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Screw driver.

posted Jan 29, 2015, 11:58 PM by Ranmini Perera   [ updated Jul 13, 2016, 1:47 PM by Upali Salpadoru ]


A Complex Machine.



Fig.1 Using a screwdriver. 

Rotate a screw driver clockwise to send a screw into the 
wood. Now try to unscrew it by hand without a tool.  

Can you do it?

It will be almost impossible, especially if the wood is hard.
Yet using the tool, screw driver it can be done quite easily.

The tools, gadgets and devices that we use to facilitate our work are the machines. ( These do not unclude energy used such as electricity)

There are three categories of Machines.  They are 1. Levers.  2. Wheels  3. Inclined plane.

Into which category will you put the screw driver ?

The handle and the blade turn together but they do not cover the same distance. The advantage comes from this difference. The two circles are shown in the figure. The two circles are similar to two wheels. Therefore we can include this under the second category. (The screw has to be taken separately)

 The advantage diven by the screw driver alone is   
                  The circumference of the handle  divided by the circumference of the blase.
 The diameter of the handle divided by the diameter of the blade. 
  The number we get is called the Mechanical Advantage of the machine.

 Mechanical Advantage   =     diameter of handle /  diameter of blade.
   (This is really the Ideal MA.)


An experiment to determine the Mechanical advantage of a Screwdriver.


Method

 

Fig.2    To find the Mechanical advantage of a Screwdriver.


Support the screw driver in a horizontal position using two boxes. Use two pans attached to threads wind the round the screw driver as shown in Fig.  2  If you pull by the blue line the screwdriver will roll to right and if you pull by the red it will roll in the other direction. The idea is to add weights so that it will reach equilibrium. Then record the results in a chart .


Recording
 

Load

 

 

 

 

Effort

 

 

 

 

M.A= L/E

 

 

 

 

 

 More on  Wheels 

Rotating wheels and gears are widely used in many appliances ranging from clocks to bicycles and motor cars. Fig.3 shows two gear wheels coupled to transmit rotation.  When the blue wheel turns CW the orange one rotates in the opposite direction. So these can change the direction of rotatory forces The blue wheel is smaller and has only 12 cogs or teeth. The other has 24. So which will rotate faster? When the blue goes one round, the orange can do only half a turn. That means For every turn of the orange the blue has to go 2 rounds.

V.R.  =   No. of cogs on effort wheel / No. of cogs in load wheel.


Fig.3  Gear wheels closely coupled. 




 
Most of the children will recognise that this as the part of the driving system in a bicycle. The push is exerted on the pedal If you consider only the pedal arm and the large sprocket wheel velocity ratio could be determined as follows:

 

The distance effort moves for 1 round of the pedal is the circumference of the dotted circle. = C1
The load is the pull of the chain.
The distance this moves is the circumference of the orange wheel =   C2


Fig.4 Initial part of the driving system of a bicycle.

V.R =   C1 / C2
C1   =  2 x pi x R2
C2    =  2 x pi x R!

Substituting these we get

V.R =   2 x pi x R2  /  2 x pi x R1

As some values cancel we get
 V.R =   R2  / R1
 
If you wish to work out the VR of the entire bicycle  easiest way is to measure the distance cycle goes forward when you turn the pedal by one round. This will be less than one. The cycle was not built to reduce the force, but it was designed to increase the speed.


  1. Which statement defines a machine most appropriately?

a.  A device that can convert one form of energy to another form.
b.  A device which can change a force.
c.  Any man made instrument.
d.  An appliance which makes it easy to do work.
e.   Any tool that we use to do work.

Fig for Q. 2 to 4.


This is a device used as a road barrier.   Distances and forces are given except the Effort.

2.  What is the moment (torque) due to the weight of the lever?
             a. 500x10 Nm    b,  200x1 Nm   c.  200x0.2 Nm   d .    500 x0.2 Nm   e  .  500x1.2 Nm.
3.  Using E as effort find the correct equation to show that the moments are balanced.

a.   E  x 10  + (500 x 0.2)   =    (200x1)             b.     10 x E  =  (200x1)  + (500 x 0.2)
c.   E =   500 – 200                                             d.     10 E   =  (500x 0.2)  +  200
e.    E  =    200 + 500

Fig for Q 4 and 5

 4.  If each pulley is 10 N and the load is 300 N find the effort required, neglecting friction.

a.   310 N    b.   155 N    c.    160 N    d.   320    e.    300 N

5.   What is the Velocity ratio of the system.

a.  5.    b.     4.   c.  3   d  2  e.   1.
             
           Figure for Q. 7 and 8.
6.   When the object is pulled to the top, what would be the amount of work done by effort?
a.  10 x 0.4 J     b.    10 x 0.5 J     c.   8.5 x 0.5 J   d.  8.5 x 0.4 J     e.    (8.5 x 0.5) + (10 x 0.4) J

7.    What is the percentage efficiency of the system?

a.  10x 0.4  /    8.5 x 0.5 x100      b.    8.5 x 0.5  / 10x 0.4  x 100

c.   10 x 0.5 /  8.5 x 0.4 x 100       d.   8.5 / 10 x 100          e.   10 x 8.5 / 0.4 x 0.5 x 100      

          
                      Fig for Q 8

8.  What is the velocity ratio when using the spanner to turn the nut? (use pi as 3.14)
a.   20/ 0.1     b.   (2 x 3.14 x 20)  /   0.1     c.   0.1 /  2x 3.14 x 20
d.   0.1 / 20    e.    20 x 3.14

Q. 9 and 10



             Fig. for Q 9 and 10.
 When the pedal goes round once, the rare wheel goes four times. The pedal arm is 25 cm long. The radius of a wheel is also 25 cm.   The  average pushing force of the leg on the pedal is 300 N,
(Take pi as 3.14)
 9.   What can you say about the velocity ratio.
a.   equal to 1   .b. equal to 4    c Greater than 1. d. equal to ¼  e. equal to 2x 3.14 x 25  

10.  If the experiment was done to find the pull necessary and the effort necessary to push what can you say about the results?

a. Effort will be equal to pull.  b. Effort will be much more than pull.  c.  The pull will be more than effort.  d.  Pull is 4 times effort   e. Nothing is certain.
 
 
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