ScientistGrades 9–11⏱ 20 min

Friction and ramps

Tilt a ramp until a block starts to slide, split gravity into parts, and compare ice, wood and rubber.

Standards: HS-PS2-1

1Wonder

A book lies still on a tilted desk. Tilt it a little more and suddenly it slides. What changed at that exact moment? And why do cars need rubber tires, not ice skates?

2Explore

Ramp lab

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Turn your phone sideways, or open this lesson on a tablet or computer. The explanation and quiz below work here.

Ramp angle θ 20 ° Mass 2 kg

Surface

Split gravity into parts

Try this

On wood, raise the angle slowly and press Let go each time. At what angle does the block start to slide?
Try the same with ice and rubber. How does the sliding angle change?
Turn on Split gravity into parts. Which part pulls the block down the ramp? Which part presses it into the ramp?
Change the mass. Does a heavy block slide down faster than a light one?

3Explain

Step 1 of 5

Friction

No surface is perfectly smooth. When one surface slides or tries to slide on another, tiny bumps catch, making a force called friction. It always points against the sliding.

Blocks on ramps

\begin{aligned} N &= mg\cos\theta \\ f_{\max} &= \mu_s N \\ a &= g\,(\sin\theta - \mu_k \cos\theta) \end{aligned}

The block starts to slide when tan θ is greater than μₛ. Friction always acts against the sliding.

4Practice

Question 1 of 5

A block slides down a ramp. Which way does friction on the block point?

5Recap

Friction acts along the surface, against sliding; the normal force is perpendicular to it.
On a ramp, gravity splits into mg sin θ (down the slope) and mg cos θ (into the slope).
The block slides when tan θ > μₛ.
Sliding acceleration: a = g (sin θ − μₖ cos θ), the same for every mass.