Moments – principle of moments and couples
PhysicsForces & DynamicsAges 15–16Virtual lab
Use it with your class: students open it from a link or a QR code, with no account needed.
Next in Forces & Dynamics
A virtual lab with a rod pivoted at its centre and equally spaced holes: drag masses onto holes on either side and the rod tilts towards the side with the larger moment, settling horizontal only when M₁ = F₁·d₁ equals M₂ = F₂·d₂. A table records each trial so the principle of moments can be checked. Couple mode shows that two equal and opposite forces only make the rod rotate, with M = F·d.
Lesson: Moment of a force, equilibrium of a rigid body about a fixed axis, couples
What it shows
A moment measures the turning effect of a force about a pivot: M = F·d, where the moment arm d is the perpendicular distance from the pivot to the line of action of the force. Here a rod pivoted at its centre has six equally spaced holes on each side. Masses hung on the left produce an anticlockwise moment and those on the right a clockwise moment; the rod stays horizontal only when the two are equal (principle of moments). In Couple mode, two equal, opposite, parallel forces have zero resultant, so the rod spins about the pivot without being pushed sideways, with M = F·d.
How to use
Start in Balancing the rod (hanging masses) mode. The rod begins tilted left, with three masses at hole 2 on the left and one at hole 3 on the right; ask students to predict where to add masses to level it, then test by dragging masses from the box. Press Record result after each arrangement and compare M₁ and M₂ in the table. Change Weight of each mass or Distance between holes, then switch to Couple and tap different holes to change d.
Parameters you can change
- Mode Balancing the rod (hanging masses), Couple
- Weight of each mass 0.1–2 N
- Distance between holes 2–10 cm
- Size of each force in the couple 0.2–3 N
- Show moment arms
Questions to explore
- With two masses at hole 3 on the left, which different arrangements of masses on the right make the rod horizontal?
- Why does the same mass tilt the rod more when it hangs at a hole farther from the pivot?
- In Couple mode, how does the moment change when you double the distance d between the two forces?
