Homework Help: Is It correct (laws of Motion)

1. Apr 9, 2010

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 $$\mu$$mg. How can it be possible as we don't have any vertical motion.

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2. Apr 9, 2010

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.

3. Apr 9, 2010

snshusat161

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

4. Apr 9, 2010

rl.bhat

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

5. Apr 9, 2010

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.

6. Apr 9, 2010

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)

7. Apr 9, 2010

snshusat161

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

8. Apr 9, 2010

snshusat161

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

9. Apr 9, 2010

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.

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• wrong concept 2.jpg
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10. Apr 9, 2010

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)