Earth ramp to the fortress at Masada
(Thanks to www.HolyLandPhotos.org)
An inclined plane is any slope or ramp, like a wheelchair ramp or a slide. It makes it easier to lift something heavy, like a rock. Instead of lifting the rock straight up, you can push it a greater distance, but with less force. The amount of work remains the same. Work is force x distance, so if you increase the distance the rock moves, you can decrease the amount of force you need.
If you make the ramp steeper, you'll have a shorter distance, but it will be harder to push the rock (you'll need more force). If you make the ramp less steep, it will have to be longer, but it will be easier to push the rock. Either way, it's the same amount of work in the end, but you have the choice of doing easier work for a longer time, or harder work for a shorter time.
We call it an "inclined plane" because it is a plane - a flat surface - and it is inclined - sloped, not level. All kinds of animals use inclined planes when they choose their path up a steep hill, and people have used them since there first were people, about 200,000 years ago. By the time of Old Kingdom Egypt, about 2500 BC, people were building their own earth ramps to move heavy stones for the Pyramids. The first staircases (a kind of inclined plane, because stairs make it easier to go up than if you had to jump) also go back to the Bronze Age.
To find out more about simple machines, check out these books and games from Amazon.com or from your library: