Block on a Friction Ramp: Finding the Static-Friction Threshold
1 · Predict
A block sits on a fixed 30° ramp, currently held in place by static friction. If you lower the friction between the block and the ramp, will it start to slide — and at what point?
- Whether the block slides depends only on μs and the ramp angle — lower μs below tan(30°) ≈ 0.58 and it slides; above that it stays perfectly put, regardless of mass.
- The block will slip a little more each time you lower μs, even while μs is still above tan(30°) — friction just gets weaker gradually.
- A heavier block needs a lower μs to start sliding than a lighter one — mass affects the threshold too.
2 · Set Up
- Open the Friction Ramp preset. Note the ramp's fixed tilt angle (30°) and the block sitting on it, currently held static by friction.
- Use the Inspector (or a shared link) to set the block's coefficient of static friction (μs) for this trial. Mass stays fixed.
- Run the simulation for a few seconds and read the block's speed off the probe. Record whether it stayed at rest or began sliding.
3 · Collect Data
| Block Mass (kg) | Static Friction (μs) | Gravity Force Along Ramp (N) | Speed After 3 s (m/s) |
|---|---|---|---|
| 1 | 0.9 | ||
| 2 | 0.7 | ||
| 3 | 0.65 |
Graph Gravity Force Along Ramp (y-axis) against Block Mass (x-axis) for your three trials. Is the relationship a straight line? What does the slope represent?
4 · Analyze
- For each row, compute tan(30°) and compare it to your μs value. Does that comparison predict whether the block stays at rest?
- Your three trials use different masses and different μs values, but every block stays put. Explain why mass drops out of the static-friction condition entirely.
5 · Extend
- This preset's ramp is fixed at 30°. Using the Inspector or a shared link, lower the block's μs from its default until it just starts to slide. What value of μs did you find, and how does it compare to tan(30°) ≈ 0.577?
- A hiking boot sole and a smooth dress shoe have very different μs values on the same wet trail. Explain, in terms of this experiment, why one is much safer on a steep slope than the other.
The Physics Behind This Experiment
Gravity Component Along the Incline
F = m·a applied with a = g sin θ gives the piece of the block's weight that pulls it down the slope — the force static friction has to fully cancel for the block to stay put.
Momentum at Rest
p = m·v stays at zero for every trial here because the measured speed never leaves zero — a quick numerical check that the block is truly static, not just moving too slowly to notice.