Atwood’s Machine
Ages 14–16 · Ages 16–18
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held
Student lab

Four steps, one string

1Balanced

Balanced

Step I · 6 min

Predict

Equal masses, one pulled lower and let go. What happens, and why?

  1. Drag A to three different heights and let go each time. Write down both gauge readings each time.
  2. Add the 20 g washer. Time the 0.80 m fall from All values. Compare it with t = √(2d/a), where a = 0.02g/4.02.
height of A / mgauge at A / Ngauge at B / N
washer: fall time from All values / st = √(2d/a) / s
2Let Go

Let Go

Step II · 8 min

Predict

What do the gauges read while 3.00 kg falls against 1.00 kg?

  1. Use the fall time to work out a = 2d/t².
  2. Work out T twice: from B’s equation, T = mB(g + a), and from A’s, T = mA(g − a). Compare both with the gauges.
  3. Repeat for 2.00 kg against 1.00 kg, and for 1.50 kg against 1.00 kg. Expected fall times: 0.699 s and 0.903 s.
mA / kgmB / kgfall time / sa / m s−2T from B / NT from A / Ngauges / N
3The Hand

The Hand

Step III · 6 min

Predict

Which hand force would make the race a dead heat?

  1. Find it with the slider.
  2. Say in one sentence how it compares with 9.81 N, and why. Check it against mB(g + a), with a from the left rig.
Your sentence
4A Tonne

A Tonne

Step IV · 8 min

  1. Fill in the table with mB = 1.00 kg: mA = 1, 2, 5, 10, 50, 1000 kg. Columns: a, T, and T ÷ 2WB.
  2. Describe what T ÷ 2WB does, and why it can never reach 1.
  3. Set mB = 0 and explain the reading.
  4. At 3 kg against 1 kg, try each g chip except no gravity. Show that a ÷ g stays the same.
mA / kga / m s−2T / NT ÷ 2WB
Exit

5 kg against 2 kg. Between which two values must T lie, and why?

The modelThis string has no mass and does not stretch; this pulley has no mass and no friction.
HistoryGeorge Atwood described this machine in 1784 to test the laws of uniformly accelerated motion. He took the distance a body falls in its first second as already known, and his pulley wheels were not massless: he allowed for their inertia in his calculations.
UnitsSI throughout: masses in kg, forces in N, g = 9.81 m s−2 on Earth