The efficiency of a screw jack is maximum, when (where α = Helix angle, and φ = Angle of friction.)
α = 45° + φ/2
α = 45° - φ/2
α = 90° + φ
α = 90° - φ
Correct Answer :
B. α = 45° - φ/2
Related Questions
Moment of inertia of a rectangular section having width (b) and depth (d) about an axis passing through its C.G. and parallel to the depth (d), is
db3/12
bd³/12
db³/36
bd³/36
The velocity ratio of a single purchase crab winch can be increased by
Increasing the length of the handle
Increasing the radius of the load drum
Increasing the number of teeth of the pinion
All of the above
Centre of gravity of a thin hollow cone lies on the axis at a height of
One-fourth of the total height above base
One-third of the total height above base
One-half of the total height above base
Three-eighth of the total height above the base
The angle of inclination of the plane at which the body begins to move down the plane, is called
Angle of friction
Angle of repose
Angle of projection
None of these
A body moves, from rest with a constant acceleration of 5 m per sec. The distance covered in 5 sec is most nearly
38 m
62.5 m
96 m
124 m
In a framed structure, as shown in the below figure, the force in the member BC is 
W/√3 (compression)
W/√3 (tension)
2W/√3 (compression)
2W/√3 (tension)
The forces which do not meet at one point and their lines of action do not lie on the same plane are known as
Coplanar concurrent forces
Coplanar non-concurrent forces
Non-coplanar concurrent forces
None of these
A cable with a uniformly distributed load per horizontal meter run will take the following shape
Straight line
Parabola
Hyperbola
Elliptical
One kg force is equal to
7.8 N
8.9 N
9.8 N
12 N
The moment of inertia of a solid cone of mass m and base radius r about its vertical axis is
3mr2/5
3mr2/10
2mr2/5
4mr2/5
The linear acceleration (a) of a body rotating along a circular path of radius (r) with an angular acceleration of α rad/s2, is
a = α/ r
a = α.r
a = r / α
None of these
The velocity ratio of a simple wheel and axle with D and d as the diameters of effort wheel and load axle, is
D + d
D - d
D × d
D / d
In order to completely specify angular displacement by a vector, it must fix
Direction of the axis of rotation
Magnitude of angular displacement
Sense of angular displacement
All of these
A framed structure, as shown in the below figure, is a 
Perfect frame
Deficient frame
Redundant frame
None of the above
According to parallel axis theorem, the moment of inertia of a section about an axis parallel to the axis through centre of gravity (i.e. IP) is given by(where, A = Area of the section, IG = Moment of inertia of the section about an axis passing through its C.G., and h = Distance between C.G. and the parallel axis.)
IP = IG + Ah2
IP = IG - Ah2
IP = IG / Ah2
IP = Ah2 / IG
If P is the force acting on the body, m is the mass of the body and a is the acceleration of the body, then according to Newton's second law of motion,
P + m.a = 0
P - m.a = 0
P × m.a = 0
P/m.a = 0
The velocity ratio of a differential wheel and axle with D as the diameter of effort wheel and d1 and d2 as the diameters of larger and smaller axles respectively, is
D/(d₁ + d₂)
D/(d₁ - d₂)
2D/(d₁ + d₂)
2D/(d₁ - d₂)
The unit of force in S.I. system of units is
Dyne
Kilogram
Newton
Watt
The slope on the road surface generally provided on the curves is known as
Angle of friction
Angle of repose
Angle of banking
None of these
The unit of force in S.I. units is
kilogram
Newton
Watt
Dyne
Two balls of equal mass and of perfectly elastic material are lying on the floor. One of the balls with velocity v is made to strike the second ball. Both the balls after impact will move with a velocity
v
v/2
v/4
v/8
The moment of inertia of a solid cylinder of mass m, radius r and length l about the longitudinal axis or polar axis is
mr2/2
mr2/4
mr2/6
mr2/8
The angle which an inclined plane makes with the horizontal when a body placed on it is about to move down is known as angle of
Friction
Limiting friction
Repose
Kinematic friction
Moment of inertia of a triangular section of base (b) and height (h) about an axis passing through its vertex and parallel to the base, is __________ than that passing through its C.G. and parallel to the base.
Nine times
Six times
Four times
Two times
When a body of mass moment of inertia I (about a given axis) is rotated about that axis with an angular velocity to, then the kinetic energy of rotation is
Iω
Iω2
0.5 Iω
0.5 Iω2
A semicircular disc rests on a horizontal surface with its top flat surface horizontal and circular portion touching down. The coefficient of friction between semi circular disc and horizontal surface is μ. This disc is to be pulled by a horizontal force applied at one edge and it always remains horizontal. When the disc is about to start moving, its top horizontal force will
Remain horizontal
Slant up towards direction of pull
Slant down towards direction of pull
None of the above
Which of the following statement is correct?
The algebraic sum of the forces, constituting the couple is zero
The algebraic sum of the forces, constituting the couple, about any point is the same
A couple cannot be balanced by a single force but can be balanced only by a couple of opposite sense
All of the above
In the shown figure, the tension (T) in the string will be 
m₁. m₂. g/(m₁ + m₂)
2m₁. m₂. g/(m₁ + m₂)
(m₁ + m₂)/ m₁. m₂. g
(m₁ + m₂)/2m₁. m₂. g
The law of the machine is (where P = Effort applied to lift the load, m = A constant which is equal to the slope of the line, W = Load lifted, and C = Another constant which represents the machine friction.)
P = mW - C
P = m/W + C
P = mW + C
P = C - mW
Frictional force encountered after commencement of motion is called
Limiting friction
Kinematic friction
Frictional resistance
Dynamic friction