Big question: why are floor joists and roof beams put on edge, not laid flat?
Make a prediction
A pine strip 30 mm × 6 mm carries 1 kg at the middle of a 500 mm span. First it lies flat, then it is turned on edge.
On edge, the sag will be…
Load and measure the sag
Hang the load. Read the sag on the dial gauge. Then change one thing at a time.
Dial gauge (sag δ)
Load W
Recorded readings
0 readings
| No. | Position | b mm | d mm | L mm | W N | δ mm | I mm⁴ |
|---|
b = width across, d = depth (up and down). Leave I empty and work it out.
Why depth matters most
Second moment of area I = b × d³ ÷ 12. Sag δ = W × L³ ÷ (48 × E × I), with E = 10 000 N/mm² for pine.
Worked solution (7 steps)
Check yourself
You double the depth d of a beam. The second moment of area I becomes…
Calculate the sag
Work out the sag, type it in mm, then the load is hung to test you. Three rounds.
Score: 0 / 3
Big question: how can a ruler measure how "grippy" two surfaces are?
Make a prediction
A wooden block starts to slip when the board is raised to a certain angle. Now we use a block with twice the mass.
The heavier block starts to slip at…
Raise until it slips
Raise the board slowly. When the block just starts to slide, read h and the base.
Recorded readings
0 readings
| No. | Block | Mass kg | h m | Base m | μ |
|---|
Leave μ empty. Work it out: μ = h ÷ base.
Friction from a ruler
At the moment of slipping, μ = tan θ = h ÷ base.
Worked solution (7 steps)
Check yourself
Why does the mass of the block not change the slipping angle?
Mystery blocks
Each round uses a block with an unknown surface. Find μ to 2 decimal places.
Score: 0 / 3