S9 Three forces in equilibrium · MVTTC Virtual Lab

Experiment workspace

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Tip: drag a peg (the end of a spring balance) to a new place on the board.

Quick set-ups

Or drag any peg. The ring moves until the three pulls balance.

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Angles at the ring Triangle of forces Force arrows

Big question: when three forces hold a ring still, how are they related?

Make a prediction

Two spring balances pull a ring at right angles: one reads 3 N, the other 4 N. A third balance holds the ring still.

What does the third balance read?

Read the balances and the angles

Move the pegs. For each set-up, read the three forces and the angles between them.

Angle F1–F2

Angle F2–F3

Angle F3–F1

Recorded readings

0 readings

No.	F1 N	F2 N	F3 N	F1–F2 °	F2–F3 °	F3–F1 °

Draw each set of forces to scale, head to tail, on your results sheet. Does the triangle close?

The triangle of forces

When three forces balance, their arrows drawn head to tail make a closed triangle. Lami's theorem: F1 ÷ sin(F2–F3) = F2 ÷ sin(F3–F1) = F3 ÷ sin(F1–F2).

Worked solution (7 steps)

Check yourself

Three forces balance a ring. You know F1 and F2. F3 must be…

Find the third force

Work out F3, type it, then the balances are set to test you. Three rounds.

Score: 0 / 3

Runs offline. No trainee data is stored or sent. MVTTC · GST 04102 · S9 v1.0 · three.js (MIT licence).

Drag	the peg at the end of any spring balance
Point at	a balance, the ring or an angle to see its value
T	show or hide the triangle of forces
R	record a reading
1 2 3	camera views
F M H	full view, sound on/off, this help
Phone	one finger turns the view, two fingers zoom

Experiment workspace

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How to use this lab

Quick set-ups

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