Suppose two children push horizontally, but in exactly opposite directions, on a wagon containing a third child. the first child exerts a force of f1 = 75.0 n, the second a force of f2 = 90.0 n, friction is 12.0 n, and the mass of the third child plus wagon is 29.0 kg. (a) what is the system of interest if the acceleration of the child in the wagon is to be calculated (b) draw a free-body diagram , including all forces acting on the system ? (c) calculate the acceleration (d) what would be the accerelation be if friction were 15.0n?