Engineering Questions with Answers - Multiple Choice Questions

# Machine Dynamics MCQ – Precessional Angular Motion

1 - Question

A disc is spinning with an angular velocity? rad/s about the axis of spin. The couple applied to the disc causing precession will be
a) 1/2Iω2
b) Iω2
c) 1/2 Iω ωp
d) Iω ωp

Explanation: Since the rate of change of angular momentum will result by the application of a couple to the disc, therefore the couple applied to the disc causing precession,
Iω ωp
where I = Mass moment of inertia of the disc, and
ωp = Angular velocity of precession of the axis of spin.

2 - Question

A disc spinning on its axis at 20 rad/s will undergo precession when a torque 100 N-m is applied about an axis normal to it at an angular speed, if mass moment of inertia of the disc is the 1 kg-m2

Explanation: Gyroscopic couple acting on the disc,
C = I. ω. ωp
100 = 1 . 20. ωp
ωp = 100/1.20

3 - Question

The engine of an aeroplane rotates in clockwise direction when seen from the tail end and the aeroplane takes a turn to the left. The effect of the gyroscopic couple on the aeroplane will be
a) to raise the nose and dip the tail
b) to dip the nose and raise the tail
c) to raise the nose and tail
d) to dip the nose and tail

Explanation: When the engine or propeller rotates in anticlockwise direction when viewed from the rear or tail end and the aeroplane takes a left turn, then the effect of reactive gyroscopic couple will be to dip the nose and raise the tail of the aeroplane. When the aeroplane takes a right turn, the effect of reactive gyroscopic couple will be to raise the nose and dip the tail of the aeroplane.

4 - Question

The air screw of an aeroplane is rotating clockwise when looking from the front. If it makes a left turn, the gyroscopic effect will
a) tend to depress the nose and raise the tail
b) tend to raise the nose and depress the tail
c) tilt the aeroplane
d) none of the mentioned

Explanation: When the engine or propeller rotates in clockwise direction when viewed from the front and the aeroplane takes a left turn, then the effect of reactive gyroscopic couple will be to raise the tail and dip the nose of the aeroplane. When the aeroplane takes a right turn, the effect of reactive gyroscopic couple will be to raise the nose and dip the tail of the aeroplane.

5 - Question

The rotor of a ship rotates in clockwise direction when viewed from the stern and the ship takes a left turn. The effect of the gyroscopic couple acting on it will be
a) to raise the bow and stern
b) to lower the bow and stern
c) to raise the bow and lower the stern
d) to lower the bow and raise the stern

Explanation: When the rotor rotates in the clockwise direction when viewed from the bow or fore end and the ship is steering to the left, then the effect of reactive gyroscopic couple will be to raise the stern and lower the bow.

6 - Question

When the pitching of a ship is upward, the effect of gyroscopic couple acting on it will be
a) to move the ship towards port side
b) to move the ship towards star-board
c) to raise the bow and lower the stern
d) to raise the stern and lower the bow

Explanation: The effect of the gyroscopic couple is always given on specific position of the axis of spin i.e. whether it is pitching downwards or upwards. The pitching of a ship produces forces on the bearings which act horizontally and perpendicular to the motion of the ship.

7 - Question

In an automobile, if the vehicle makes a left turn, the gyroscopic torque
a) increases the forces on the outer wheels
b) decreases the forces on the outer wheels
c) does not affect the forces on the outer wheels
d) none of the mentioned

Explanation: The gyroscopic couple will act over the vehicle outwards i.e. in the anticlockwise direction when seen from the front of the vehicle. The tendency of this couple is to overturn the vehicle in outward direction.

8 - Question

A motor car moving at a certain speed takes a left turn in a curved path. If the engine rotates in the same direction as that of wheels, then due to the centrifugal forces
a) the reaction on the inner wheels increases and on the outer wheels decreases
b) the reaction on the outer wheels increases and on the inner wheels decreases
c) the reaction on the front wheels increases and on the rear wheels decreases
d) the reaction on the rear wheels increases and on the front wheels decreases