Include a Missing Force

Supplied materials

• Use the supplied fictional observations or model. Paper and pencil are optional; spoken answers are welcome. No equipment or real experiment is needed.

Learn the idea

A model is useful only when you know what it includes. A real rolling cart can experience friction or resistance. For this supplied paper case, the cart is moving right and friction acts left. Include that left force with the other forces before deciding whether the horizontal effect is balanced.

Initial notes list a 5-unit right push and a 2-unit left pull. Updated notes add 3 units of leftward friction and say there are no other horizontal forces. The first incomplete list suggests 3 units right, but the complete list balances: 5 right against 2 + 3 left. The update changes the conclusion without changing the rule for combining forces.

1. Calculate the result using only the initial two-force list.
2. Add the newly supplied 3-unit left friction arrow.
3. Combine the two left forces before comparing with the right force.
4. Explain why the complete model gives a different result.

Check your reasoning

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Take it further

Change friction from 3 to 4 units left. The left total becomes 6 and the net becomes 1 unit left; a cart moving right initially slows while that leftward effect acts.

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