NDA (II) 2016 General Ability Test · Question 80 of 150

Correct — B, gravitational force. Gravity is the standard textbook example of a force that acts at a distance: two masses attract each other across empty space with nothing touching and nothing in between. The Sun holds the Earth in orbit across 150 million kilometres, and a dropped stone begins accelerating the instant it is released, with no surface pressing on it. Every other option in the list needs two surfaces in contact before the force can exist at all, so gravity is the odd one out.

• (a)Push force — A push is applied by one body directly against another — a hand on a door, a bat on a ball. Remove the contact and the push simply stops existing, which is the definition of a contact force.
• (c)Frictional force — Friction arises at the interface between two surfaces sliding, or tending to slide, over each other. It is generated by the irregularities and molecular attraction of those touching surfaces, so it cannot act unless the surfaces are in contact.
• (d)Strain force — This is the restoring force that appears inside a body once it has been stretched, squeezed or bent — a stretched spring pulling back, a loaded beam pushing up. It only comes into being when something is in contact with the body and deforming it, so it belongs with the contact forces. The wording is loose, since strain is strictly the measure of deformation rather than a force in its own right, but the intended meaning is the elastic restoring force and its classification is not in doubt.

Forces are grouped by whether the two interacting bodies have to touch. Contact forces — push and pull, normal reaction, friction, tension, air resistance, buoyancy, the restoring force of a deformed body — require physical contact between surfaces. Non-contact forces, also called forces of action at a distance or field forces, act across a gap through a field: gravitational, electrostatic and magnetic forces are the three met at school level.

The item is designed so that three options are unmistakably contact forces and only one is not, so it rewards a candidate who has memorised the two lists rather than one who reasons from scratch. The safest test is to ask whether removing all contact removes the force. A magnet still pulls an iron nail through a sheet of paper, an electrified comb still lifts paper bits from a distance, and the Earth still pulls a falling stone — none of these needs touching. A hand cannot push a door it is not touching, and friction cannot act on a surface it has left. The one genuinely unfamiliar term in the list is 'strain force', which is not standard vocabulary; read it as the elastic restoring force in a deformed body, which is a contact force either way.

• Gravitational, electrostatic and magnetic forces are the three non-contact forces normally taught at school level.
• Normal reaction, friction, tension, applied push or pull, buoyancy and air resistance are all contact forces.
• Non-contact forces are also called field forces, because they are described as acting through a field that fills the space between the bodies.
• Gravitational force acts between any two masses and is always attractive, whereas electrostatic and magnetic forces can attract or repel.

• Assuming that a force you can feel is necessarily a contact force; weight is felt continuously and is not one.
• Being thrown by an unfamiliar label such as 'strain force' — classify by whether contact is needed, not by whether you recognise the name.

NDA usually asks this as an odd-one-out among four named forces, or as a true-or-false statement pairing friction with magnetism, so keep the two lists ready rather than reasoning them out under time pressure.

• practice — not a real PYQ

  Which one of the following is a non-contact force?

  • (a)Tension in a string
  • (b)Normal reaction
  • (c)Electrostatic force
  • (d)Air resistance

  Answer(c) Electrostatic force — charges attract or repel across a gap through the electric field, while the other three need surfaces in contact.

• practice — not a real PYQ

  The force that opposes the relative motion of two surfaces sliding over each other is

  • (a)gravitational force
  • (b)frictional force
  • (c)magnetic force
  • (d)buoyant force

  Answer(b) frictional force — it acts at the interface between the two surfaces and always opposes their relative sliding.