How does light have momentum but we assume it has no mass?


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alexfwang3g
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Joined: Thu Jul 11, 2019 12:16 am

How does light have momentum but we assume it has no mass?

Postby alexfwang3g » Thu Oct 17, 2019 12:31 am

We say light has no mass, therefore it moves at the speed of light. However, it is able to affect small particles such as electrons and can affect them due to its momentum. Since momentum is calculated by m*v, it is naturally contradictory. Can someone explain??

Emma Popescu 1L
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Joined: Wed Sep 11, 2019 12:16 am

Re: How does light have momentum but we assume it has no mass?

Postby Emma Popescu 1L » Thu Oct 17, 2019 12:53 am

Light does have momentum however the equation m*v cannot be used in this case to calculate momentum

005206171
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Joined: Wed Sep 18, 2019 12:20 am

Re: How does light have momentum but we assume it has no mass?

Postby 005206171 » Fri Oct 18, 2019 1:56 am

Light doesn't have rest mass at least. So we can't really use the mv equation. But it does have momentum in the form of energy. We can use another equation instead to find its momentum, E=pc where E is the energy of the photon, and c is the speed of light.

Kassidy Ford 1I
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Re: How does light have momentum but we assume it has no mass?

Postby Kassidy Ford 1I » Fri Oct 18, 2019 9:02 am

Light has no resting mass, but electrons do. That's why de Broglies equation can be used for electrons, and it is also why electrons don't move at the speed of light

Sanjana Munagala_1j
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Re: How does light have momentum but we assume it has no mass?

Postby Sanjana Munagala_1j » Sat Oct 19, 2019 9:44 am

According to a video that was shown to me in discussion, manipulating equations causes there to be a discrepancy when solving for the mass of a photon. This is why we assume photon's have a different type of momentum, one that is entirely dependent on energy and not mass.

Hope that helps!


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