Friction — Set 2

Correct Answer: D. Limiting friction

Limiting friction is the maximum static friction that acts when an object is just about to slide. Once the applied force exceeds this limit, the object starts moving. It depends on the nature of the surfaces and the normal force.

Correct Answer: B. Making surfaces rough

Making surfaces rough increases the number of irregularities that can interlock with each other. This increase in interlocking directly results in higher frictional resistance. Athletes often use chalk on their hands to increase friction for a better grip.

Correct Answer: A. Friction produces heat

Friction produces heat as a result of energy dissipation during the rubbing of surfaces. This effect is used when we rub our hands together to stay warm in winter. Excessive heat from friction can sometimes damage industrial machinery.

Correct Answer: B. Normal force

The coefficient of friction is defined as the ratio of the frictional force to the normal force pressing the surfaces together. It is a dimensionless quantity that characterizes the pair of materials in contact. Higher coefficients indicate a stronger frictional resistance between the materials.

Correct Answer: B. To increase friction on wet roads

Treads on tires help to expel water and increase friction on wet or slippery roads. This prevents the vehicle from hydroplaning or losing control in rainy conditions. Worn-out treads are dangerous as they significantly reduce the car's braking efficiency.

Correct Answer: B. Area of contact

The magnitude of friction is generally independent of the apparent area of contact between the surfaces. It mainly depends on the nature of the materials and the normal force acting between them. This observation is one of the classic laws of dry friction.

Correct Answer: A. Speed of the object

Fluid friction increases as the speed of the object moving through the fluid increases. It also depends on the shape of the object and the viscosity of the fluid. Streamlining helps reduce this speed-dependent resistance.

Correct Answer: B. Writing with a pencil

Writing with a pencil is possible because friction allows the lead to rub off onto the paper. Without friction, the pencil would simply slide over the paper without leaving a mark. Friction is also essential for lighting a matchstick.

Correct Answer: D. Adhesion

Highly polished surfaces can experience increased friction due to molecular adhesion between the atoms. When surfaces are very flat, more molecules come into extremely close contact and attract each other. This is often observed with very clean glass or metal plates.

Correct Answer: B. Heavier objects have more friction

Heavier objects have more friction because they press the surfaces together with greater normal force. This increased force makes the irregularities of the surfaces interlock more tightly. Thus, more force is required to move a heavy crate than a light one.