PHYSICS WORTH SHARING
Thanksgiving is two weeks away, and grocery carts are about to get heavy. Unit tests and semester review are coming up too, so here is a review question with a cart in it.
Give an empty cart and a full cart the same net force. Which one speeds up faster, and by how much?
If the full cart has twice the mass, it speeds up at exactly half the rate. A 24 N net force gives a 12 kg cart 2 m/s² of acceleration. Load it to 24 kg and the same 24 N gives only 1 m/s².
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Grocery Cart Physics practice sheet (Units 1 and 2: Motion and Forces). One printable page, six problems with whole-number answers, built on Fnet, a = Fnet / m and a = (vf − vi) / t with our GUESS steps. It works as unit test or semester review. Page 2 is the answer key, with a teacher note for the demo below. Get the free Grocery Cart Physics practice sheet
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Where students slip on Newton's 2nd Law
The rule is a = Fnet / m. Four common slips, each with a question to ask and a fix:
1. Applied force instead of net force. Ask: "A 30 N push, 6 N of friction. What do you divide by the mass?" Fix: find the net force first, 30 N − 6 N = 24 N.
2. "It needs a force to keep moving." Ask: "A cart rolls at a steady 1 m/s. What is the net force?" Fix: constant velocity means Fnet = 0. Inertia keeps it moving; the push only cancels friction.
3. Dropping the minus sign when it stops. Ask: "A 20 kg cart goes from 3 m/s to rest in 1.5 s. Which way is the net force?" Fix: keep the sign. a = −2 m/s², so Fnet = (20 kg)(−2 m/s²) = −40 N, opposite the motion.
4. Mixing up mass and weight. Ask: "Is a 12 kg cart's weight 12?" Fix: mass is in kg, weight is a force in N, W = mg. The cart weighs about 118 N, but only the 12 kg goes on the bottom.
The cart-push test (run it Monday)
For: Unit 2 Forces, Newton's 2nd Law. Time: 10–15 minutes. Materials: a rolling lab cart (or a rolling office chair with no one sitting on it), a spring scale or a long rubber band, a stack of textbooks, tape and a stopwatch.
Tape a start line and a finish line about 2 m apart on a smooth floor.
Work in small groups. One student pulls the empty cart from rest with the spring scale or rubber band. A partner walks alongside, watches the scale and calls out readings (or checks that the band stays stretched the same amount), and another student times the trip. Do 2–3 trials.
A steady pull is the hard part, and that's fine. The readings will wobble. Aim to hold about the same reading each time, for example around 5 N, and redo any trial where the pull clearly drifted.
Add textbooks until the cart has about twice its mass. Pull again with the same scale reading or band stretch, and time 2–3 trials.
Compare. The loaded cart speeds up more slowly and takes longer to reach the line.
Land it: the pull stayed the same and the mass went up, so the acceleration went down. Then ask why the result is not exactly half. Friction grows with the load, so the loaded cart's net force is a little smaller than the empty cart's.
Review games for unit tests and semester exams
Review-day tip: start with the questions students missed on this week's practice sheet. Put them into your review game, play in teams, and close with a two-question exit check on the problem they missed most.
Featured: Physics Jeopardy Review Game Bundle, $52. It has two review games for each of the 8 units, a Concept Review and a Math Review, in PowerPoint and Google Slides, so it fits whichever unit you are reviewing.
Also in the store: the Physics Escape Room Bundle (8 printable investigations), $58.80, and the Physics Test Bundle (8 NGSS-aligned unit tests), $36.
In case you scrolled past it: get the free Grocery Cart Physics practice sheet.
Sources: The Physics Classroom, Newton's Second Law and OpenStax Physics, 4.3 Newton's Second Law of Motion.
Forward this to a colleague who is reviewing Motion and Forces this month.
Stay Wildly Curious — Lauren & Shawn
