Conversation started Apr 22, 2020 at 7:00.
Apr 22, 2020 07:00
@JohnRennie: Hi sir. Good morning :-)
@GuruVishnu hi :-)
Are you free now sir? I'm having a doubt about coefficient of friction.
Yes I'm free. In fact I finished work early today so I'm free for the rest of the morning :-)
Ok sir :-)
Here a ball is rotating between a rough wall and a smooth inclined plane.
The following is the free body diagram of the ball:
Yes, that looks good to me.
Apr 22, 2020 07:04
Ok sir. On equating the forces in the vertical direction, I get:
$$\mu N_1+N_2\cos\theta=W$$
Similarly for the horizontal direction:
$$N_1=N_2\sin\theta$$
From these two, we get:
$$N_1=\frac{W}{\mu+\cot\theta}$$
Or in other words, the normal contact force exerted by the wall depends on the coefficient of friction. As $\mu$ increases, $N_1$ decreases and vice versa.
I haven't checked the algebra, but that makes sense.
We know that the coefficient of friction is independent of the normal contact force. But here the normal contact force depends on coefficient of friction. What is the reason for this?
The ball is pressed against the wall by its own weight. Yes?
Yes sir.
But the effect of the frictional force is to reduce the weight of the ball because it applies an upwards force that counteracts the downwards force due to gravity.
But if the (net) weight of the ball is reduced then the force with which it presses against the wall is also reduced.
Apr 22, 2020 07:11
Ok sir. I understand the apparent weight is less than the true weight due to the upward frictional force. But I think here we need to compare the degree of roughness of the wall and the normal contact force.
Or in other words, how can the wall repel away the ball when the roughness (coefficient of friction) increases?
The wall doesn't repel the ball.
Suppose the friction was very high then as soon as the rotating ball touched the wall it would fly upwards so the (net) weight would be zero. If the ball is not in contact with the slope then there is nothing pressing it against the wall so the normal force is zero.
Fine sir. I get the idea. So is it incorrect to say $N_1$ depends on $\mu$ when in general $\mu$ doesn't depend on the normal contact force?
$N_1$ and $\mu$ are related so they both depend on each other.
Although normally $\mu$ is a constant because it's a property of the surface.
So I guess I'd probably say $N_1$ depends on $\mu$ (as well as the mass of the ball and the slope angle).
Apr 22, 2020 07:28
Ok sir. Thank you :-)
I think the problem with my comparison of the normal force and coefficient of friction is here there are other factors disturbing these two quantities where under normal conditions only these two are present.
 
Conversation ended Apr 22, 2020 at 7:29.