Which of the following statements is NOT correct according to the simple friction theory?

Question

BlackTom AI · Accepted Answer

To analyze the statements, I will assess each claim in the context of simple friction theory.
Option 1: 'Friction forces are always proportional to the normal reaction force.' In simple friction, kinetic friction is proportional to the normal force via F_f = mu_k N, but static friction is not necessarily proportional; it can take any value up to mu_s N depending on the applied tangential force. Therefore, claiming 'always proportional' is not strictly correct, making this option the likely NOT-correct statement.
Option 2: 'Friction forces tend to oppose motion.' This is a fundamental aspect of friction: friction acts to oppose the relative motion (or impending motion) between surfaces, so this is correct.
Option 3: 'Friction forces are independent of the contact area.' According to Amontons' laws (and the simple friction model), friction does not depend on the contact area, so this is also correct.
Option 4: 'Friction forces always act parallel to the surface.' Friction acts along the plane of contact, opposing motion, i.e., parallel to the surface, so this statement is correct.
In summary, the nuance lies with static friction: it is not strictly proportional to the normal force in all cases, whereas kinetic friction is proportional to N. Since the prompt asks for the NOT correct statement, option 1 is the misleading or incorrect one under the simple friction framework.

INU1127 (24/25) Friction - Quiz 2

Which of the following statements is NOT correct according to the simple friction theory?

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