forces

Four charges are placed at the corners of a square of side $$10 \mathrm{~cm}$$，as shown in Figure 7． $$ \begin{array}{l} \mathrm{A}=+10 \times 10^{-9} \mathrm{C} \\ \mathrm{B}=+8 \times 10^{-9} \mathrm{C} \\ \mathrm{C}=-12 \times 10^{-9} \mathrm{C} \\ \mathrm{D}=-6 \times 10^{-9} \mathrm{C} \end{array} $$

A point charge of magnitude $$2 q$$ exerts an electrostatic force of magnitude $$F$$ on a point charge of magnitude $$q$$ that is a distance $$R$$ away．What is the magnitude of the force that the $$2 q$$ point charge exerts on a point charge of magnitude $$3 q$$ that is a distance $$R / 5$$ away？( )

A beam of weight $$6.0 \mathrm{~N}$$ is suspended from two strings $$\mathrm{P}$$ and $$\mathrm{Q}$$． String $$\mathrm{P}$$ is $$30 \mathrm{~cm}$$ from the left-hand end of the beam，as shown．String $$\mathrm{Q}$$ is not shown． The tension in string $$\mathrm{P}$$ is $$2.0 \mathrm{~N}$$． What is the tension in string $$\mathrm{Q}$$ and where is it attached so that the beam is in equilibrium？( )

Which object is in equilibrium？( )

The diagram shows a wooden beam $$PQ$$，of negligible weight，which is attached to a wall by a hinge at $$P$$ and kept in a horizontal position by a vertical rope attached at $$Q$$ The beam is $$3.0 m$$ in length． A man of weight $$800 N$$ walks along the beam from $$P$$ to $$Q$$． What is the distance of the man from $$P$$ when the tension in the rope at $$Q$$ becomes equal to $$500 N ?$$ ( )