1. A force of 18 N accelerates an object at 3 m/s. What is the object's mass? 2. A force acting on a given mass produces an acceleration of 20 m/s. What value of acceleration is produced by twice the force acting on half the mass? 3. A 10-kg bookcase is pushed across the floor with an acceleration of 4 m/s. If the force of friction acting on the bookcase is 20 N, how much force is required to push it? 4. The Earth exerts a force of 1 N on a falling object. The mass of the object is 1 kg. How much force does the object exert on the Earth? 5. Two horizontal forces act in opposite directions on a 35-kg object. One of the forces is 200 N. What must the other force be for the object's acceleration to equal that of gravity?