Push a heavy box across the floor and at first it will not budge, because friction appears where the box and the floor touch and pushes back against you.
Friction grows when the surfaces are rougher and when the object presses harder on the floor, and it hardly depends on how much area is in contact.
Thanks to friction we can walk without slipping and stop a car with its brakes, and when friction gets in the way we reduce it with oil or wheels.
Increase the push little by little and friction grows with it to hold the box, until the push passes the largest friction that floor can give and the box moves; switch the floor to ice and that moment comes much sooner.
Friction Pushes Back
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Pushing a box: friction acts the other way at the face touching the floor
10 N
Wooden floor
💡 How Friction Holds
①Friction acts between two surfaces in contact, opposite to the way the object tends to slide.
②While the box stays still, friction is exactly as large as the push; with no push there is no friction.
③Each floor has a maximum friction it can give. The 50 N box in the picture is held up to 5 N on ice, 15 N on a wooden floor and 30 N on rough concrete.
④Once the push exceeds the maximum friction, the box starts to move.
What Sets the Size of Friction
What increases friction
friction ∝ roughness of the surfaces × pressing force
The rougher the surfaces and the harder a heavy object presses on the floor, the greater the friction.
What hardly affects friction
contact area, sliding speed
A brick pushed lying flat or standing on end feels almost the same friction (school-level model).
🔍 When to Increase or Reduce Friction
①Increase: grooves in tires and hiking boots, non-slip strips on stairs, sand spread on icy roads.
②Reduce: oil in machines, wheels and ball bearings, the layer of air on an air hockey table.
③Icy roads are dangerous because the maximum friction between ice and shoes is so small that a small push makes you slip.
④Bowling lanes are oiled so the ball drags less and slides farther.
Where We Need Friction
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Wherever two surfaces rub, friction is at work
🌍 A World Without Friction
①When you walk, your foot pushes back on the ground; with nothing to hold it, you would slip on the spot.
②A car's brake pads could press on the discs and the wheels still would not stop.
③Pencil lead would not rub off onto paper, so nothing could be written.
④With nothing holding string against string, every knot would come undone.
Exam Key Points
Summary
friction: a force at the contact surface opposite to the direction of sliding
It grows with roughness and pressing force and hardly depends on contact area.
🎯 Exam Points
①Friction acts opposite to the direction an object moves or tends to move.
②Rougher surfaces give more friction.
③A greater pressing force (weight) gives more friction.
When a box on a wooden floor is pushed to the right, in which direction does friction act?
1
Friction acts at the surface where the box touches the floor, opposite to the way the box tends to slide.
2
The box is being pushed to the right, so friction acts to the left, even while the box is not yet moving.
▸ To the left (opposite to the push)
To find the direction of friction, first ask which way the object tends to slide.
Example 2
Pushing the same box on rough concrete and on smooth ice, where is friction greater? And what happens to friction if the contact area is made larger?
1
Friction grows with roughness and with pressing force; the same box presses equally, so only roughness needs comparing.
2
Rough concrete gives more friction than ice. Contact area hardly affects friction, so a larger area leaves it about the same.
▸ Rough concrete is greater / a larger contact area changes almost nothing
With the same weight, the same force is just spread over a larger area, so the friction stays about the same.
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
1
Friction grows as the contact surfaces get rougher, so ② is correct.
2
Friction acts opposite to motion (①), contact area has almost no effect (③), a greater pressing force gives more friction (④), and oil reduces friction (⑤).