3. A car travelling at 20. m/s stops in a distance of 40. m. What is its acceleration in m/s^2? (Please pay attention to direction and do not include
units in your answer.)

Suppose that three astronomical objects, 1, 2, and 3 are observed to lie on a line, and the distance from object 1 to object 3 is d. object 1 has four times the mass of object 3, and seven times the mass of object 2. find the distance between objects 1 and 2 for which the net force on object 2 is zero.