[katieHaylor (OP)]

Raf says:

Anyone ever consider the weight of light?

What do you think?

[geordief]

Raf says:

Anyone ever consider the weight of light?

What do you think?

Is weight defined as mass times acceleration?Then light would have gravitational  acceleration but no mass.

[guest45734]

Anyone ever consider the weight of light?

Generally light is considered to have inertia and no mass, however that is not the only view.

Peter Brown posted the following document on "On the concept of relativistic mass" thread, giving some of the different definitions of mass.
 http://arxiv.org/abs/0709.0687

[Bill S]

Pete's contributions are always worth reading.  My trouble is I tend to get lost in the maths.  He's always willing to explain, but I often get  more lost in the explanation.  On one occasion, I had to consult Chris Baird; http://wtamu.edu/~cbaird/sq/category/physics/  to untangle my mental knots.

[jeffreyH]

Light can power a solar sail so it can impart a force. Isn't that weigh by another name. In this case the energy would be the determining factor rather than mass.

[Bill S]

https://en.wikipedia.org/wiki/Weight

In science and engineering, the weight of an object, either due to gravity or to a reaction force that holds it in place.[1][2][3]
Some standard textbooks[4] define weight as a vector quantity, the gravitational force acting on the object. Others[5][6] define weight as a scalar quantity, the magnitude of the gravitational force. Others[7] define it as the magnitude of the reaction force exerted on a body by mechanisms that keep it in place:

It looks as though weight has a few definitions; chances are there is one somewhere that would fit the powering of a solar sail.

[raf21]

my apologies for the rookie looking mistake of using weight when referring to mass,  but the impact imparted on a solar sail is exactly the weight I'm inquiring about.

[raf21]

it may be my subconscious desire to coin the phrase ' lightweight'.....

[Colin2B]

my apologies for the rookie looking mistake of using weight when referring to mass,  but the impact imparted on a solar sail is exactly the weight I'm inquiring about.

What you are looking for is not weight but momentum.
Newtons original equations of motion used change of momentum rather than F=ma. Although light does not have mass it can, like many waves, transfer momentum and hence apply a force to eg a solar sail.

[geordief]

my apologies for the rookie looking mistake of using weight when referring to mass,  but the impact imparted on a solar sail is exactly the weight I'm inquiring about.

What you are looking for is not weight but momentum.
Newtons original equations of motion used change of momentum rather than F=ma. Although light does not have mass it can, like many waves, transfer momentum and hence apply a force to eg a solar sail.

Is there a relationship between momentum and mass?

[jeffreyH]

Momentum is mass times velocity. Mass is an inherent part of momentum. This is not true for photons, where it is energy that is important. Effectively, E/c is the momentum of the photon. Where E is the energy of the photon.
https://en.m.wikipedia.org/wiki/Photon_energy

[raf21]

perhaps my rudimentary understanding of physics is not allowing me to see the "light"...... but is not mass and energy the same thing?

[raf21]

e=mc squared,  therefore mc squared=e...........

[jeffreyH]

Mass is measured in kilograms and energy is measured in joules. They are not the same thing. You have rest energy, which is based on mass, and equals m times c^2. This is not a relativistic term. It gets more complicated when relativity is involved.

[raf21]

So e=mc squared is not correct? I thought the equation meant that matter is just cold energy. ....

[raf21]

x amount of joules= 10bs iron times speed of light in a vacuum squared

[Colin2B]

...... but is not mass and energy the same thing?

No, they are clearly not the same thing. A lump of rock is very different to the kinetic energy created by a moving object.
They are however equivalent as described by E=mc²

[Bill S]

This is another case of things that are equivalent not necessarily being identical.  A bit like gravitational force and spacetime curvature, perhaps.

[raf21]

then the flaw is the equal sign,  an equivalent sign should be used instead. .....?

[jeffreyH]

You cannot state that E = m, where E is energy and m is mass. The units are not equivalent. That is why kinetic energy is the product of mass and the square of the velocity the object is travelling at. Rest energy is slightly different. Instead of a velocity we use the speed of light. This is not a vector quantity. Squaring a vector gives the square of a magnitude which is a scalar. The speed of light is already a scalar. It has no directional component. Therefore the square of the speed of light is defining an energy at rest.