Difficult Physics Problem? Assistance greatly appreciated. :)?

Outline of a solution:

(1) The sodium lamp emits energy, in fact 139 Joule/sec = 139 Watt

(2) each photon has also an energy computed with E = h * nu where nu is 1240 nm

divide the total energy (in 1) by each photon and you will get an enormous number.

divide the number of photons by 0.260 sec

Examine the forces acting on each block individually, and use "F_net = ma" on each block individually. Also, make use of the fact that each block has the same "a" as the others (since they're all tied together, they're forced to accelerate at the same rate); and each block has the same "m" (as given in the problem). This will give you a set of equations that you can then solve simultaneously to get the answer. Block "A" -------------- Only one force (T_AB) acts on it: F_net = T_AB = ma Block "B" -------------- Force T_AB pulls it to the left, and Force T_BC pulls it to the right: F_net = T_BC - T_AB = ma Block "C" -------------- Force F_vec pulls it to the right; and force T_BC pulls it to the left: F_net = F_vec - T_BC = ma Recap: ----------- You now have these equations: T_AB = ma T_BC - T_AB = ma F_vec - T_BC = ma The rest is algebra. You are given the values of "T_BC" and "m", so that means you have three unknowns ("T_AB", "F_vec" and "a"). That's 3 equations in 3 unknowns, so you can use the algebra of simultaneous equations to get the values of all three.