If the value of static friction for a couch is 400N, what could be a possible value of its kinetic friction? *

Less than 400 N

Explanation:

The force of friction is a resistive force that opposes the motion of an object. It is caused by the presence of microbumps on the surfaces of the object, so that when for instance we try to make an object sliding along the surface, there is a force opposite to the push we are applying, and this force is the force of friction.

There are two types of force of friction:

- Force of static friction: this occurs when the object is still at rest. Its value changes according to the push applied: therefore, its value ranges from zero up to a maximum value given by

where

is the coefficient of static friction

m is the mass of the object

g is the acceleration due to gravity

- Force of kinetic friction: this occurs when the object is already in motion. Its magnitude is given by

where

is the coefficient of kinetic friction

Generally for most surfaces, , so the force of static friction is usually larger than the force of kinetic friction (that's why it generally requires more force to put the object in motion, rather than keeping it in motion).

Therefore, since here the force of static friction is 400 N, the force of kinetic friction must be smaller than that.

the duration of a photographic flash is related to an rc time constant, which is 0.100 μs for a certain camera.