a not so whopping 50 Joules

[alphabet song]

Hello, guys!

Please look at this sentence.

'Its kinetic energy is a not so whopping 50 Joules.'

'a' and 'Joules' is grammatically correct?

 

[entangledbank]

Yes, because the amount is being qualified by an adjective:

Its kinetic energy is 50 joules.
Its kinetic energy is a 50 joules. (and would still be wrong if it was just one joule)
Its kinetic energy is an amazing/impressive/negligible 50 joules.

 

[alphabet song]

Yes, because the amount is being qualified by an adjective:

Its kinetic energy is 50 joules.
Its kinetic energy is a 50 joules. (and would still be wrong if it was just one joule)
Its kinetic energy is an amazing/impressive/negligible 50 joules.

Oh! Thank you so much. I have understood enough!