of an object is directly proportional to the force applied to it and inversely proportional to its mass. F = m x a. 7. How much force does a 20, 000 kilogram rocket develop to accelerate 1 m/s2? Ans. F = m x a = 20,000 kg x 1 m/s2 = 20,000 N 8. What is the cause of friction and in what direction does it act with respect to the motion of a sliding object? Ans. Friction is caused by the contact force between two objects and the irregularities between the surfaces of the objects. It always acts opposite to the direction of motion. 9. If the force of friction acting on a sliding crate is 100 N, how much force must be applied to maintain a constant velocity? Ans.