Balanced & Unbalanced Forces

Activities to try

1. 1

   Run a real fair-test tug: how does the size of the pull change the motion?

   Why it works: Changing only one variable and repeating readings lets you read results as evidence: if the trolley starts at rest and a small pull leaves it still, that pull is balanced by friction, so the forces are balanced (no change in motion); once the pull is bigger than the friction the forces become unbalanced and the trolley starts to move and speed up toward the pull, and a bigger unbalanced force makes a bigger change in motion.

   1. Set up a wheeled trolley at rest on a table, a string over a pulley at the table edge, a small pot you can add identical masses to, plus a metre ruler and stopwatch.
   2. Choose ONE thing to change — the size of the pulling force (add masses one at a time) — and list everything you will keep the same: same trolley, same surface, same starting point.
   3. Predict, using what you already know, what will happen as the pull grows, and say why (reason from prior knowledge, not a blind guess).
   4. Run each setting at least three times, recording distance travelled in a set time (or speed) in a results table.
   5. Point to a moment when the trolley was NOT changing its motion (for example, a setting where the pull was too small to move it, if your data has one) and say what the forces were doing; point to a moment when the motion WAS changing (for example, the trolley speeding up after starting from rest).
2. 2

   Virtual investigation bench: lock the variables and see why only one may change

   Why it works: The variable-control logic can be experienced, not just stated: equal and opposite forces produce no change in motion; an unequal pair produces a resultant that changes motion (here, from rest, the cart starts moving toward the larger force); and changing two things at once makes the cause unreadable.

   1. On a virtual tug-of-war or cart-with-pushers bench (cart starting at rest), pick ONE independent variable (e.g. the force on the left); the bench keeps the others the same by default (same cart, same surface, same number of pulls), though you can unlock them to test what happens if you change more than one.
   2. Run the trial three times and watch a results table fill in; note that with equal, opposite forces the two arrows match and the cart stays still.
   3. Nudge one force bigger and observe the cart, starting from rest, begin to move toward that force; connect the unequal arrows (a resultant) to the motion changing.
   4. Deliberately try to change two things at once and respond to the bench's prompt ('you changed the surface AND the force; which one caused it?').
   5. State from the table when forces were balanced (equal, opposite, no change in motion) and when unbalanced.
3. 3

   Sort and sequence the fair-test reasoning (retrieval companion)

   Why it works: Fair-test reasoning can be rehearsed: one variable changes, the rest are controlled, readings are repeated, and balanced/unbalanced is judged by whether motion changes.

   1. Put the steps of a fair test in order: question with one variable → decide what to change, measure, and keep the same → run repeated trials → read results → conclude.
   2. Categorise described scenarios as balanced (no change in motion) or unbalanced (motion changing), including a still object held by a small pull cancelled by friction.
   3. Answer 'which variable should you change, and which must stay the same?' prompts for a given question.
   4. Explain in a sentence why repeating readings matters.