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Lesson: Chapter 9 — Resultant Force

9.2. The resultant of two collinear forces 2 of 7

Activity 1 — two pulls, one line

How much two forces in the same direction come to together can be measured with three balances.

Activity 1

Apparatus: a trolley, three Newton balances, two pulleys, a ring.

  1. Fix the ring to one side of a trolley placed on the table, and pass two strong threads attached to that ring over two smooth pulleys.
  2. Connect the far ends of those two threads to two Newton balances, B and C.
  3. Connect the other end of the trolley to a wall through a Newton balance A.
  4. Pull with two forces on balances B and C and note their readings. Note the reading on balance A as well.
  5. Repeat several times with different forces on B and C, and look for the relationship between the readings.

Figure to be added

Figure 9.5 — applying two forces in the same direction to the trolley: the ring, the two pulleys and the three Newton balances A, B and C.

Observation

Whatever forces are applied, you will find that the readings of B and C added together equal the reading of A.

What is balance A measuring?

B and C pull the trolley towards the ring; A holds the trolley to the wall. Since the trolley does not move, A must be applying exactly the force that balances the pull coming from the other side. So A's reading is a measure of the resultant of B and C — it is the one place the resultant can be read off directly.

Why the pulleys?

A pulley does not change the size of the force — all it does is turn its direction. That lets the two balances be kept out to the side while both pulls still reach the trolley along one line, in one direction.