1.   The x,y coordinates in meters of the center of mass of the three-particle system shown below are:

A) (0, 0)        B) (1.3, 1.7)         C) (1.4, 1.9)          D) (1.9, 2.5)             E) (1.4, 2.5)

6*(1,3)+5*(3,2)+4(0,0) = (4+5+6)(x_cm,y_cm)   so    x_cm = (6+15)/15 = 1.4    y_cm = (6*3+5*2)/15 = 1.87     (1.4, 1.9)

2. A 0.20-kg rubber ball is dropped from the window of a building. It strikes the sidewalk below at 30 m/s and rebounds at 20 m/s. The change in momentum of the ball as a result of the collision with the sidewalk is (in kg.m/s)

                 A) 10                 B) 6                C) 4                 D) 2                         E) 1
   DP = m*Dv = 0.20*(v_final - v_initial) = 0.2[+20 - (-30)] = 10
 

3. A 140-lb woman stands on frictionless level ice. She kicks a 0.2-lb stone backward with her foot so that the stone acquires a velocity of 3.5 ft/s. The velocity (in ft/s) acquired by the woman is:

A) 3.5 forward           B) 0.005 backward                C) 0.005 forward              D) 3.5 backward             E) none of these

g = gravity constant  v_w assumed positive (forward)
total initial momentum = 0 == total final momentum = (0.2/g)*(-v_s) + (140/g)*v_w     >>  v_w = (0.2*3.5)/140 = + 0.005 guess was correct.