When you open a door what force do you expert on it

It depends, if u are pulling the door or if u are pushing the door

It normally takes up to 5lbs of force to open a door, if u use the handle.

But if u are pushing from somewhere else, away from that side of the door, suppose the center of the door, you'll have to apply more force, since it won't have the same amount of momentum, and there'll be more resistance form the door.

But it also depends on the type of the door, if it's a door that doesn't have a door closer, it'll be easier to open the door, but if it has a door closer, it'll be harder to open that door, due to the resistance.