# Stress Tensor - MTW Ex 5.4

1. Mar 2, 2014

### TerryW

I've been working on Ex 5.4 in MTW. The maths is fairly straight forward, but I don't really understand what is going on!

In part (b) what are the 'forces' pushing the volume through a distance? If they are forces, they must produce an acceleration but we have a constant velocity. Are these forces producing the stress in the volume?

What would be an example of a 'stressed' medium as used in this example?

If mjk is 'inertial mass per unit volume' equation 5.51 will sum to produce the total momentum in the direction of v, not one of the components.

Can anyone shed any light?

Regards

TerryW

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2. Mar 2, 2014

### Bill_K

Anything where the spatial components of the stress tensor are nonzero. Could be a gas under pressure. Could be a squeezed rubber ball.

The forces, for example, could be the forces acting on this element of the medium by neighboring elements. Whether the velocity is constant depends on whether the forces are balanced.

It will be the three spatial components of a vector, maybe not in the same direction as v.

3. Mar 4, 2014

### TerryW

Problem sorted

"If mjk is 'inertial mass per unit volume' equation 5.51 will sum to produce the total momentum in the direction of v, not one of the components."

didn't really help me, but I took another look at it and I re-wrote the expression for T01 in terms of the stress tensor in the volume's rest frame:

T01 = (T0'0' + T1'1')v1 + T1'2'v2 + T1'3'v3

and it then becomes much clearer.

For a unit volume, T01 is the x-component of momentum which is made up from

1. the contributions from the x-component of momentum arising from the rest mass moving at v1 + the x-component of flux of x-momentum x vx

2. the contribution from the y-component of the flux of x-momentum x vy and

3. the contribution from the z-component of the flux of x-momentum x vz

The rationale for the contributions 2 and 3 come from the equation preceding (5.16) on page 139.

Regards

TerryW