No the can go faster downhill or uphill
Gravity. The car is heavy and doesnt want to move, so you must excelerate to make the car go uphill. If a car is going downhill, the is no need to excelerate because gravity is pulling the car downhill.
cool
TON -TAN
Because your legs, just like the engine of a car, works best a certain rate of movement and a certain load. A gear that gets you moving easily will have you cranking your legs like crazy w/o getting up to speed while a gear that keeps your legs spinning at a reasonable rate when going fast will be very heavy to turn from a standstill. The ability to change gears allows you to keep pushing with a force and a rythm that suits you, regardless if you're riding on the flat, uphill, or downhill.
Don't really know what you mean by a jump bike... Generally speaking a MTB is intended to be fairly versatile, and it will usually have gears enough both for slow slogs uphill, for going along the flat, and for powering downhill. Something like a dirt bike is usually adapted to run in pretty much one condition only, so taking it on a longer trail with both uphills and descents is likely to be heavy going.
Because car is lighter and has less gravitational force to counter when moving up in comparison to a heavy loaded truck.
Mudflow
Water from the rain mixes with loose sediment, forming a dense slurry that can flow downhill.
Glaciers advance when the weight of the new snow that piles on top of the glacier gets heavy enough to push it downhill.
I bet it does, because the energy of heavy metal music is bound to make your heart beat faster. It does for me. ;P
Glacial movement starts near the tops of mountains and ridges when the snowfall is heavy enough to start sliding downhill.
Sometimes it does matter but it is easier to go faster when you are a kid and are lighter!