# Search results

1. ### Pion decay in QED

For the diagram that just has the up/anti-up annihilating to a photon I don't know what else the coupling could be... For the two photon diagram you'd get (2/3*e)+(-2/3*e)=0?!
2. ### Pion decay in QED

The coupling strength is the charge of the up quark multiplied by e, the electronic charge, so (2/3)*e?
3. ### Pion decay in QED

Hi, I was wondering whether the decay of the Pi-0 meson in QED to an electron positron pair can occur as follows: Pi-0 -> virtual photon -> e+e- or does it have to go via Pi-0 -> two virtual photons -> e+e- (the Feynman diagram has a 'square' of virtual electrons/photons)? I have...
4. ### Particle exchange and parity

Yes that makes sense, thank you. If we had a system of say 3 protons the the overall angular momentum would not tell us about the symmetry with respect to particle exchange - is that correct?
5. ### Particle exchange and parity

I'm struggling with the relation between particle exchange and parity with the case of para- and ortho-hydrogen. The overall wavefunction must be antisymmetric with respect to particle exchange and so for para-hydrogen (an antisymmetric spin state) the spatial part of the wavefunction must be...
6. ### Components of 4-momentum

Firstly, apologies for the notation. The 4-momentum of a massive particle (rest mass m) is defined by p=mu where u is the 4-velocity. Thus in a frame S in which a particle has 3-velocity u the components of p are [p]=gamma*(mc,mu) How can we then identify the zeroth component of...
7. ### Conservation of angular momentum of a falling particle

Thanks for your help diazona. That improves my confidence in the integration. I wonder if anyone can see my mistake in setting up the equation? Thanks.
8. ### What is the displacement of the blue piston?

I guess you have a two cylinder system with an incompressible fluid? For a) you may call, generally, the diameter of the red cylinder D, and thus the diameter of blue cylinder 2D. What are the areas associated with diameters? We thus may work out the volume V=Area x distance through which the...
9. ### Work done on a block by net force

Yes it is in this case. It is pointing vertically upwards.
10. ### Work done on a block by net force

I would start by drawing a diagram of the situation. Draw the 4 forces in, i.e. weight W, reaction force R from the floor, friction F' and the force we are pulling with F. Since the trunk isn't moving the vertical direction, we may equate the 'downwards' forces with the 'upwards' forces, i.e...
11. ### Conservation of angular momentum of a falling particle

Homework Statement A stone is dropped from a stationary helicopter 500m above the ground, at the equator. How far from the point vertically below the helicopter does it land? Homework Equations Conversation of AM The Attempt at a Solution Let the height above the ground it is...
12. ### What the hell is a coulomb

The Coulomb is the SI unit of charge. It is just a standard against which we can compare the relative charges of difference objects; just like we might say this object has a mass of 1kg, and this object has a mass of 3kg, we could say one object has 1 coulomb (C) of charge, and another has 10C...
13. ### Rocket Equation with varying gravity

So, the rocket equation is F_ext = m(dv/dt) + u(dm/dt) where m is the mass of the rocket, v the velocity, u the effective exhaust gases speed, and F_ext the external forces on the system. If we take a constant mass ejection rate p, and take the external force to be the gravitational...
14. ### How do you write in scientific notation ?

How do you write in scientific notation "the (SI) units of x are..."? For example, the SI units of time are seconds. Is there any succinct way I can write this? Maybe it's like this: {time} = s ?
15. ### Resolving forces

Apply F = ma down the slope by calculating the component of the weight down the slope to find the accleration. Then use v^2 = u^2 + 2as to find the velocity v after s = 3 metres. Hope this helps.
16. ### Calculus Highway Construction Problem

You've recognised the general equation of a parabola to be y = Ax^2 + Bx + C. Because we're dealing with gradients, I suggest you differentiate this equation with respect to x. In this situation, y is the elevation, and x is the horizontal distance. A sensible co-ordinate system would be to...
17. ### Prove that the Given Equation is an (Trig) Identity

Remember, you're trying to PROVE that tan(x/2) = sinx/(1+cosx), so don't start the proof by writing this! I would suggest by writing down the right hand side (i.e. sinx/(1+cosx)) because this looks intuitively like it can be simplified, and then manipulate this to find it equal to tan(x/2) My...
18. ### Prove that the Given Equation is an (Trig) Identity

Okay so you know the identity cos2A=(cosA)^2-(sinA)^2, as you've written above. Remember (sinA)^2+(cosA)^2=1 ? Try substituting (cosA)^2 from the second equation into the first, then rearrange to find the denominator. Use sin2A=2sinAcosA for the numerator, and you should be okay! Hope this...
19. ### Physics help average acceleration

You need to know the accleration to find the time using the s = ut - 0/5at^2 equation, so that's no good surely, because that's ultimatley what you're trying to find. Use your initial velocity as the one you calculated, and write down what the final velocity must be. You know the displacement...
20. ### Physics help average acceleration

Try using one of the equations of motion which doesn't involve time, that is, v^2 = u^2 + 2as where v is the final velocity, u is the initial velocity, a is the average acceleration and s is the displacement between when vel = u and vel = v
21. ### Gauss's Law

Thanks very much.
22. ### Simplifying with Pythagorean identites.

That is an option! haha
23. ### Simplifying with Pythagorean identites.

The answer is 1, if you didn't already know. Try again but write sinx as S, and cosx as C; this makes the expression a whole lot more manageable. I suggest taking a factor of 1/S^2 out and then forming a single fraction. Hope this helps.
24. ### Newton`s Law

Draw yourself a little diagram of the situtation. Include the forces acting. You should have a force(say, F) coming out of the loco, between the loco and the first car call the tension T1; remember to draw two arrows for this tension, one pulling the loco towards the car, the other pulling the...
25. ### Finding Final Speed Using

Energy by virtue of its motion is called kinetic energy, and kinetic energy is given by the formula K.E. = 0.5mv^2 where m is the mass of the body and v is the velocity.
26. ### Finding Final Speed Using

There are two ways of approach. One is using the constant accleration equation (v2)^2=(v1)^2+2as where a is the acceleration of the crate and s is the displacement caused by the accleration, i.e. v1 = 0, a = F/m (Newton's Second Law) = 25/4.5 = 50/9, s = 1.2, so v2 = sqrt (2.4*50/9)...
27. ### Complex numbers help

Okay, your first question was to show that |y + x|^2 = |y|^2 + |x|^2 + 2|yx|cos(a1-a2) Using the definitions of y and x given. Start by writting down that y = |y|exp(ia1) = |y|(cos(a1)+isin(a1)) x = |x|exp(ia2) = |x|(cos(a2)+isin(a2)) Using the series definitions of the exponential, sine and...
28. ### Gauss's Law

Hi, can you derive Gauss's Law without using Coulomb's Law? If so, how? Thanks
29. ### Rotational acceleration

Oh yes of course. Thanks for your help.
30. ### Rotational acceleration

Cheers that makes things clearer. I'm afriad my experience at differentials is sufficiently small that I don't know how to derive both sides. V_t goes to a_t by definition of acceleration I suppose, but I haven't got any t's on the RHS of the equation, and as its differentiating w.r.t t, I'm...