NDA (I) 2024 General Ability Test · Question 88 of 146

Correct — C, static friction > kinetic friction > rolling friction. For the same pair of surfaces and the same normal force, the maximum static (limiting) friction is the largest, because it is the force that must be overcome to start motion. Once the body is sliding, kinetic (sliding) friction is smaller than that limiting static value, which is why an object first resists, then slides more easily once moving. Rolling friction, which opposes a body rolling over a surface, is smaller still — typically one to two orders of magnitude less than sliding friction — because a rolling wheel only slightly deforms the surface rather than dragging across it. Hence the descending order is static, then kinetic, then rolling.

• (a)Kinetic friction > Static friction > Rolling friction — Reverses the top two — kinetic friction is smaller than limiting static friction, not larger; that is precisely why less force is needed to keep a body sliding than to start it moving.
• (b)Static friction > Rolling friction > Kinetic friction — Places rolling above kinetic, but rolling friction is the smallest of the three — far below sliding (kinetic) friction, which is why wheels and ball bearings are used to reduce resistance.
• (d)Static friction > Kinetic friction = Rolling friction — Rolling friction is not equal to kinetic friction; it is much smaller, so the two cannot be equated.

Dry friction between solid surfaces appears in static, kinetic (sliding) and rolling forms. For a given pair of surfaces and load, their limiting values fall in a fixed order that explains everyday experience — hard to start a heavy box sliding, easier to keep it sliding, and easiest of all to roll it.

The trap is guessing the order or assuming kinetic exceeds static because a moving object feels resistance. The reliable memory hook is that it is always hardest to start motion (static), and that rolling is engineered precisely because it is the least resistive.

• Limiting static friction is the maximum friction just before motion begins; the coefficient of static friction mu_s is greater than the coefficient of kinetic friction mu_k for the same surfaces.
• Kinetic (sliding) friction acts on a body already in motion and is roughly independent of sliding speed for ordinary conditions.
• Rolling friction is far smaller than sliding friction — commonly one to two orders of magnitude less — which is why wheels, rollers and ball bearings reduce effort.
• For the same surfaces and normal force the ordering is static friction > kinetic friction > rolling friction.

• Assuming kinetic friction exceeds static friction — the reverse is true.
• Treating rolling friction as comparable to sliding friction rather than far smaller.

Asked as an arrange-in-correct-order item — commit the chain static > kinetic > rolling to memory.

No directly related past PYQ was found.

• practice — not a real PYQ

  It is easier to keep a heavy box moving once it has started sliding than to start it from rest because:

  • (a)kinetic friction is greater than static friction
  • (b)limiting static friction is greater than kinetic friction
  • (c)there is no friction once the box moves
  • (d)rolling friction takes over

  Answer(b) limiting static friction is greater than kinetic friction — so less force is needed once the box is already sliding.

• practice — not a real PYQ

  Ball bearings are used in machinery mainly because:

  • (a)they increase static friction
  • (b)rolling friction is much smaller than sliding friction
  • (c)they eliminate friction completely
  • (d)they increase the normal force

  Answer(b) rolling friction is much smaller than sliding friction — bearings convert sliding contact into rolling contact to cut resistance.