Force — Class 6 Science MCQs

Quick Revision: Force

• Force: A push or a pull that can change the state, speed, direction, or shape of a body. It has both magnitude and direction (vector quantity).
• Effects of Force: (1) Move a stationary body, (2) Stop a moving body, (3) Change speed, (4) Change direction of motion, (5) Change shape/size. Force does not change mass.
• SI Unit: Newton (N). Gravitational unit: kilogram-force (kgf). 1 kgf ≈ 10 N.
• Spring Balance: Measures weight (force). Works on the principle that the increase in spring length is directly proportional to the weight suspended.
• Contact Forces: Forces that require actual contact between bodies. Examples: Muscular force (push/pull by muscles), Force of tension (in a string/wire), Force of reaction (acts normal to the surface).
• Non-Contact Forces: Forces that act from a distance without physical contact. Examples: Gravitational force (always attractive — Earth pulls objects down), Magnetic force (attract/repel — unlike poles attract, like poles repel), Electrostatic force (charged objects attract/repel).
• Weight vs Mass: Weight is the force with which Earth attracts a body (SI unit: N). Mass is the quantity of matter (SI unit: kg). Force changes weight but not mass.
• Friction: The force that opposes relative motion between two surfaces in contact. Always acts opposite to the direction of motion.
• Cause of Friction: Interlocking of irregular projections (hills and valleys) on the surfaces in contact.
• Static Friction: Maximum opposing force while the body is stationary. It is self-adjusting — increases with applied force up to a limit.
• Sliding Friction: Opposing force when a body slides over a surface. Less than static friction.
• Rolling Friction: Opposing force when a body rolls over a surface. The least of the three types.
• Order of Friction: Static > Sliding > Rolling (for the same mass).
• Factors Affecting Friction: Depends on the nature of surfaces (rough = more friction, smooth = less) and weight of the body (heavier = more friction). Does not depend on the area of surfaces in contact.
• Methods to Reduce Friction: Polishing, Lubrication (oil/grease), Ball bearings (convert sliding to rolling friction), Streamlining (pointed shape to reduce fluid friction).
• Methods to Increase Friction: Making surfaces rough (sprinkling sand on icy roads), using brake pads, treads on tires.
• Advantages of Friction: Walking (reaction of friction moves us forward), Writing, Braking, Holding objects, Lighting a matchstick.
• Disadvantages of Friction: Causes wear and tear of moving parts, produces unwanted heat (reduces efficiency), requires more energy to overcome.
• Fluid Friction: Friction experienced by bodies moving through liquids or gases. Least in gases, more in liquids, most between solids. Streamlining reduces fluid friction (e.g., sharp arrows, fish shape).
• Balanced vs Unbalanced Forces: Balanced forces are equal and opposite, resulting in zero net force (no change in motion). Unbalanced forces cause a change in the state of motion.
• Non-Contact Force & Distance: Magnitude of non-contact forces (gravitational, magnetic, electrostatic) depends on the distance between bodies. If distance doubles, force becomes one-fourth.