It takes a set amount of energy to move something, like a door. That energy can be from high effort over a short distance or low effort over a long distance. When you’re further from the hinge, you have to push a few feet compared to a few inches near the hinge.
Energy = force (effort) x distance
Edit: this is true for a set door speed.
Other answers are sort of correct, but they’re missing an important component: time. When you push on the door from farther away from the hinge, you’re often pushing the door over a longer period of time than if you’re pushing right at the hinge. It’s easier because you’re exerting that energy over a large period of time, which means that your rate of producing the energy (the power, in physics terms) is smaller. So you effectively don’t have to push as hard to get the same result because you’re pushing for longer.
It’s the same amount of “work”–pushing hard for a short distance or pushing less hard for a longer distance–, but that doesn’t answer your question.
“Why is it easier?”
Imagine moving a large bucket of pennies. You could struggle to lift the whole bucket or you could scoop them out a handful at a time or you could pick each penny out one by one. You would probably pick the middle option, because that’s “easier”. It’s a task sized for you. If you were much stronger, you’d just lift the whole bucket. If you were much weaker, you would move one penny at a time. In all cases, you pick the “easy” option.
Back to doors. Why is it “easy” to push where the knob is? Because we deliberately put knobs where it would be easiest for a human to use. When we have very heavy doors, like on ships or in banks, we use wheels and cranks to make it less like lifting a heavy bucket and more like scooping handfuls of pennies. It becomes “easy” because we made it easy.
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