Gears are wheels with teeth that fit together - like the gears on your bike. On a bike you can see the gears, but on a car you can't, however they work on the same principles.

You have a small wheel that you turn (with your feet on the bike, or with the motor of the car) and you connect it with a cable to the gears, and the gears get connected to the wheels that touch the ground; that's basically what you have going in your bike, right? To make the tires turn through one rotation, you would have to turn the small wheel many times, and you would get really tired. But if you use in-between-size wheels (that is, the gears) to connect the small wheel you turn to the larger tires that touch the ground, then you are turning the smaller wheel, which turns the medium sized wheel, which turns the larger wheels (that is, the tires) that touch the ground. The larger the gear wheel, the farther you go on one turn of the smaller wheel.

I don't know about the car that your sister's boyfriend got, but this is basically how gears work. Try it with your bike - when you use the smaller gears, it is relatively easy to pedal, but you don't go that far in one turn of your foot; in higher gear, it is harder to make your feet go around, but you go farther for every push. In a car, it is the motor that is doing that work for you, so that is why it will go faster,and you won't feel it the way you would on a bike!

Try the experiment yourself on level ground: using the smallest gear, if you turn your feet around the pedals at a steady rate, you will go "so far." Now shift to the larger and larger gears - it will be harder to get your feet going, but you will find that if you turn your feet around the pedals at the same rate as you did before, that you will indeed go faster.

Now, for your second question:
A ground cable connects your electrical devices to "ground" - literally the earth. That way any stray charge goes into the ground, and not into your hand or back into the device. Stray charges, if you happen to touch them, will "go to ground" through YOU, and that is when you feel a "shock!"

I found a great web site at:
gear ratio

A car engine performs best at a certain number of cycles per minute or revolutions per minute (rpm). The gear ratio basically determines how fast the tires on a car rotate with respect the rpm of the engine. Cars have multiple gears so they can go a variety of speeds. With a manual transmission ("stick shift") you have to change gears manually while an automatic transmission changes gears for you.

Changing the gear ratio would affect how fast a car would go in each gear so this would be a way to make it so the car can go faster for a given gear and engine rpm.

Since electricity from the power company is referenced to "Earth Ground" (literally the earth, which is a very big reservoir of electrons) the ground wire is a good safety feature that allows any excess charge to return to ground.

The ground wire if often attached to the "case," meaning the outside surface of your blender, toaster, etc. In case there is an electrical short inside the device, the ground connection prevents the outside of the device become "hot" thereby preventing someone from getting a nasty shock.

Can also get more info at:
electricity_1 .