The answer is Newton’s first law of motion.
Any thing in uniform motion will continue to be in uniform motion.
Any thing in rest will continue to be at rest.
To change the state of rest or state of uniform motion a force is necessary.
Uniform motion means constant velocity or uniform velocity.
Only to increase or decrease the velocity a force is necessary; to keep it in uniform motion no force is necessary.
Once a body is set into a uniform motion no more force is necessary; it will continue to be in uniform motion.
But in practice a force called friction comes into play always and this force brings the body to rest.
If this frictional force is small, the body comes to rest only after moving through a long distance or long time.
In the case of golf ball, the friction is so small that it moves through a long distance before coming to rest.
Any moving body will have velocity (speed) and hence momentum (mass x velocity) and kinetic energy.
The statements that a body is moving, a body is having velocity, a body is having momentum and a body is having kinetic energy are all one and the same.
A body is having kinetic energy means it is moving and vice versa.
In the case of a car or train moving with some constant speed, it is a fact that we are using motor power. But the motor power (force) is just applied to over come the frictional force and not to accelerate the car or train.
If there is no friction between the wheels and road, no motor power is necessary and the car or train once set into motion the engine can be switched off and the train or car will continue to move with constant speed indefinitely.
To stop it we must apply a force from out side.
Once a base ball is hit nothing is needed to keep it moving. In fact it is slowing down due to friction.