do airplanes travel at top speed when in the air?

just like cars and trains.......NO

They travel at 'cruising speed', which is the most fuel efficient speed for the altitude they are flying at. A 747 at 35,000 feet will usually cruise at 555 mph (895 km/h).

No... they figure a good cruising speed that will yield the best fuel efficiency and least wear and tear.

seldom. it consumes too much fuel to maintain the maximum speed. however at the service ceiling operations, the maximum speed approaches the minimum one, and it is easier to reach the maximum speed.

Generally said it is not efficient to fly at the maximum speed for any other than tactical purposes.

its like travelling by a car.. you use the maximum speed only in a hurry.

Unless you're in a hurry (with enough fuel for that hurry) fly a bit slower cos the closer you fly to max speed, the more the air (you're flying through) becomes too thick/dense for your plane to fly through without using more fuel. Also the more you expose your airframe to physical stresses of flying through increasingly thicker air.

Yes they do - if required.

Unless we're trying to stretch our range, we generally fly within .02 mach of our max speed during cruise. This depends on traffic restrictions and turbulence etc...

No. For most jets it is between .79 mach(521 mph and .86 mach(586 mph) at 35,000

Yes. It's not safe to travel at top speed on the ground.