          Distance moved by the effort = 20m

          Distance moved by the load = 3.5m

LEVERS

Levers is a rigid body which when in use turns about a fixed point called a fulcrum or pivot.

It is used to shift heavy loads. A lever is designed such that a small force applied at one point overcomes a large force at another point. A lever is therefore a simple machine.

Classes of lever

Levers are divided in to three classes or orders namely, first class lever, second class lever and third class levers.

Classification of levers depends on the position of the fulcrum, load and effort.

1. First class lever
       This is a class of lever whereas fulcrum between the load and effort.

 Examples: claw hammer, crow bar and pair of scissors.

3. Third class levers
        This is a class of lever whereas effort between the fulcrum and the load.

Examples: tongs, fishing rod and a spade.

Pulleys

A pulley's block consists of two or more pulleys in a wooden or metal frame.

A pulley is a grooved wheel which is free to turn about an axle fixed in a frame. There are about four types of pulleys which are;

1. The single fixed pulley
2. Single movable pulley
3. Movable pulley
4. Block and tackle system.

1. The single fixed pulley

A single pulley is a fixed wheel with a rope passing round a groove in the wheels. Circumference, it is used to raise flag to the top of a flag – pole and builders use this type of pulley to lift cement bricks.

3. Block and tackle

Block is made up of two of more pulleys fixed in a wooden or metal frame. The pulleys in each block are fixed on separate axles. One string is used. It is tied to the top of the block and then passes around each of the pulleys. In block and tackle system pulleys, it is only the lower block which is free to move.

Figure, the tension in each section of the rope is equal the effort applied at the free end. The total tension in the rope sections supporting the movable block is equal to the load. That is load L=4T since there are four sections.

The tension (T) in the string equals to the load applied effort (E), that is E = T, then it follows that:-

Load (L) = 4 x Effort, which means that the load is equal to the effort multiplied by a number of rope sections.

In figure (b)

Load = number of rope sections that supports the load x Effort (number of pulley)

Load = 3 x effort

M.A = Number of pulleys and

V.R = Number of pulleys of the system

Example 

1.A block and tackle pulley system has a velocity ratio of 4. If a load of 100N is raised by using a force of 50 N. Calculate the mechanical advantage and efficiency of the system.

Solution

Data given

Velocity ratio   = 4

Load = 100N

Effort = 50N

Example

1. A loaded wheel barrow weight 800N is pushed up an inclined plane by a force of 150N parallel to the plane. If the plane rises 50cm for every 400cm length of the plane, find the velocity ratio, mechanical advantage and efficiency of the plane.

Solution

Data given

L = 800N

E = 150N

Height of inclined plane   = 50cm

Height of inclined plane = 400cm         

The screw jack

A screw jack consists of a rod in which there is thread. The thread of the screw is regarded as continuous inclined plane mapped round a cylinder. The distance between two successive threads is called the pitch of the screw.

The thread of a screw jack threads into base is turned by means of handle. The load acts on top of the screw, at the same time effort is applied to the handle. When the handle makes one complete turn, the distance moved 2R. The effort raises the load through the distance equal to the pitch p of the screw, making the length of the handle then velocity ratio can be found as follows;

Solution

Data given

R = 35cm

P = 0.5cm

L = 2200N

E = 40%

WHEEL AND AXLE

A wheel and axle is a simple machine which consists of a wheel and axle mounted with the same axis of rotation the radius of the wheel is always greater than that of the axle. When in operation, the effort E is applied to a string wound round the wheel while the load is attached to a string round the axle in opposite direction to that of the string in the wheel.

Data given

Load = 2000N

E = 80%

R = 50cm

   r = 2cm

V.R =?

M.A =?

Effort =?

Effort = 100N

   MECHANICAL ADVANTAGE, VELOCITY RATIO, AND EFFICIENCY OF HYDRAULIC PRESS

In a hydraulic press a small force (effort) applied on the small piston is used to overcome greater force (load) on the large piston. When a small effort E is applied downwards on the effort piston of radius r, the load piston of radius R lift the load L.

By principle of transmission of pressure in liquids, the pressure on effort piston equal to that on the load piston.