Question

A ruler, balanced at its center point, has two coins placed on it, as shown in the figure. (Intro 1 figure) One coin, of mass M_1= 10g, is placed at the zero mark;the other, of unknown mass M_2, is placed at the 4.7 mark. The center of the ruler is at the 3.0 mark. The ruler is in equilibrium; it is perfectly balanced.

[IMG]http://i40.tinypic.com/15cj2g9.jpg[/IMG]

http://tinypic.com/view.php?pic=15cj2g9&s=5

A)

Does the pivot point (i.e., the triangle, in the diagram, upon which the ruler balances) exert a force on the ruler? Does it exert a nonzero torque about thepivot?
Choices:
yes/yes
yes/no
no/no
no/yes

B)
Although the pivot exerts a force on the ruler it does not exert a torque. Why not?
Choices:
Because the force is in a different direction.
Because the distance from the exerted force to the pivot is zero.
Because the exerted force is along the ruler.

C)
Find the mass $M_2$ . (Express your answer for the unknown massnumerically, in grams, to two significant digits.)

D)

Now do a similar problem, but without numbers: We have a ruler with a total length $L_r$ and a pivot at its middle. An object of known mass $M_1$ is placed a distance $L_1$ from the end of the ruler, as shown. An object of unknown mass $M_2$ is placed at the end of the ruler.
[IMG]http://i43.tinypic.com/200pdtv.jpg[/IMG]
http://tinypic.com/view.php?pic=200pdtv&s=5

Find the mass $M_2$ . (Express the mass in terms of $M_1$ , $L_1$ , and $L_r$ .)

Answer 1

A) yes/no B) because the distance from the exerted force to the pivot is zero C) 18