Forces_Concept_4

Subject Facts

The nature of friction

The odd thing about friction is that you don't really notice it until you try to move or try not to. Friction either acts between surfaces or between an object and a medium: it happens when things rub together or when something moves through something else. Friction is a force that always resists movement. Sometimes this causes problems, and sometimes it actually helps by assisting movement on a different level.

The book and the table, part 3

Between the book and the table there is the potential for friction, since the cover of the book is in contact with the surface of the table. Slide the book gently across the table notice how it fails to keep going. How could you increase the friction? Make the surfaces rougher; or use a book with bigger covers (a greater surface area). So to reduce the friction, you would do the reverse: reduce the surface area of the book in contact with the table, or make the surfaces smoother.

To reduce friction, you might also try lubrication: placing some fine solid particles (such as sand) or a slippery liquid between the two surfaces. Press your finger to the tabletop and try to rub it across the surface: not so easy! Now lick your finger (check it clean first) and try again: is it easier? You should find that the smoother the tabletop, the more effective\y the lubricant (your saliva) works..

Get a grip

Now place the book in the middle of the tablet and slowly lift one end of the table. As the tilt increases, the book begins to slide. Why doesn^'t it begin to slide immediately? The reason is static friction, or grip'. If the tabletop were perfectly smooth, the book would start sliding down at the merest hint of a slope.

We have already seen how an application of spit can make a surface more slippery - so why do mechanics sometimes spit on their hands to get a better grip of something? It is a mater of quantity . Plenty of water forms a barrier between two surfaces, allowing freedom of movement. A little bit of water only creates surface tension between the water and each surface, thus increasing the grip or suction between the two surfaces. For example, water on the bottom of a glass often causes a mat to stick to the bottom,

A very small amount of water between the book and the table would, in effect, increase the frictional force, so that you would have to tilt the table even more to get the book going. Using a dot of water might not work either: the book might go soggy as it absorbed the water; reducing the amount of water acting as a barrier with the table and creating more surface tension.

Some static friction is always useful: it would be awkward if things always slid off surfaces that weren't perfectly flat. Even the slightest variation in the thickness of the carpet at one end of the room would be enough to send a bowl of fruit on the table crashing to the floor. When you attempt to stand up on ice, there is very little friction between the soles of your shoes and the ice surface (unless the shoes have tiny nails which penetrate the surface). Without friction between your shoes and the surface, you won't get very far except by uncontrollable sliding.

Most means of transport on land depend on creating static friction between the ground surface and the moving surface. You can wear different types of shoe to get better static friction on different surfaces. A sole with a minimum of tread and a maximum of surface area is ideal for smooth surfaces hence gym shoes and 'slick' tyres on racing cars. When the ground is lubricated with water, these smooth surfaces can become very slippery, The water can form a layer, preventing contact and causing 'aquaplaning' cars. Normal road tyres and training shoes have grooves to channel the water away so that contact with the ground can be made. On more uneven and yielding surfaces such as grass or mud, you need shoes with spikes or studs and tyres with coarse, knobbly treads,

Rollers and balls

Instead of using a liquid lubricant between two surfaces, it is sometimes easier to use rollers or ball-bearings, if you want the movement to be only forwards or backwards, placing rollers between the two surfaces will reduce the level of friction. The movement of the rollers is such that the top and bottom are always moving in opposite directions (see Figure 6), If movement in other directions is required, ball-bearings can be used: they can move on a surface in any direction, but the top of the ball is always moving in the opposite direction to the bottom.

Rollers and ball-bearings are used in many mechanical devices: swivel chairs, bicycle wheels, steam engines, washing machines and so on. It is important to note that this form of friction reduction only works between two hard surfaces that are pushing past each other: If more than the very top and bottom of the rollers or balls comes into contact with either surface, a considerable degree of friction returns. The use of marbles or pieces of dowel under a heavy box will ease it across a tiled floor, but wont be as effective in moving it over a carpeted surface. The balls or rollers come into greater contact with the carpet as they begin to dig in', causing some friction (see Figure 7).