When the two surfaces in contact have a very thin layer of lubricant in between them, it is known as ________.
- (1)Solid friction
- (2)Rolling friction
- (3)Greasy friction
- (4)Film friction
The Commission cancelled this question. It carries no answer in the final key, and this card names none — no option is nominated here, nothing below should be read as a reconstruction of the withdrawn answer, and the vocabulary discussed is set out as the textbooks set it out rather than as a key. The physics the item stood on is straightforward and belongs in every candidate's preparation. Friction is the force that opposes relative motion, or attempted relative motion, between two surfaces in contact. It arises because no surface is truly smooth: at the scale that matters, both surfaces are ranges of microscopic peaks, and the real area over which the two actually touch is a small fraction of the area they appear to share. At those points of true contact the materials adhere, and sliding requires those junctions to be sheared and the peaks to be climbed or ploughed through. Three classical laws follow from that picture and are examined regularly. The frictional force is proportional to the normal reaction pressing the surfaces together, which is written F = μN, where μ is the coefficient of friction and is itself a dimensionless number. The frictional force is independent of the apparent area of contact, which is why a brick slides no more easily on its side than on its face. And the kinetic frictional force is, to a good approximation, independent of the speed of sliding. Friction is usually classified first by whether motion has begun. Static friction acts while the surfaces are still at rest and adjusts itself to whatever applied force it must balance, up to a limiting value; once that limit is exceeded, motion starts and kinetic friction takes over, and kinetic friction is smaller than limiting static friction, which is why a heavy box is harder to start than to keep moving. Kinetic friction is then divided by the kind of motion: sliding friction, where one surface slides over another, and rolling friction, where a body rolls, which is very much smaller and is the reason for the wheel, the roller and the ball bearing. Motion through a fluid produces its own resistance, called fluid friction or viscous drag, and it depends on speed in a way that dry friction does not. Lubrication is the practical response to unwanted friction, and it works by interposing a fluid between the surfaces so that the shearing happens inside the fluid instead of between the solids. Engineers describe it in terms of regimes that depend on how thick the interposed film is. In BOUNDARY lubrication the film is extremely thin, a matter of molecular layers, the surface peaks still touch through it, and the chemistry of the lubricant's contact with the metal governs the result. In FULL-FILM or HYDRODYNAMIC lubrication the moving surfaces generate enough pressure in the lubricant to keep themselves entirely apart, no solid contact remains, and the resistance is set by the viscosity of the fluid rather than by the metals. Between the two lies a mixed regime. The Stribeck curve is the standard picture of how friction varies across these regimes, falling steeply as the film thickens and then rising again as viscous losses grow. One point about vocabulary is worth making plainly, because it bears on why a question of this shape is fragile: the older engineering textbooks used in India attach words such as 'greasy' and 'film' to these regimes, and their usage is not uniform from book to book, whereas the terms boundary, mixed and hydrodynamic are used consistently in the modern literature. Learning the physics of the regimes, rather than one book's labels for them, is what survives that inconsistency.
Friction is both a nuisance and a necessity, and MPSC's science section tends to examine it from the second side as often as the first. Nothing could be walked on, gripped, written with, braked or knotted without it: a shoe grips the ground by static friction, a brake pad stops a wheel by converting kinetic energy into heat through kinetic friction, a nail holds in wood by friction, and a belt drives a pulley by it. Against that, friction is what wears machinery out and what turns useful work into waste heat, so a great deal of engineering is devoted to reducing it where it is unwanted. The methods are worth knowing as a set: polishing surfaces so there are fewer and smaller asperities to interlock; replacing sliding with rolling by using wheels, rollers and ball or roller bearings, since rolling friction is far smaller than sliding friction; streamlining bodies that must move through air or water so that fluid resistance falls; and lubricating, which replaces solid-on-solid shearing with shearing inside a fluid. Lubricants themselves range from oils and greases to solids such as graphite and molybdenum disulphide, whose layered structures shear easily, and to gases in specialised bearings. The coefficient of friction is dimensionless because it is the ratio of two forces, and its value depends on the pair of materials and the state of the surfaces rather than on either material alone — which is why tables of friction coefficients are always given for a pair, such as rubber on dry concrete or steel on ice.
Seven questions in this paper were withdrawn by the Commission, more than in any other of its recent prelims papers, and this is one of them. A cancelled question is not a topic that can be dropped. The syllabus material behind it stays in the syllabus and returns in later papers in better-drafted form, so the useful response is to learn the ground the question stood on rather than to note the withdrawal and move past it. This item was also a fill-in-the-blank, printed with a rule of underscores at the end of the sentence in both language columns — a shape MPSC uses occasionally and which offers a candidate no statements to reason from, only a definition to recognise. That is worth noting as a matter of technique: in a fill-in-the-blank item there is nothing to eliminate by internal logic, so it is pure recall, and a candidate who does not hold the term has no route into the question at all. Where the recall depends on terminology that differs between textbooks rather than on physics that does not, the honest response in the hall is to decide quickly and move on, since it is the Commission alone that decides afterwards whether such an item stands at all.
- The Commission cancelled this question and published no answer for it in the final key; friction and lubrication remain part of the syllabus and are examined regularly in other forms.
- Friction opposes relative motion between surfaces in contact and arises from adhesion and interlocking at microscopic peaks; the frictional force is proportional to the normal reaction, F = μN, is independent of the apparent area of contact, and for kinetic friction is roughly independent of sliding speed.
- Static friction adjusts itself up to a limiting value before motion begins; kinetic friction, which acts once motion has started, is smaller than limiting static friction, which is why a heavy object is harder to start than to keep moving.
- Rolling friction is very much smaller than sliding friction, which is the reason for the wheel, the roller and the ball bearing; motion through a fluid produces viscous drag, which unlike dry friction depends strongly on speed.
- Lubrication regimes are distinguished by the thickness of the interposed film: boundary lubrication, where the film is only molecular layers thick and the surface peaks still touch through it; a mixed regime; and full-film or hydrodynamic lubrication, where the surfaces are kept entirely apart and the resistance is set by the lubricant's viscosity. The Stribeck curve is the standard picture of how friction varies across them.
- The coefficient of friction is dimensionless, being a ratio of two forces, and belongs to a PAIR of materials and the state of their surfaces rather than to either material alone.
- WHY A LUBRICANT WORKS AT ALL — no surface is truly smooth. At the scale that matters both surfaces are ranges of microscopic peaks, and the real area over which they actually touch is a small fraction of the area they appear to share; at those junctions the materials adhere, and sliding requires them to be sheared and the peaks to be climbed or ploughed through. Interposing a fluid moves the shearing INSIDE the fluid instead of between the solids
- BOUNDARY LUBRICATION — the film is extremely thin, a matter of molecular layers. The surface peaks still touch through it, so what governs the result is the chemistry of the lubricant's contact with the metal rather than the bulk properties of the fluid. This is the high-friction end of the range
- MIXED — the intermediate regime, in which part of the load is carried by the fluid film and part still passes through asperities that are touching
- FULL-FILM, also called HYDRODYNAMIC — the moving surfaces generate enough pressure in the lubricant to keep themselves entirely apart. No solid contact remains, and the resistance is set by the VISCOSITY of the fluid rather than by the metals at all
- THE STRIBECK CURVE — the standard picture of how friction varies across the three regimes: it falls steeply as the film thickens, passes through a minimum, and then rises again as viscous losses grow. Friction is least not where the film is thickest but where it is just thick enough
The Commission cancelled this question and published no answer for it. Nothing in this figure maps any term printed on the page onto any regime, and no choice is nominated here — the older engineering textbooks used in India attach words such as 'greasy' and 'film' to these regimes, their usage is not uniform from book to book, and that inconsistency is itself the likeliest reason an item of this shape could not stand. Boundary, mixed and hydrodynamic are used consistently in the modern literature, which is why the physics is worth learning in those words rather than in one book's labels. A cancelled question is not a topic that can be dropped, and the rest of this one stays squarely in the syllabus: friction is proportional to the normal reaction, F = μN; it is independent of the apparent area of contact, which is why a brick slides no more easily on its side than on its face; kinetic friction is roughly independent of sliding speed and is smaller than limiting static friction, which is why a heavy box is harder to start than to keep moving; rolling friction is very much smaller than sliding friction, which is the reason for the wheel, the roller and the ball bearing; and μ is dimensionless, belonging to a PAIR of materials and the state of their surfaces rather than to either material alone.
- Treating a cancelled question as a topic that can be skipped, when the material behind it returns in later papers in a better-drafted form
- Assuming friction depends on the apparent area of contact, when the classical laws state it does not
- Confusing rolling friction with sliding friction, or supposing that a rolling body experiences no friction at all
- Giving the coefficient of friction a unit, when it is the ratio of two forces and is dimensionless
- Learning one textbook's labels for the lubrication regimes rather than the physics of the regimes themselves, since the older terminology is not uniform between books
Friction reaches MPSC papers in four shapes. The first is definitional — which kind of friction acts in a described situation, or which of two named kinds is larger. The second is a numerical one-step item using F = μN, or a statement about what happens to a body on the point of sliding. The third is applied, asking why ball bearings are used, why a lubricant reduces friction, why vehicles skid on wet roads, or why the soles of shoes are grooved. The fourth is a laws-of-friction verification item in which two or three claims about proportionality, area and speed have to be sorted. All four are answerable from one page holding the classification, the three laws, the formula and half a dozen everyday applications, and that page is worth writing out once rather than assembled from fragments.
No directly related past PYQ was found.
- practice — not a real PYQ
Which of the following statements about friction is correct ?
- (a)Kinetic friction is greater than limiting static friction for the same pair of surfaces
- (b)The force of friction is proportional to the apparent area of contact between the surfaces
- (c)Limiting static friction is greater than kinetic friction for the same pair of surfaces
- (d)The coefficient of friction is measured in newtons
Answer(c) Limiting static friction is greater than kinetic friction for the same pair of surfaces — which is why a heavy crate takes a larger push to start moving than to keep moving once it is sliding. The classical laws also state that friction is proportional to the normal reaction rather than to the apparent area of contact, so the second option fails, and the coefficient of friction is the ratio of two forces and therefore a dimensionless number with no unit at all, so the fourth fails as well.
- practice — not a real PYQ
Ball bearings are used in machinery chiefly because
- (a)rolling friction is much smaller than sliding friction
- (b)they eliminate friction entirely
- (c)they increase the normal reaction between the moving parts
- (d)they convert kinetic friction into static friction
Answer(a) Rolling friction is much smaller than sliding friction — a ball bearing replaces the sliding of one surface over another with the rolling of hardened spheres between two races, and rolling resistance for the same load is a small fraction of sliding resistance. Friction is reduced, not eliminated: bearings still lose energy to rolling resistance and to whatever lubricant they run in, which is why they are lubricated and why they eventually wear out.