In the bucket experiment when the bucket reaches the top of the circle why will it have a normal force acting on the water downwards? Doesn't normal force oppose any other force? There is no force acting upwards… (As we are an observer in an inertial frame so we wont consider the centrifugal force because it is a pseudo force)
[Physics] Rotating the bucket in circular motion without spilling water
forceshomework-and-exercisesinertianewtonian-mechanics
Best Answer
I want to focus on one thing, here. The nature of the normal force.
You write
which is a easy impression to get when you are introduced to the normal force in the context of things sitting on other things in a gravitational field, but that's not the best way to think about it.
The normal force keeps things from occupying the same space.
So consider a book sitting on the lab bench. It's subject to gravity, and in the absence of other forces would fall. But to fall it would have to occupy the same space as the solid top of the bench. The normal force is the interaction between to two objects that resists their inter-penetrating. In this case it has to provide a force that is equal and opposite to that of gravity to make that happen.
In the case of the water in the bucket, it's inertia would take it in a straight line, gravity modifies that into a parabola, but the sides and bottom of the bucket are moving in a circle for both those things to be true (the bucket goes in a circle and the water goes in a parabola) the water would have to move through the bottom of the bucket. The normal force serves to prevent the water from penetrating the solid material of the bucket and must supply whatever forces (beyond that of gravity) are necessary to cause the motion to be in a circle.