2. a woman prevents a 3kg brick from falling by pressing it against a vertical wall. the coefficient of friction
is 0.6
a what force must she use?

Explanation:

The downward force is the weight of the block which can be calculated by W = mg = 3×10 = 30 N

So the frictional force must be equal to 30 N to prevent the block from fall.

So to calculate the normal reaction ( the force exerted by the women)

F = xR

Where F = frictional force

x = coefficient of friction

R = normal reaction.

By substituting you can get,

30 = 0.6× R

R = 50 N

when insulating materials rub against each other, they may become electrically charged . electrons , which are negatively charged, may be 'rubbed off' one material and on to the other. the material that gains electrons becomes negatively charged. the material that loses electrons is left with a positive charge.