If the spheres have the same volume they displace the same weight of water and experience the same buoyant force. The buoyant force pushes the sphere up. The N3L interaction has the sphere push the water (and the pan) down with the same magnitude as the buoyant force.
I think this is one of those questions that has a few factors of which we should be aware (aside from physics):
- these can be fun
- if we don't build to these, these can be damaging to student thought processes.
If we build to these (how do you find Fb of the ping pong ball? How does the buoyant force affect the force the table experiences? What happens if another ping pong ball is used with a slightly larger Volume? What happens if a metal sphere is used with the same volume?) we can build problem solving skills for kids on these topics.
If we have physics puzzle day where we ask the scale problem without anything building to this (on the multiple levels of Fb depending on Volume and on analyzing the scale or the bottom of the beaker to determine what happens to the scale) then we are always asking them what they find to be "trick questions." They develop the sense that whatever they think, the answer will be opposite. Dangerous.
It is easy to say we are teaching them to think or to analyze. It is more effective to build that process up to these questions than to give them these sorts of questions in isolation , resulting in making students think physics doesn't make sense.
Sorry to preach.
Have a good Wednesday.
Paul (where the temp is up to -4deg F and school is on)