A straight rod of length L extends from x=a to x=L+a. Find the gravitational force exerts on a point mass m at x=0 is (if the linear density of rodμ=A+Bx2)

If two students are running down the hall toward each other, trying to get to class, and they have the same mass and acceleration, what will happen when they collide? will their forces cancel out or will each one experience a reaction?