Mechanics
Friction ramp
Watch the block stay put — then lower its static friction (μs) until it slides past the threshold (μs = tan 30° ≈ 0.58).
Friction ramp — interactive Mechanics simulation. Watch the block stay put — then lower its static friction until it slides past the critical threshold (μs = tan θ).
Static friction on a ramp
Friction opposes sliding. On a ramp, the block stays put until the downhill component of gravity exceeds the maximum static friction.
The critical angle satisfies tan θ = μ. Below that angle the block remains at rest; above it the block slides. This preset's ramp is fixed at 30°; lower the block's static-friction coefficient (μs) in the Properties panel until it drops below tan 30° ≈ 0.58 and find the threshold.
- μₛ = tan θ_critical
- F_friction ≤ μₛ N
Investigation brief
Plan the question before you open the lab
The brief mirrors the prerendered page: driving question, competing predictions, variable roles, governing laws, setup, analysis and extension prompts remain visible and in this order.
Driving question
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?
Predictions to weigh
- Two rules apply in order. The block holds for any μs ≥ tan(30°) ≈ 0.58, regardless of mass — and once it lets go, it is μk, not μs, that sets how fast it accelerates.
- 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.
Variable roles
What you set:
- Block Mass (kg)
- Static Friction (μs)
- Kinetic Friction (μk)
What you measure:
- Gravity Force Along Ramp (N)
- Speed After 1 s (m/s)
How the investigation runs
- 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 Properties panel (or a shared link) to set the block's coefficients of static (μs) and kinetic (μk) friction for this trial. Keep μk ≤ μs — the app enforces it, since a surface cannot grip less when at rest than it does while sliding. 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.
Governing equation
Gravity Component Along the Incline — F = m·a
F = m·a applied with a = g sin θ gives the piece of the block's weight that pulls it down the slope. Static friction has to cancel it completely for the block to stay put; once the block slides, kinetic friction only cancels part of it.
Kinetic Friction Force — f_k = μ_k·m·g·cos θ
Once the block is actually sliding, friction stops being a threshold and becomes a definite force: f = μk·m·g·cos θ, where μk is the coefficient of KINETIC friction. It is subtracted from the gravity component along the slope, so the block accelerates at g(sin θ − μk·cos θ) rather than the frictionless g·sin θ. Mass cancels from that expression entirely, which is why the sliding rows below do not depend on it.
What the printable worksheet asks students to work out
- For each row, compute tan(30°) and compare it to your μs value. Does that comparison predict whether the block stays at rest?
- Rows 1–2 stay put; rows 3–5 slide, at three different rates. Explain why mass drops out of BOTH the static-friction condition and the sliding acceleration, even though these rows use three different masses.
Where this shows up beyond the lab
- There are two ways to make the block let go: lower its μs until it falls below tan(30°) ≈ 0.577, or steepen the ramp until the angle passes atan(μs) ≈ 35°. Try both. Are they really the same condition written two ways?
- 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.
- AP Physics 1 — Unit 3: Work, Energy, and Power
- AP Physics C: Mechanics — Unit 3: Work, Energy, and Power
- IB Physics — A.3 Work, energy and power
- General High School Physics — Work, energy & power
- NGSS High School Physics — Energy accounting in systems
- Middle School Physical Science — Force, mass, and motion
- Welcome to Friction Ramp
- Select the block
- Press Play
- Find the critical friction
- Open the data
- You did it!
Open the interactive simulation to build the scene, press Play, and explore with live measurements and a guided tutorial.