Calculating Photon Momentum & Force

AI Thread Summary
To determine the momentum of a photon with a wavelength of 400nm, the calculation yields a momentum of approximately 1.66x10^27 kgm/s. To support a 1mg mass using this photon momentum, a force of 9.81x10^-3N is required. The relationship between force and momentum change is expressed through the equation F = dP/dt. To find the number of photons needed per second, the total momentum change must be divided by the momentum of a single photon. The discussion emphasizes understanding the connection between force and momentum in the context of photon interactions.
ride4life
Messages
33
Reaction score
0

Homework Statement


a) Determine the momentum of a photon of light of wavelength 400nm.
b) A mass of 1mg is being held up by a beam of 400nm light. How many photons per second would have to hit the mass (from below) to do this, assuming the photons bounce back with the same momentum as they hit.


Homework Equations


a) p=h/(wavelength)
b) F=mg

The Attempt at a Solution


a) p=6.64x10^-34/400x10^-9=1.66x10^27 kgm/s
b) F=10^-3x9.81=9.81x10^-3N
I can only figure out how much force is needed to keep the mass still, I don't see how p and F are related :(
 
Physics news on Phys.org
Try using Newtons second law in its "original" form.
 
yes i think it will solve the problem!

Its F = dP/dt
 
Don't be confused, i am sure you know the formula cupid.callin mentioned, F = dP/dt,
the problem asks you to find photons per second so you found one of the photons momentum, you try to reach dP/dt so you have to product p with dn/dt what is asked, then reach dP/dt which is force
consider P is the total momentum and p is the momentum of one photon
 
TL;DR Summary: I came across this question from a Sri Lankan A-level textbook. Question - An ice cube with a length of 10 cm is immersed in water at 0 °C. An observer observes the ice cube from the water, and it seems to be 7.75 cm long. If the refractive index of water is 4/3, find the height of the ice cube immersed in the water. I could not understand how the apparent height of the ice cube in the water depends on the height of the ice cube immersed in the water. Does anyone have an...
Thread 'Variable mass system : water sprayed into a moving container'
Starting with the mass considerations #m(t)# is mass of water #M_{c}# mass of container and #M(t)# mass of total system $$M(t) = M_{C} + m(t)$$ $$\Rightarrow \frac{dM(t)}{dt} = \frac{dm(t)}{dt}$$ $$P_i = Mv + u \, dm$$ $$P_f = (M + dm)(v + dv)$$ $$\Delta P = M \, dv + (v - u) \, dm$$ $$F = \frac{dP}{dt} = M \frac{dv}{dt} + (v - u) \frac{dm}{dt}$$ $$F = u \frac{dm}{dt} = \rho A u^2$$ from conservation of momentum , the cannon recoils with the same force which it applies. $$\quad \frac{dm}{dt}...
Back
Top