B Equivalence of Frictional and Applied Force

[mopit_011]

TL;DR Summary

We can measure frictional forces. By placing the body on a horizontal surface where it experiences a frictional force, we could attach a spring and pull the body with just the right force so that it moves at constant velocity. Why would the body begin moving when the frictional force becomes equivalent to the applied force by the spring?

The following passage is from Halliday Resnick Krane in Chapter 3 which is about dynamics in one dimension.

"We can measure frictional forces. By placing the body on a horizontal surface where it experiences a frictional force, we could attach a spring and pull the body with just the right force so that it moves at constant velocity."

I had assumed that the passage was talking about static frictional force and do not understand why when the frictional force is equal to the applied spring force, the body would move at constant velocity. Wouldn't the net force still be 0 Newtons on the body meaning that it does not accelerate? Thank you so much!

 

[kuruman]

I had assumed that the passage was talking about static frictional force and do not understand why when the frictional force is equal to the applied spring force, the body would move at constant velocity. Wouldn't the net force still be 0 Newtons on the body meaning that it does not accelerate? Thank you so much!

Are you confusing acceleration and velocity? A body that moves at constant velocity has zero acceleration. "Zero" is a special case of constant velocity.

 

[Lnewqban]

... I had assumed that the passage was talking about static frictional force and do not understand why when the frictional force is equal to the applied spring force, the body would move at constant velocity. Wouldn't the net force still be 0 Newtons on the body meaning that it does not accelerate? Thank you so much!

I believe that the passage was referring to both, static and kinetic forms of friction between the static body and surface first, and then to the relative movement.

Clarification: Because the third law of Newton, the magnitude of the static frictional force is always equal to the magnitude of the spring force, from zero up to its limit (normal force times coefficient of static friction).

Once the magnitude of the force pulling or pushing our spring becomes greater than that limit, the body "detaches" from the "static grip" of the surface, and starts slidding over it.

After a small period of aceleration, the body should start sliding at constant velocity, as a new balance between kinetic friction and spring force is reached.

Because the value of the kinetic coefficient of friction is always smaller than the static one, the spring force should be constant and of lower value than the earlier reached max limit while the body keeps sliding at constant velocity (same velocity at which the force pulling or pushing the spring keeps moving).

You can find a better explanation here:
https://courses.lumenlearning.com/suny-osuniversityphysics/chapter/6-2-friction/