Notes

Write a note in this area. Any 4 silly mistakes in GATE can take down marks down by almost 10 and affect your rank severely. This article can increase your marks in GATE by 9-10!!

These common mistakes have been collected overtime by us for all the GATE aspirants.

Here you go:

1. In calculating power in rolling. Do not forget to multiply 2 to the final answer..(2 is for two rollers)

2. While calculating Prandtl no, do not take specific heats in kJ/kg.K Take in J/kgK. You will get wrong ans of delta thermal

3. In Sommerfeld No. always put N (speed) in RPS not in RPM!

4. When the problem involving diameter and radius, Always Make sure that you haven’t solved the problem taking dia. Instead of radius. And vice versa…

5. When cone angle is given, it is actually 2(alpha). Divide it by 2 for calculation.

6. In Queuing theory when we find Probability of n-units its actually n-units in SYSTEM not in only QUEUE!

7. In flywheel, when fluctuation is x% about avg. Speed take it directly. If it is x% either side, take it as “2x%”

8. In HMT, when thickness of insulation is asked, don’t find just radius of insulation.

9. If thermal conductivity of material & thermal conductivity of insulation both are given, use K of insulation to calculate critical radius.

10. While calculating specific speed of a turbine speed should be in rpm, power is in kW.

11. In psychrometry(RAC), every formula uses temp. in °C, don’t convert it into Kelvin

12. In maxima & minima between [x,y] problems, check the value for boundary points also. Sometimes max. Or min. Occurs at boundary points.

13. While calculating shear stress due to torque is stress is in N/mm^2 then convert torque in N-mm and all dimension put in mm u always get dia in mm directly.

14. In PERT Standard Deviation doesn’t show additive properties, so S.D(of project) is not equal to summation of S.D of critical path. So S.D of project should be found ONLY by calculating Variance of project and den taking square root of it.. and variance is also only sum of variance on critical path and then take square root to get standard deviation…

15. In questions of SFEE for enthalpy calculation, unit conversion is a common mistake as heat, work, enthalpy are given / asked in KJ.s and velocity is generally given in m/s, so term of velocity is to be divided by 1000 if others are in kJ.s

16. Critical radius of insulation for cylinder is k/h and for sphere it is 2k/h. Also, k is of insulation (and not of the wire or pipe on which insulation is done).

17. When viscosity is given in Kg/(m.s), it is equal to N.S/m², so don’t waste your time in converting units.

18. 1 poise = 10^-1 N.S/m² 1 stoke =10^-4 m²/s One trick for this: since 1 poise = 0.1 N.S/m² Remember this : “In POISE, one digit after POINT.” Other you will automatically remember.

19. Taylor’s tool life formula, put speed in m/min and time in minutes. If speed given in m/s, convert accordingly.

20. In laminar flow always check either coefficient of friction or friction factor is given. As, friction factor = 4 × coefficient of friction.

21. Do not apply ideal gas equation in wet zone

22. In casting, while calculating the velocity of molten metal at the bottom of sprue take care of converting units since height of sprue given usually in mm and g(acceleration due to gravity) is given in m/s².

23. While calculating stresses in SOM. Always take the dimensions in mm and load/force in N. After calculating using these units You will get stress value in Mpa.
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