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Chapter 4: Problem 7

Which of the following observations about the friction force is (are) incorrect? a) The magnitude of the kinetic friction force is always proportional to the normal force b) The magnitude of the static friction force is always proportional to the normal force. c) The magnitude of the static friction force is always proportional to the external applied force. d) The direction of the kinetic friction force is always opposite the direction of the relative motion of the object writh respect to the surface the object moves on. e) The direction of the static friction force is always opposite that of the impending motion of the object relative to the surface it rests on f) All of the above are correct.

Short Answer

Expert verified
Answer: C) Static friction force proportionality with external applied force and F) All statements are correct.

Step by step solution

01

A) Kinetic friction force proportionality with normal force.

This statement is true. Kinetic friction force (\(f_k\)) is always proportional to the normal force (\(N\)). The proportionality constant is the coefficient of kinetic friction (\(\mu_k\)). So mathematically, it can be written as \(f_k = \mu_k N\).
02

B) Static friction force proportionality with normal force.

This statement is also true. The maximum static friction force (\(f_{s,max}\)) is proportional to the normal force (\(N\)). The proportionality constant is the coefficient of static friction (\(\mu_s\)). Mathematically, it can be written as \(f_{s,max} = \mu_s N\). However, it's important to note that the actual static friction force can vary from 0 to the maximum value.
03

C) Static friction force proportionality with external applied force.

This statement is incorrect. The static friction force is not always proportional to the external applied force. For a given normal force, the static friction force will adjust to balance the applied force up to a maximum value of the static friction force (\(f_{s,max} = \mu_s N\)). Beyond that, the object may start moving due to an external force that exceeds the static friction force.
04

D) Kinetic friction force direction.

This statement is true. The kinetic friction force always acts in the opposite direction of the relative motion of the object with respect to the surface the object is moving on.
05

E) Static friction force direction.

This statement is true. The static friction force always acts in the opposite direction of the impending motion of the object relative to the surface it rests on.
06

F) All statements are correct.

This statement is incorrect. As we have analyzed, statement C is incorrect. Therefore, all the statements are not correct. In conclusion, the incorrect observations about friction force are C) and F).

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Most popular questions from this chapter

A 0.50 -kg physics textbook is hanging from two massless wires of equal length attached to a ceiling. The tension on each wire is measured as \(15.4 \mathrm{~N}\). What is the angle of the wires with the horizontal?

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