physics : class11 Force Direction : Moment of a Force : Motion of System of Particles and Rigid Body MCQ Questions and Answers
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Force Direction
MCQ questions and answers.
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MCQ Questions
1. For a force acting on a rigid body, the moment of the force about a point is maximum when the force is directed at what angle to the position vector?
A.
0°
B.
45°
C.
90°
D.
180°
Correct Answer:
90°
Explanation:
The moment of a force is given by τ = rF sin θ. It is maximum when sin θ is maximum, which occurs at θ = 90°.
2. When a force acts along the line joining the point of application to the pivot, its moment about the pivot is
A.
maximum
B.
zero
C.
negative maximum
D.
equal to the force
Correct Answer:
zero
Explanation:
When the force is directed along the position vector, the angle is 0° or 180°. Since sin 0° and sin 180° are zero, the moment is zero.
3. A force produces an anticlockwise moment about a pivot when its direction tends to rotate the body
A.
clockwise
B.
anticlockwise
C.
without rotation
D.
only in a straight line
Correct Answer:
anticlockwise
Explanation:
The direction of the moment is described by the rotational tendency of the force. A force tending to turn the body anticlockwise produces an anticlockwise moment.
4. For a force applied at a fixed distance from a pivot, which direction gives the greatest turning effect?
A.
Along the rod toward the pivot
B.
Along the rod away from the pivot
C.
Perpendicular to the rod
D.
At zero force direction
Correct Answer:
Perpendicular to the rod
Explanation:
The turning effect is maximum when the force is perpendicular to the position vector from the pivot to the point of application.
5. A force directed through the pivot produces
A.
a maximum moment
B.
a zero moment
C.
a constant clockwise moment
D.
a constant anticlockwise moment
Correct Answer:
a zero moment
Explanation:
If the force direction passes through the pivot, its line of action has zero perpendicular distance from the pivot, so its moment is zero.
6. If the direction of a force is reversed while its point of application remains unchanged, the direction of its moment
A.
remains unchanged
B.
also reverses
C.
becomes zero in every case
D.
becomes perpendicular to the force
Correct Answer:
also reverses
Explanation:
Reversing the force reverses its tendency to rotate the body, so the direction of the moment changes from clockwise to anticlockwise or vice versa.
7. A force acting tangentially at the edge of a circular disc produces its greatest moment about the centre when it is directed
A.
toward the centre
B.
away from the centre
C.
perpendicular to the radius
D.
along the radius
Correct Answer:
perpendicular to the radius
Explanation:
The radius from the centre to the point of application is the position vector. The moment is greatest when the force is perpendicular to this radius.
8. The moment of a force about a point is unaffected by which change in the force direction?
A.
Changing from perpendicular to parallel
B.
Changing from clockwise tendency to anticlockwise tendency
C.
Reversing the force along the same line of action
D.
Changing the force to act through the pivot
Correct Answer:
Reversing the force along the same line of action
Explanation:
When a force is reversed along the same line of action, its moment generally changes sign, but its line of action and perpendicular distance from the pivot remain the same. Thus, the magnitude is unaffected.
9. A force makes an angle of 30° with the position vector from a pivot. To increase its moment without changing its magnitude or point of application, the force should be turned toward
A.
0° with the position vector
B.
30° with the position vector
C.
90° with the position vector
D.
180° with the position vector
Correct Answer:
90° with the position vector
Explanation:
The moment is proportional to sin θ. Increasing the angle from 30° toward 90° increases sin θ and therefore increases the moment.
10. A force acting on a spanner produces a clockwise turning effect about the nut. If the force direction is reversed, the turning effect becomes
A.
clockwise
B.
anticlockwise
C.
zero in all cases
D.
directed along the spanner
Correct Answer:
anticlockwise
Explanation:
Reversing the force reverses the rotational tendency about the nut, changing a clockwise moment into an anticlockwise moment.
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