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/sci/ - Science & Math

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3904000 No.3904000 [Reply] [Original]

/sci/, i need your help in a high school physics problem. I think the textbook has the wrong answer. Here's the full question:

A rocket has a mass of 400 kg, 75% being fuel. It develops a thrust of 8000N.
(a) Calculate its initial acceleration

Is it not just F=ma --> a=F/m
a=8000/4000
a= 20 ms^-2?

The answer in the book is 10.2 ms^-2

Contributions would really be appreciated

>> No.3904005

EFF GEE EQUALS EMM GEE

>> No.3904007

8000/400* not 4000

>> No.3904010

Fnet = Fthrust - Fgravity
Fnet = 8000N - (400)(9.8)
Fnet = 4080N

F = ma
4080N = 400kg*a
a = 10.2 m/s^2

>> No.3904014

You should take gravity into account.

>> No.3904016

hurr durr the problem doesn't say the rocket's on earth so obviously it is in space under negligible gravity

obviously we can't just subtract the force of gravity to find the net force and acceleration because we don't know where we are

NOT ENOUGH INFORMATION TO SOLVE

>> No.3904020

Subtract gravity since it is acceleration in the opposite direction

20-9.8=10.2

>> No.3904025

>>3904010
fuck, embarrassed that i did not realise that. thank you anon