Review Force and Motion-4

I put a paper plate on top of a glass of water. I turned it over, and the water stayed in the glass.

The weight of the water is pushing down on the paper plate, but the plate stays in the glass because the pull of gravity is being balanced by another force. What is that force?

  1. Attraction

    No. The slight attraction between the water and the glass is not enough to balance the pull of gravity.
  2. Air pressure

    Yes! Because the plate is keeping outside air from entering the glass, outside air pressure is keeping the plate in place. As long as the outside air pressure is enough to balance the weight of the water and the plate, it will stay in place. If you made a small hole in the glass to let outside air get in, that would unbalance things, and the water would fall out.
  3. Surface tension

    No. The water tension at the surface of the water would not balance the force of gravity.
  4. The weight of the paper card

    No. Gravity is pulling down on the paper plate and the water. The weight of the paper does not help balance the force of gravity.

Click to see which state standards this question tests, and which of my videos, experiments, and other resources support that topic.


SC.2.P.13.3 Recognize that objects are pulled toward the ground unless something holds them up.

SC.3.E.5.4 Explore the Law of Gravity by demonstrating that gravity is a force that can be overcome.

SC.5.P.13.4 Investigate and explain that when a force is applied to an object but it does not move, it is because another opposing force is being applied by something in the environment so that the forces are balanced.

SC.6.P.13.3 Investigate and describe that an unbalanced force acting on an object changes its speed, or direction of motion, or both.


UT.3.III.2.c Compare the relative effects of forces of different strengths on an object (e.g., strong wind affects an object differently than a breeze).

UT.4.II.1.c Investigate evidence that air is a substance (e.g., takes up space, moves as wind, temperature can be measured).


3-PS2-1 Plan and conduct an investigation to provide evidence of the effects of balanced and unbalanced forces on the motion of an object.

MS-PS2-2 Plan an investigation to provide evidence that the change in an object’s motion depends on the sum of the forces on the object and the mass of the object.