Because of friction
Could be but probably an engine problem.
Are you going up a hill? Did you take your foot off the gas and is the compression of the engine slowing you down? Are your brakes sticking? Is your car getting the correct amount of air to fuel ratio? there are lots of reasons why a car would slow down while you're driving it. start with the easiest.
a car is much smaller and easier to shoulder check and when driving a bus you just have to rely on you mirrors, it takes longer to slow down or stop and they are larger which makes them harder to turn
gravity!
Not necessarily. A tree sloth moves slower than a slow, slow walk while a cheetah can run as fast as 60 miles per hour. That's as fast as your car moves while driving down the highway. Most mammals are somewhat fast moving.
the car should slow down and eventually stop if you keep on holding it
I am not a car professional but I think that installing brake shoes simply means installing brake pads. And brake pads are what helps you slow down and stop while driving.
Gravity is trying to push the car down the hill. It takes more power to overcome that push of gravity. Unless you give the car more gas it will slow down.
That it's old and destroyed, needs to be fixed. The car sucks.
A lot of possible causes, I recommend a careful investigation.
your alternator is dead.