The Block on Three Surfaces
A fifth-grade class pulls a wooden block with a spring scale, adjusting the pull each time so the block slides at a steady speed. On smooth tile the scale reads 1.6 N; on carpet, 3.4 N; on waxed paper, 1.2 N. The class then stacks a second identical block on the first and repeats the test on tile: the scale reads 3.2 N at steady speed. A student writes, "The block isn't speeding up or slowing down, so the forces on it are balanced. That means nothing is pushing it forward - the scale reading is just the block's weight." The teacher asks the class to use the readings to judge the student's statement.
1. List the forces acting on the sliding block and decide whether the student is right that the scale reading is the block's weight. Use the readings as evidence.
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Forces on the block
- The scale pulls forward (horizontal).
- Friction from the surface pushes backward (horizontal).
- Gravity pulls down; the floor pushes up. These two are equal, so the block does not rise or sink.
Steady speed means balanced, not zero Steady speed means the forward pull exactly equals the backward friction. The scale is reading the friction, not "nothing." If no forward force acted on a moving block, friction alone would slow it down - but the speed stayed steady, so a forward pull must be present and equal to friction.
Is 1.6 N the block's weight? No. The scale is horizontal, so it measures the pull needed to overcome friction. Weight acts downward and would be measured with the block hanging, not sliding. The same block gives 1.6 N, 3.4 N, and 1.2 N on three surfaces - its weight cannot change with the surface, so the scale cannot be reading weight.
Evidence summary Changing only the surface changed the reading (1.2 N → 3.4 N), so the reading tracks surface roughness, which is friction.
2. Predict the scale reading for two stacked blocks sliding at steady speed on carpet. Explain your reasoning using the tile data, and state any assumption you are making.
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Step 1: use the tile result to find the pattern
- One block on tile: 1.6 N. Two blocks on tile: 3.2 N.
- 3.2 ÷ 1.6 = 2. Doubling the mass doubled the friction force at steady speed.
Step 2: apply the same pattern to carpet
- One block on carpet: 3.4 N.
- Two blocks on carpet: 3.4 N × 2 = 6.8 N.
Step 3: state the assumption This assumes the doubling pattern holds for carpet too - that friction grows in proportion to how hard the surfaces press together. That is a reasonable assumption given the tile data, but the class should test it rather than assume it, because carpet fibers can mat down or the blocks can tilt.
What a check would look like Pull the two stacked blocks on carpet at steady speed, take three readings, and average them. A result near 6.8 N supports the pattern; a result far from it (say 4.0 N) means something else changed, such as the bottom block sinking into the carpet.
3. A classmate claims waxed paper "has no friction at all, because the reading is lowest." Use the steady-speed condition and the 1.2 N reading to argue for or against that claim.
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Against the claim.
- The block moved at a steady speed on the waxed paper, which means the horizontal forces were balanced: the 1.2 N pull forward equalled the friction backward.
- If friction were truly zero, the 1.2 N pull would have been an unbalanced force, and the block would have kept speeding up instead of sliding steadily. That did not happen.
- So friction on waxed paper is real and equals about 1.2 N - it is just the smallest of the three friction forces measured.
What the comparisons support Ordering the readings (waxed paper 1.2 N < tile 1.6 N < carpet 3.4 N) ranks the surfaces by how strongly they resist sliding. It does not show any surface reaching zero.
A fair way to test "zero friction" Give the block a push on waxed paper and then stop pulling. If friction is zero, the block should keep a constant speed for a long distance. If it slows and stops, friction is present.