Complete each stage to unlock the next one.
You shove a heavy sofa across carpet and your arms burn; your friend slides the same sofa across floorboards with one hand. Same sofa, same person — only the floor changed. Where does all that resistance come from?
You shove a heavy sofa across carpet and your arms burn; your friend slides the same sofa across floorboards with one hand. Same sofa, same person — only the floor changed. Where does all that resistance come from?
Friction isn't fundamental — it emerges from microscopic surfaces gripping each other. Two numbers set its strength: surface roughness (μ) and how hard the surfaces press (the normal force N).
Friction is a contact force opposing relative sliding. Its size is proportional to the normal force N pressing the surfaces together, with constant μ encoding how rough the surface PAIR is. Static friction (prevents sliding starting) is a bit larger than kinetic friction (resists sliding once underway).
μ depends on the pair: rubber on asphalt (≈ 0.8) grips far more than ice (≈ 0.03). Static μ_s > kinetic μ_k, which is why it's harder to GET something moving than to keep it moving. On a flat surface N = mg, so — this is why friction grew when you added mass. On a slope of angle θ the surface feels only the perpendicular part of the weight, N = mg cos θ, so ; sliding starts when the along-slope pull mg sin θ overtakes friction, i.e. when tan θ > μ. Note what's missing from : contact area. **Limiting case:** the decisive moment is breakaway — static friction climbs with your push until , then drops to kinetic ; and on a slope the whole slide/stay question reduces to versus . **Connect it:** is not a law of its own — it comes from the same force balance ( on the flat, on a slope), and microscopically friction is electromagnetic: surface bumps welding and tearing, which is why it scales with how hard the surfaces press.
Thinking friction depends on contact area or speed — it depends on the surfaces (μ) and the normal force: f = μN.