Is the Law of Motion Explanation Correct?

[snshusat161]

View this attachment and tell me whether it is correct or not? It is written there that to move the block we need F Sin\theta > f_s. I agree but then they have given the value of f_s equal to \mumg. How can it be possible as we don't have any vertical motion.

 

[rl.bhat]

Frictional force is a self adjusting force which depends on the normal reaction. The maximum friction force ( when the object starts moving) is mu*mg. This frictional froce acts in the opposite direction of the relative motion of the two objects.

 

[snshusat161]

wow, but I'm already familiar with this concept. Please have a look on attachment

 

[rl.bhat]

wow, but I'm already familiar with this concept. Please have a look on attachment

How can it be possible as we don't have any vertical motion
In this problem there is no question of vertical motion.

 

[snshusat161]

You are not understand what I mean to say or may be you are acting too lazy to look on the picture I've given.

 

[rl.bhat]

I agree but then they have given the value of LaTeX Code: f_s equal to LaTeX Code: \\mu mg. How can it be possible as we don't have any vertical motion.
I have gone through the attachment and your above statement. In the attachment there no suggestion of vertical motion.
Actually the expression should be
fs = mu[Fcos(theta) + mg], because R = mg + f*cos(theta)

 

[snshusat161]

yes, that's what I wanted to confirm. Thanks

 

[snshusat161]

Can you tell me what should be minimum angle if it derived correctly.

 

[snshusat161]

And see this, if the frictional force is greater than the applied force then how can a body move. What rubbish they have printed on the book.

 

[rl.bhat]

In the first problem, if theta is zero, the block will not move in the horizontal direction. Net normal reaction is mg + F. As the angle increases, f*sin(theta) increases and R decreases.
Body starts moving when Fsin(theta) = mu[mg + Fcos(theta)].
The formula derived in the attachment is true when the object placed on the horizontal plane starts moving when the angle of inclination of the plane to the horizontal satisfies the relation mu = tan(theta)