Grade 10 physics – Moments and equilibrium Quiz
1. What is the definition of the moment of a force (turning effect) about a pivot?
Moment (or torque) is defined as force × perpendicular distance from the pivot. It measures the turning effect of the force.
2. What are the SI units of moment (torque)?
Moment is force × distance so its unit is N·m (newton metre).
3. According to the principle of moments, when is a beam in rotational equilibrium about a pivot?
Rotational equilibrium requires clockwise and anticlockwise moments about the pivot to balance.
4. Which two conditions must be satisfied for a rigid body to be in complete equilibrium?
Complete equilibrium requires no resultant force (no translation) and no resultant moment (no rotation).
5. A seesaw with the pivot in the middle is an example of which class of lever?
In a first-class lever the pivot is between the effort and the load, as in a seesaw.
6. Where is the centre of gravity of a uniform straight rod located?
For a uniform rod mass is evenly distributed so the centre of gravity is at the middle.
7. What is a couple in mechanics?
A couple consists of equal and opposite parallel forces separated by a distance; they produce rotation without net force.
8. A force of 10 N acts at a perpendicular distance of 0.2 m from a pivot. What is the moment about the pivot?
Moment = force × perpendicular distance = 10 N × 0.2 m = 2 N·m.
9. A seesaw is 2.0 m long with its pivot at the centre. A child of weight 300 N sits 0.5 m to the left of the pivot. What weight must sit 0.8 m to the right to balance it?
Take moments: 300 N × 0.5 m = W × 0.8 m, so W = (300×0.5)/0.8 = 187.5 N.
10. What describes stable equilibrium for a body?
In stable equilibrium a small displacement raises the centre of gravity and the body tends to return to equilibrium.
11. How does the turning effect (moment) change if the same force is applied further from the pivot?
Moment = force × distance, so increasing the distance increases the moment for the same force.
12. Where is the centre of gravity of a uniform rectangular sheet located?
For a uniform rectangle the mass is symmetric, so the centre of gravity is at the centre where the diagonals meet.
13. A force of 50 N is applied at a lever arm of 0.40 m making an angle of 30° with the lever. What is the moment about the pivot (use perpendicular component)?
Perpendicular component = 50×sin30° = 50×0.5 = 25 N. Moment = 25 N × 0.40 m = 10 N·m.
14. A homogeneous plank of negligible mass is pivoted 0.2 m from its left end. A 40 N weight at the left end is balanced by a weight at the right end 1.0 m from the pivot. What must the right-hand weight be?
Moments about pivot: 40 N × 0.2 m = W × 1.0 m ⇒ W = 8 N.
15. Which statement about the moment of a couple is correct?
A couple produces pure rotation and its moment (force × separation) is the same about any point.
16. A beam of negligible weight has two weights 25 N and 35 N hung on opposite sides of a pivot at distances 0.6 m and 0.5 m respectively. Is the beam in equilibrium, and why?
Compute moments: 25×0.6 = 15 N·m, 35×0.5 = 17.5 N·m. They are unequal so not in rotational equilibrium.
17. Why is it easier to undo a stubborn nut with a longer spanner?
Moment = force × distance. A longer spanner gives larger turning effect for the same push.
18. A rigid body suspended from a point hangs so that its centre of gravity is directly below the suspension point. What type of equilibrium is this?
When CG is below the suspension point, a small displacement raises CG and the body returns to original position: stable.
19. For an equilateral triangular lamina, where is the centre of gravity located?
Symmetry places the centre of gravity at the intersection of the medians (also centroid) for an equilateral triangle.
20. Two equal and opposite forces 10 N separated by 0.3 m form a couple. What is the magnitude of the moment of this couple?
Moment of a couple = one force × separation = 10 N × 0.3 m = 3 N·m.
21. If a beam is free to rotate about a pivot and two forces produce moments of 12 N·m anticlockwise and 9 N·m clockwise, what is the resulting motion?
Net moment = 12 (anticlockwise) − 9 (clockwise) = 3 N·m anticlockwise, so the beam turns anticlockwise.
22. A horizontal uniform bar is supported at two points. If a small load is placed between the supports, which condition ensures the bar remains horizontal (no rotation)?
For no rotation about any point, anticlockwise and clockwise moments must balance.
23. Where should you push on a door to get the largest turning effect with the least force?
Pushing far from the hinge gives a larger distance from pivot so larger moment for the same force.
24. A uniform metre rule balances on a pivot at 40 cm. Where is its centre of gravity located?
A uniform 1 m rule has CG at 50 cm. If it balances at 40 cm something else (extra weight or non-uniformity) is causing torque.
25. Why must the perpendicular distance be used when calculating a moment?
Only the perpendicular component to the lever arm causes turning; hence moment = force × perpendicular distance.