An elevator can be moving up while the scale under your feet reads less than usual.

Going up. Feeling lighter. Physics has entered the elevator.

The trick is to separate where the elevator is going from how its velocity is changing. Here's a quick way to find out whether your students can.

For: conceptual or algebra-based high-school physics, after introducing net force. Time: plan about five minutes for the predictions and discussion; add time for the optional algebra. Materials: board or projector. No actual elevator needed.

A student stands on a scale in an elevator. In each situation, is the reading greater than, equal to, or less than the reading at rest?

Give everyone 30 seconds to choose answers. Then have pairs discuss the one they disagree about most.

Teacher key: equal, greater, less, greater.

The scale responds to the contact force between the student and the scale. The student's gravitational weight has barely changed. What changes is the contact force needed for the acceleration.

Situation three is the trap: upward motion does not require an upward net force. Slowing while traveling upward means acceleration points downward, so the scale pushes up with less force than gravity pulls down.

The diagram to draw: isolate the student. Draw the upward scale force and downward gravitational force. Ask which arrow is longer in each situation. Velocity is not another force arrow.

Optional algebra: take upward as positive: N − mg = ma. For a 60 kg student with g = 9.8 m/s², the resting reading is 588 N. An upward acceleration of 1.0 m/s² gives 648 N; a downward acceleration of 1.0 m/s² gives 528 N. These readings are expressed as forces in newtons.

One more question: “The scale reads less than it does at rest. Can you tell whether the elevator is moving up or down?”

Answer: No. The acceleration points downward. The elevator could be moving upward and slowing down, or downward and speeding up. The scale reading alone does not tell you the direction of motion.

Our Forces Unit Bundle is $71.20 and includes seven lesson decks—including a bonus deck on what scales measure—nine assignments, sixteen warm-ups, and assessments with answer keys.

Use the lesson slides to introduce a concept, the matching practice to work through it, and the warm-ups and assessments to check what stuck. The download includes editable PowerPoint decks plus Word and PDF materials.

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Stay Wildly Curious,
Lauren & Shawn

Science reference: OpenStax: applying Newton's laws.