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Here the object does not spin on the surface. (point $O$ has $M_o = 0$) However, there are non-zero net torques on the object at points $A$ and $B$ ($M_a$, $M_b \neq 0$). Why do these net torques not cause the object to start turning?

My opinion is that the normal force is constantly displacing each instant of time. Rotations are prevented in these displacements.

enter image description here

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  • $\begingroup$ Why do you have a single normal force vector? Why don't you have a distribution of normal forces across the whole surface? You can't model a contact surface with a single force if you're calculating torques. $\endgroup$
    – Bill N
    Commented Mar 22, 2020 at 15:28
  • $\begingroup$ How was the point O decided? In general, the line of action of the reaction is such that not tipping happens. $\endgroup$
    – John Alexiou
    Commented Mar 22, 2020 at 15:39
  • $\begingroup$ @ ja72 To prevent the object from falling, If at any other point, the Object falls $\endgroup$
    – Bilgehan Yılmaz
    Commented Mar 22, 2020 at 15:49
  • $\begingroup$ @ Bill N There is only one normal force, this is the location of the total normal force.. $\endgroup$
    – Bilgehan Yılmaz
    Commented Mar 22, 2020 at 15:50
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    $\begingroup$ @BilgehanYılmaz, a small technical note. I don't think adding a space between @ and the username (like @ user) will cause others to receive pings. Just plainly type @user without spaces in between @ and user. $\endgroup$
    – Vishnu
    Commented Mar 22, 2020 at 15:54

1 Answer 1

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Since there is not tipping moment and the block remains level, it means that the normal force and friction combined must pass through the center of mass

fig

With point O such that $\tan \theta = \frac{10}{40}$.

Using similar triangles from geometry you can find the location of O relative to A.

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  • $\begingroup$ How does the friction force pass through the center of mass? $\endgroup$
    – Bob D
    Commented Mar 22, 2020 at 16:06
  • $\begingroup$ @ja72 So will my object fall over in my drawing? $\endgroup$
    – Bilgehan Yılmaz
    Commented Mar 22, 2020 at 16:37
  • $\begingroup$ @BilgehanYılmaz the friction force DOES NOT act through the center of mass (point A). It acts horizontal to the surface. $\endgroup$
    – Bob D
    Commented Mar 22, 2020 at 16:49
  • $\begingroup$ @Bob D I know that. Here, the force passing through the center of mass is the combined force... $\endgroup$
    – Bilgehan Yılmaz
    Commented Mar 22, 2020 at 16:52
  • $\begingroup$ @BilgehanYılmaz That's not what you said. You said to ja72 "the friction force, not the normal and friction force, passes through the center of mass". That's what you said. The applied force acts through the center of mass. The weight and normal reaction force acts through the center of mass. The friction force does not. It provides a moment about the center of mass. Any way, I have not more time to devote to this. $\endgroup$
    – Bob D
    Commented Mar 22, 2020 at 16:58

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