At the solution of tasks in physics concepts of the mobile and motionless block often meet. Normally it is simple tasks in mechanics which solution comes down to understanding of the scheme of connection of blocks and knowledge of the basic working formulas. Mobile and motionless blocks are two different concepts. Respectively, and regularities for calculation of these spirit of systems will be different.
The term ""block"" is understood as some mechanical device which is roller which is fixed on perpendicular axis. This roller or can freely move, or on the contrary – is fixed strictly. Let's simplify definition - if the rotation axis of roller moves in space, then the block mobile. On roller there is fillet into which the rope or rope is inserted. The picture shows exterior of the block below.If the roller is fixed, for example, on ceiling - it is the motionless block. If the roller moves together with cargo - it is the mobile block. In a general sense difference only in it.
Point of use of the mobile block – prize in force at rise or transfer of cargo and physical bodies. The motionless block of prize does not give, however often strongly simplifies movement of body and is used in systems together with the mobile block.
Use of mobile and motionless blocks
The system of blocks meets everywhere. It both cranes, and various devices for cargo transfer in garage, and even drive belts in the modern car. Often the block is used even without clear understanding of the fact that it is that mechanism.
For certain on construction sites to you the mobile wheels fixed on upper floors of the house under construction met. Through such wheel the rope or chain and the worker is thrown, fixing bucket on the first floor, lifts it on upper floor, moving rope. It is simple example of use of the motionless block. If to add one more wheel to bucket, then the system of blocks - will turn out mobile and motionless.One more more rare example of use of the motionless block. When the person pulls out the car from dirt, having enveloped towing rope around tree trunk. It becomes for bigger convenience as the towing winch will easily be hooked for the small end of the rope enveloped around trunk. From the most such block of prize is not present and as the tree does not rotate vokurg the axis, resistance force increases loading.
Around us there is a lot of examples of use of these simple mechanisms.
The most known device which works at the principle of blocks it is polyspast. It is actively used in lifting gears. The system of blocks reduces force and the general work is reduced by 4-8 times.
The solution of tasks with mobile and motionless blocks
In tasks in physics it is often necessary to define what total prize in force will be received when using blocks. The difficult scheme where several blocks of different type are connected in a row is offered to the pupil.The key to the solution of similar tasks lies in ability to understand interaction of these devices. Each block pays off separately, and then is added to the general formula. The rated formula for all task is formed according to the scheme which was drawn by the pupil, reading condition.
For the best understanding of similar tasks it is necessary to remember that the block is peculiar lever. The won force gives loss in distance (in case of the mobile block).Rated formula very simple.
For the motionless F=fmg block where F is force, f – coefficient of resistance of the block, m – the mass of cargo, g – gravitational constant. In other words, F is the that force which needs to be put to lift, for example, box from the earth with use of the motionless block. As you can see, dependence direct and there is no coefficient.
For the mobile block we have double prize in force. Rated formulaf=0.5fmg where alphabetic references are similar to formula slightly above. Respectively, when using the mobile block, such box with mass of m will be twice easier to be lifted with the block, than with use only of only one own back.
Pay attention that the coefficient of resistance is that counteraction which arises in the block when moving of rope on it. Usually these sizes are set in statement of the problem or are table value. Sometimes in school tasks these coefficients fall at all and are not considered.Besides, it is worth to remember that if force is applied at an angle, then it is necessary to use standard method of calculation of force triangle. If in task it is told that the person pulls cargo for rope which is under 30 degrees to the line of the horizon, then it certainly has to be considered and designated on the rated scheme.