# Engineering Mechanics – Section 4

1. Three forces acting on a rigid body are represented in magnitude, direction and line of action by the three sides of a triangle taken in order. The forces are equivalent to a couple whose moment is equal to

A. area of the triangle
B. twice the area of the triangle
C. half the area of the triangle
D. none of these

Correct Answer: B. twice the area of the triangle

2. If the tension in the cable supporting a lift moving downwards is half the tension when it is moving upwards, the acceleration of the lift is

A. g/2
B. g/3
C. g/4
D. none of these

Correct Answer: D. none of these

3. The efficiency of a screw jack is given by(where α = Helix angle, and φ = Angle of friction.) 4. When a train is rounding a curve, the side thrust on the wheel flanges is prevented by raising the outer edge of the rail.

A. Yes
B. No

5. The frequency of vibrations means the number of cycles per second.

A. Yes
B. No

6. The time of flight (t) of a projectile on a horizontal plane is given by 7. The velocity ratio for the third system of pulleys is

A. n
B. n2
C. 2n
D. 2n – 1

Correct Answer: D. 2n – 1

8. If the gravitational acceleration at any place is doubled, then the weight of a body will be 9. The moment of inertia of a thin spherical shell of mass m and radius r, about its diameter, is

A. mr2/3
B. 2mr2/3
C. 2mr2/5
D. 3mr2/5