guncamp

In my high school physics class we are working with energy. We had the following problem that puzzled us slightly: A 70 kg box is pulled horizontally with a constant force of 200N. It is pulled for the first 10 meters across a frictionless surface, and then is pulled another 10 meters over a surface with a coefficient of of friction that is equal to .3. Find the final velocity of the box. so the the total energy done is equal to: (200)(10) + (200)(10) - (70)(9.8)(10)(.3) = 1954 Therefore, it is equal to the final kinetic energy 1954= 1/2 (70)(v2) v= 7.5 m/s. This is all fine and dandy, but the problem appears to arise in the second part where the box is moved across the surface with coefficient of friction of .3. μ=.3 (9.8)(70)(.3) = 205.8 N of frictional force is acting against the 200 N of constant horizontal force applied to the box. Now, since the amount of kinetic friction is greater than the force being applied, how could the box traverse the final 10 meters? The only thing I could think of is the energy built up from the first 10 meters was enough to help carry it across the final 10 meters with friction. Are we missing something here? *Note that I don't have my textbook with me, and I think I provided all the information that was given to me in the problem. Bear with me if I forgot a piece, I did my best to recall all the information. Thanks in advance.

I must have misread it. Looking back it indeed does say photons of small energy. Silly mistake, I must have just overlooked it. Thanks for the clarification.

My apologies, I for some reason put proton instead of photon. i corrected the mistake.

I am reading a book on an introduction to quantum physics and I came across what appears to me as two conflicting statements. I believe it said that a single photon cannot have enough energy to knock loose an electron due to the fact the the photon is quantized. Yet a few pages after this the book claims that "The energy necessary to knock loose an electron is provided by a single photon in an incoming electromagnetic wave". So which statement is true?