Activity 2 — three cases
This time the two balances pull on opposite sides of the trolley — in opposite directions.
Activity 2
Apparatus: a trolley, two Newton balances, two smooth pulleys, balance weights.
- Place the trolley on the table, fix a thread to each side, pass the far ends of the threads over smooth pulleys and connect two spring balances (A and B) to the two ends.
- Observe and record the motion of the trolley for three cases.
Figure to be added
Case 1 — A: 4 N, B: 4 N
The trolley does not move. Equal forces act on it in opposite directions, and under those forces the trolley is in equilibrium.
Case 2 — A: 4 N, B: 6 N
The trolley moves towards balance B. The extra force applied in the B direction over the A direction is 2 N. So in this case the resultant is 2 N in the direction of B.
Case 3 — A: 6 N, B: 6 N
The trolley does not move. The forces on the two sides are equal and the resultant is zero.
Are cases 1 and 3 not the same?
The trolley stays still in both, so at a glance they look identical. But the pull on the threads is greater in case 3 — 6 N each, not 4 N. Which means a zero resultant is not the absence of force; it is forces cancelling each other. Keep increasing the pull and the thread will snap — yet the resultant stays zero right up to the moment it does.