grade 7 Friction

Friction

Friction opposes motion and acts opposite to the direction of movement. Rougher surfaces and greater pressing force increase it, while contact area has little effect. Walking and braking need it, and lubricants or wheels cut it down. Change the applied force and the surface and the slide-off moment appears.

What is Friction?

Applied force10 N

Surface typeWood floor

💡 What Friction Really Is

①Friction: opposes object motion

②Acts opposite to motion

③Rougher surfaces produce more friction

④Applied force must exceed friction to start motion

⑤This sketch uses normal force 50; the threshold is μ×50. Ice μ=0.1, wood 0.3, concrete 0.6.

Factors Affecting Friction

Friction Determinants

Friction ∝ surface roughness × normal force

Rougher and heavier → larger friction

What Does NOT Affect Friction

Contact area, sliding speed → no effect

Wider contact area does not change friction (middle-school approximation)

🔍 Increasing vs Decreasing Friction

①Increase: tire treads, anti-slip pads, hiking soles

②Decrease: lubricants, bearings, air hockey

③Icy roads are dangerous because friction is tiny

④Bowling lanes oiled → reduces friction so balls roll well

Useful Roles of Friction

🌍 A World Without Friction?

①Cannot walk — feet slip

②Cars cannot stop — brakes useless

③Cannot write — pencils slip

④All knots untie — no rope friction

Exam Key Points

Core Summary

Friction: a force acting opposite to motion

Proportional to surface roughness and normal force; independent of contact area

🎯 Exam Key Points

①Friction acts opposite to motion

②Rougher surface → larger friction

③Greater pressing force (weight) → larger friction

④Contact area does not affect friction

⑤Reduce: lubricant, wheels / Increase: treads, rough surface

Examples and Unit Test

Example 1

When you push a box to the right on a wooden floor, in which direction does friction act?

Friction is a force that opposes the motion of an object, acting opposite to the direction of motion.

If you push the box to the right, friction acts to the left, the opposite direction.

▸ To the left (opposite the push)

Friction always acts opposite to the direction of motion.

Example 2

Pushing the same box on rough concrete versus smooth ice, where is friction greater? And if you widen the contact area, what happens to friction?

Friction is greater when the surface is rougher and when the pressing force (weight) is larger.

Rough concrete gives more friction than ice. Contact area does not affect friction, so widening the area does not change it.

▸ Rough concrete is greater / contact area has no effect (no change)

Friction depends on roughness and pressing force, not on contact area or speed.

Unit-test style

Which statement about friction is correct?

①Friction acts in the same direction as the motion

②The rougher the surface, the greater the friction

③The larger the contact area, the greater the friction

④The heavier the object, the smaller the friction

⑤Applying lubricant increases friction

▸ ② The rougher the surface, the greater the friction

Friction increases as the surface gets rougher, so ② is correct.

Friction acts opposite to motion (①), contact area has no effect (③), a larger pressing force increases friction (④), and lubricant reduces friction (⑤).

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Resultant Force & Equilibrium

Density

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