Question 7-1.

If there is frictional force acting on an object, and there is no other force to counter balance the frictional force, then the object will slow down and eventually stop. The momentum of an object is conserved ONLY IF the total force acting on the object is zero. Otherwise, momentum will not be conserved and the object will accelerate (or decelerate). Let us consider a moving car at a speed of 55 mph. Frictional force and air resistance (opposite direction to velocity) will act against the force from the engine (same direction as velocity). If these forces add up to zero, momentum will be conserved and the car will move forward with a constant velocity. If the driver steps on the gas pedal, the engine force will be larger and the total force will not be zero any more. Momentum will change (i.e. "not conserved") and the car will accelerate forward. Similarly, if the driver release the gas pedal and there are only frictional forces acting on the car. Momentum will not be conserved and the car will slow down and eventually stop. After the car stops, the frictional forces will disappear and the car will remain stationary. Momentum is conserved (=0) again.