example question:
a can will only just fall over if the tennis ball has an energy greater than 0.1 J. find the kinetic energy of the can if the tennis ball has an incoming energy of:
a) 0.01 J
b) 0.5 J
answer:
a) 0.01 J is not enough to knock over the can, so the can remains stationary (has 0 kinetic energy)
b) 0.1 J has to go into knocking the can over, the energy left over goes into the cans kinetic energy:
cans kinetic energy = 0.5 - 0.1 = 0.4 J
==================================================
example question:
an electron will only emit from a certain surface if the incoming photon has an energy greater than 1.9eV. find the kinetic energy of the electron if the photon has an energy of:
a) 1 eV
b) 2.3 eV
answer:
a) 1 eV is not enough to emit the electron, so the electron remains bound to the surface (has 0 kinetic energy)
b) 1.9eV has to go into emitting the electron, the energy left over goes into the electrons kinetic energy:
KE = E - work function
= 2.3 - 1.9
= 0.4 eV
==================================================
NOTE: i may, for example, instead of given the incoming photons energy, have given you its wavelength or frequency. in which case you would have to convert to energy using E = hf or E = hc / λ
