Which force prevents your foot from sliding backward as you walk?
Think of the force acting before any sliding begins.
Yes—static friction grips the sole and stops backward slip.
Review what force acts just before the foot starts to move.
Microscopic origin of friction
Surfaces are never perfectly smooth; even polished metal has tiny peaks and valleys.
These irregular peaks interlock, so micro-joints must break before sliding begins.
Atoms in close contact create weak adhesive bonds, adding extra resistance to motion.
Static friction is the force that resists the impending motion between two contacting surfaces and keeps them still.
It increases up to a maximum, then motion begins.
Source: NCERT Fig.4.10a
Kinetic friction is the force that opposes the relative sliding motion between two contacting surfaces once they are in motion.
It appears only after motion begins; it is absent while the body remains at rest.
Source: NCERT Fig.4.10b
Static friction rises with push up to its limit \( \mu_s N\). Once motion starts, kinetic friction stays constant at \( \mu_k N\). Both forces scale with the normal force \(N\).
\(N\) equals the object’s weight only on a horizontal surface; on an incline, calculate \(N\) first.
For most surfaces, \( \mu_s \ge \mu_k \).
How do the two types differ?
Tip: Decide direction first; then use \(\mu_s\) or \(\mu_k\) as needed—remember \(\mu_s > \mu_k\) for most surfaces.
For dry wood-on-wood contact, which coefficient is usually larger?
Recall the extra force needed to start motion.
Right! Static friction demands more force to start motion.
Remember the first push needs more effort than steady sliding.
Determine the angle of repose using the static-friction limit.
On the inclined plane show \(mg\sin\theta\), \(mg\cos\theta\), normal \(N\), and static friction \(f_s\) up the slope.
At impending slide \(f_s = \mu_s N\). Balance along the plane: \(mg\sin\theta = \mu_s mg\cos\theta\).
Solve to get \( \theta_r = \tan^{-1}\mu_s \). Sliding starts when \(\theta \ge \theta_r\).
Remember: the angle of repose links the friction coefficient directly to the slope angle—no mass term appears.
Top mistakes & fixes — avoid friction misconceptions.
Normal force ≠ weight when surface tilts or extra pushes act.
Friction opposes relative motion tendency, not always velocity.
Coefficient \( \mu \) is a ratio and therefore unitless.
Remember these six essential points.
Force that resists sliding between two touching surfaces.
Always opposite to actual or impending motion.
Maximum static friction \(F_s^{\text{max}} = \mu_s N\).
Sliding friction \(F_k = \mu_k N,\; \mu_k < \mu_s\).
\(\mu\) depends only on surface pair, not area or speed.
Limiting angle: \(\tan\phi = \mu_s\); equals angle of repose.
A 2 kg block rests on a horizontal floor. What is the maximum static friction force? Given \( \mu_s = 0.40 \).
Use \(f_{s,\text{max}} = \mu_s N\) and \(N = mg\).
Great! You’re set.
Rework using weight 19.6 N.