4. (a) Write down (in symbols) the equation relating the resultant force F, mass m and acceleration a of a body. (b) A trolley of mass 3 kg is pulled by a constant force on a frictionless surface. (1) If the acceleration of the trolley is 2 m s, calculate the resultant force. (H) If the force applied is 3 N, calculate the acceleration of the trolley. (c) When the trolley in (b) is pushed along a different surface with a force of 6 N, it moves with a constant speed of 0.40 m s. When the trolley is pushed along the surface with a force of 15 N, describe and explain what happens in terms of the acceleration of the trolley.