1. A cart of mass 4 kg moves at a speed of 10 m/s to the right when it encounters a horizontal spring of spring constant 250 N/m. The maximum potential energy stored the spring in bringing the cart to rest is (A) 80 J (B) 200 J (C) 280 J (D) 400 J

The answer is 4f======since electrostatic force is given by    we let f be the electrostatic force when  r = 1 m    if we let  denote the electrostatic force when the distance is decreased so that r = 0.5m, then    we established earlier that f = kq₁q₂. therefore    the force will now be 4f.