حصريا : شرح درس Simple machines- علوم لمدارس اللغات للصف الاول الاعدادى

تاريخ التسجيل : 30/03/2010

Introduction

Man depends in his life on many simple and complicated machines, the use of simple machines saves effort or time or both. Objectives:

Recognize the simple machine.
Recognize some kinds of simple machines.
Recognize the benefits of simple machines.

The Machine

The machine is a tool used by man to do work and the use of simple machines saves effort or time or both.
Examples of simple machines:

Door openers.
Car jacks.
Scissors, hammers, pliers, pulleys and chisels are kinds of simple machines..

You can find complicated machines in workshops, factories and other production places.
Kinds of simple machines:

Levers.
Pulleys.
Wheels and axles.
Inclined surfaces.

The Lever

The lever consists of:

A fulcrum: that the lever rests on.
Effort: A point which is affected by the effort, this is called the point of effort effect.
Load: A point which is affected by the load, this is called the point of load effect..
The effort arm: the distance between the effort and the fulcrum.
The load arm: the distance between the load and the fulcrum.

Kinds of levers:

Levers of the first kind:
- The fulcrum is located between the load and the effort.
- Examples: the scissors and the swing.
- Have a mechanical benefit since it saves effort when the length of effort arm is larger than the length of the load arm.

Levers of the second kind:

The load is located between the effort and the fulcrum.
Examples: the garden car and the bottles opener.
Have a mechanical benefit because it always saves effort since the length of effort arm is always larger than the length of the load arm.

Levers of the third kind:

The effort is located between the load and the fulcrum.
Examples: the tweezers and the ice holder.
Don't have any mechanical benefit because it don't save effort since the length of effort arm is always smaller than the length of the load arm but it facilitates some work like the tweezers.

Pulleys

Pulleys specifications:

A pulley is a wheel around which a rope moves and it::

Consists of A wheel that turns on an axle that is fixed in a frame.
Around the rim of the wheel, there is a groove through which the rope moves without sliding out of the wheel during the pulling of the rope and causing the rotation of the pulley.
Pulleys are used to raise the load upwards.

Types of pulleys:

Fixed pulleys:
- Is fixed by its anchor in a holder or the ceiling.
- The load is at one end of the rope while the effort is at the other end.
- The fixed pulley is a first order lever, the fulcrum is located at the pulley’s center while the point of effort effect is located at one side of the pulley’s circumference and the point of load effect is located at the opposite point on the pulley’s circumference.
- The effort arm is equal to the load arm because each of them equals the pulley’s diameter, therefore, there is no mechanical benefit by using this pulley because the effort is equal to the load but the use of the fixed pulleys saves time.

The wheel and the axle

The wheel and the axle specifications:

Are examples of levers, it is a rotating lever and is considered one of the most important simple machines, it has many examples like door knobs and it's:

Composed of a large wheel that is connected to a small wheel and a cylindrical rod called the axle.
Part of the wheel may be enough (that is the handle) to do job of lifting objects and each full cycle rotates the axle a similar cycle.
The fulcrum of this rotative lever is the common center of the wheel and the axle.
The wheel diameter is the effort arm and the axle diameter is the load arm.
The effort arm is always larger than the load arm, that's why there will be a benefit for the wheel and the axle.
The wheel and the axle save effort and time.

The inclined surface

The inclined surface properties:

Is any incline that connects two levels one is low and the other is high like the screw and the chisel.

Moving objects upwards on an inclined surface requires less effort than the effort needed to raise them vertically.
It is possible to say that the work of the inclined surface is similar to the work of levers, the raised object is the load and the vertical distance is the load arm.
The effort which moves the load has an arm, that is the length of the inclined surface, witch is always larger than the load arm.
When the incline decreases, the required effort to move the object will be less.
The inclined surface saves effort and time.

Force and Motion

To describe the motion of a body in the space we find two types of motions:

Transferring motion:
- If the position of the body in the space changes on observing its motion in different intervals of time.
- From the main components of the transferring motion: displacement and velocity.

Vibrating motion:

If the body moves in its place with the change of time.

The velocity is the relation between the displacement & time, it is a vector quantity which has magnitude & direction & is calculated by the relation:
The velocity = change in displacement / change in time
there are two types of velocity:

Uniform velocity:
- If the body moves where it covers equals distance (displacements) in equal intervals of time.

Non-uniform velocity:

If the body moves where it covers different distances (displacements) in equal intervals of time.

The change of the motion as a result of the effect of the force:

If a force affects a body in the same direction of its motion, its velocity increases in the same direction.
If the force affect the body in an opposite direction to its motion, the velocity of this body decreases till it stops.
If a moving body is affected by a vertical force on its direction, the body moves in a circular path.
If the body is affected by two parallel equal forces in the magnitude but opposite in the direction, the body is affected by what is called couple or torque which rotates the body.

تاريخ التسجيل : 04/08/2010

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