A batsman taps the ball gently along the pitch. The ball rolls, slows down, and stops a few metres away. Nobody pushed the ball backward. On a smooth marble floor the same tap sends the ball much farther. On thick grass the ball stops almost at once. Something between the ball and the ground slows it down, and that something depends on the ground.
A force along the surface
When two bodies touch, each pushes on the other. The push across the surface, at right angles to it, is the normal reaction. The push along the surface is friction. Both are contact forces: they exist only while the surfaces touch, and they vanish the moment the surfaces separate.
Friction is the force that one surface exerts on another surface in contact with it, along the surface of contact. It resists the sliding of one surface over the other.
Why surfaces grip
A table top looks smooth, but under a microscope every surface is a range of hills and valleys. Place a block on the table and the block does not rest on its whole lower face. The block rests only on the tops of a few hills, as Fig 1.1 shows. At these few points the pressure is very large. The atoms of the two surfaces come so close that they attract each other and form tiny welds.
To slide the block, these welds must be broken and the hills must be pushed over or through one another. That costs a force, and the force is friction. A heavier block presses the hills harder together. More hills touch, the welds are larger, and the friction is larger. This is why friction grows with the normal reaction, a result we will use in the next lesson.
The true area of contact is often less than one ten-thousandth of the area of the face. It grows in proportion to the load, because the hills flatten until they carry the weight. Friction depends on this true area, so friction depends on the load and hardly at all on the size of the face. Frank Bowden and David Tabor measured this in Cambridge in the 1940s using electrical conduction through the contact points.
A book lies on a table. You lift it clear of the table and hold it still in the air. What happens to the friction between the book and the table?
The direction of friction
Friction resists sliding, so its direction is decided by the sliding. What matters is the relative motion of the two surfaces, the motion of one surface as seen from the other. A block sliding to the right over a fixed floor feels friction to the left. A block sliding to the left feels friction to the right.
A body need not slide for friction to act. Push a heavy almirah gently and it does not move. The almirah still tends to slide in the direction of the push, and friction points against that tendency. It balances the push exactly. If nothing pushes the almirah and nothing tilts the floor, there is no tendency to slide, and friction is zero.
Friction on a body acts along the surface of contact, opposite to the motion of the body relative to the other surface. If the body does not slide, friction acts opposite to the direction in which the body tends to slide.
Friction does not always oppose motion. It opposes relative motion, the sliding of one surface over the other. In the last lesson of this chapter, friction on a walking foot points forward, in the direction in which the person moves. The foot tends to slip backward, so friction pushes it forward.
Now return to the batsman's tap. The ball rolls forward, and the grass and soil it touches resist its motion. The resisting forces act at the contact with the ground and point backward, against the motion, so the ball slows and stops. Thick grass has more contact and resists more, and polished marble resists less.
A ball that rolls without slipping does not slide at the point of contact. The force that stops a rolling ball is mostly rolling friction. The ball and the ground both dent a little, and the ground in front of the ball pushes back on it. Rolling friction is usually far smaller than sliding friction, which is why wheels were invented.
A carrom striker slides across the board towards the left. In which direction does friction from the board act on the striker?
You push a heavy trunk to the east with a small force, and the trunk does not move. What is the friction on the trunk?
Which of these statements about friction are correct?
A box rests on a level floor, and nothing pushes it. A friend says that friction still holds the box in place. Is the friend right? Explain.
No. Friction acts only against sliding or a tendency to slide. On a level floor with no push, nothing tends to make the box slide, so friction is zero. The weight of the box is balanced by the normal reaction, which acts at right angles to the floor. If someone pushes the box, friction appears at once and opposes the push.
Leonardo da Vinci filled his notebooks with sketches of blocks pulled across a table by a cord over a pulley. He found that the friction did not change when he turned a block onto a narrower face. It changed only with the weight of the block. Leonardo never published these results, and they lay unread for centuries. In 1699 the French engineer Guillaume Amontons found the same laws again and presented them to the Paris Academy. Friction depends on the load, and not on the apparent area. The contact points of Fig 1.1 explain both laws.
- Saying that friction always opposes motion.Friction opposes relative motion, or the tendency to it. On a walking foot or a driving wheel, friction points in the direction of motion.
- Drawing friction on a body that is at rest with no push on it.No tendency to slide means no friction. Look for a push, a pull or a slope before drawing friction.
- Drawing friction at right angles to the surface.Friction lies along the surface. The force at right angles to the surface is the normal reaction.