Tips and Tricks of The Day

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Ex: The weight of a person is 60 kg. If he gets 105 calories heat through food and the efficiency of his body is 28%, then upto how much height he can climb (approximately):
A) 100 m                                      
B) 200 m
C) 400 m                                      
D) 1000 m
Suppose person climbs upto height h, then by using                    
W=JH | mgh = JQ 
60×9.8×h = 4.2×(10^5 * 28/100) 
h = 200 m
Ex: In a gravity free space, a man of mass M standing at a height h above the floor, throws a ball of mass m straight down with a speed u. When the ball reaches the floor, the distance of the man above the floor will be
A)  h(1+m/M)
B)  h(2−m/M)
C)  2h
D)  a function of m, M, h and u
As in gravity free space displacement of centre of mass of man and ball system should not move. If displacement of the ball be h then the displacement of man in upward direction. 
mh = Mh′ ⇒ h′ = mh / M  -----  Hence the position of man from ground
H = h + h′ = h = h[1 + m / M]
Ex: Movement of colloidal particles under the influence of electrostatic field is:
A) Electrophoresis
B) Electrolysis
C) Dialysis
D) Ionisation
Explanation: Movement of (charged) colloidal particles under the influence of electrostatic field is called electrophoresis due to opposite charge.
Ex: When propionic acid is treated with aqueous sodium bicarbonate CO2 is liberated. The 'C' of CO2 comes from:
A) Methyl group   
B) Carboxylic acid group
C) Methylene group           
D) Bicarbonate
Explanation: CH3CH2COOH(aq) + NaHCO3(aq)     -------> CH3CH2COONa + CO2 + H2O
                       Propionic acid           sod.bicarbonate
Ex: One mole of O2 gas having a volume equal to 22.4 litres at 0o C and 1 atmospheric pressure in compressed isothermally so that its volume reduces to 11.2 litres. The work done in this process is
A) 1672.5 J                         
B) 1728 J
C) -1728J                              
D) -1572.5 J
Explanation: W = -µRT loge V2 / V1 = -1×8.31×(273+0)loge (22.4/ 11.2)
= -8.31 × 273 × loge2 = -1572.5J 
loge2 = 0.693
Ex: A body of mass 10 kg is lying on a rough plane inclined at an angle of 30o to the horizontal and the coefficient of friction is 0.5.  the minimum force required to pull the body up the plane is:
A) 914 N
B) 91.4 N
C) 9.14 N
D) 0.914 N
Explanation: F = mg(sinθ + μcosθ)                
= 10 × 9.8(sin30 + 0.5cos30) = 91.4 N
Ex:  A catalyst is a substance which:
A) Increases the equilibrium concentration of the product
B) Changes the equilibrium constant of the reaction
C) Shortens the time to reach equilibrium
D) Supplies energy to the reaction
Explanation: A catalyst is a substance which alters the rate of reaction and shortens the time to reach equilibrium.
Ex: The thermal conductivity of a material in CGS system is 0.4. In steady state, the rate of flow of heat 10 cal/sec-cm2, then the thermal gradient will be
A) 10C/cm            
B) 12C/cm
C) 25C/cm            
D) 20C/cm
Explanation: ΔQ / Δt = KA Δθ / Δx
Thermal gradient Δθ / Δx
=(ΔQ/Δt)/ K = 10 / 0.4 = 25∘C/cm
Ex: On centigrade scale the temperature of a body increases by 30 degrees. The increase in temperature on Fahrenheit scale is:  A) 50°
B) 40°
C) 30°
D) 54°
Explanation: Difference of 100°C = difference of 180°F \ Difference of 30°= 180 / 100 × 30 = 54^∘
Ex: When a 4 kg mass is hung vertically on a light spring that obeys Hooke's law, the spring stretches by 2 cms. The work required to be done by an external agent in stretching this spring by 5 cms will be (g = 9.8metres/sec2):
A) 4.900 joule           
B) 2.450 joule
C) 0.495 joule
D) 0.245 joule
K = F / x = 40 / 2 × 10−2 = 0.2 N/m
Work done = 1 / 2 Kx2 = 1/2 × (0.2)×(0.05)2 = 2.5 J