You push on a skateboard and it moves. What type of forces are responsible for its motion? Balanced forces
Friction forces
Gravity forces
Unbalanced forces

Which theory was contradicted by experiments with the photoelectric effect? light spreads out after it passes through a small opening. as soon as light strikes metal, electrons will be ejected. visible light, regardless of color, will cause the ejection of electrons when striking metal. the kinetic energy of ejected electrons depends on the frequency of light that strikes the metal.