A 83 kg person is hanging off the edge of a cliff, holding a rope. A 101 kg person is on the top of the cliff holding the other end of the rope. Find the minimum coefficient of static friction that would prevent the 83 kg person from falling.

Answer 1

You have to use F=ma and draw a force diagram for each person.

For the person hanging down
F = ma
83*g - T = 83*0
(you don't want it to accelerate)
(T = tension in the string)

For the person standing on top.
F = ma
T - Ff = 101*0
(Ff = force of friction = (mu)*R
where mu is the Greek letter representing the coefficient of friction and R is the force exerted by the ground on the person standing on the ground. R = mg = 101*g for the person on top)

You get two equations:

T = 83*g (from the 1st equation)
T = (mu)*101*g (from 2nd equation)

Therefore:
83*g = (mu)*101*g
83 = 101*mu

mu = 83/101

Hope that helps.

Answer 2

the velocity of a car A relative to a car B is 15km/h in a direction