Rocket acceleration and g force (1 Viewer)

[Scheldor]

A certain model rocket has a pre-launch mass of 87.3 g, of which 10.5g is propellant. It is able to deliver a thrust of 6.1 N. Assuming the rocket is fired directly up, calculate:

a) the initial rate of acceleration and g force
b) the final rate of acceleration and g fore just prior to exhaustion of fuel.

I worked it out, but my answers werent the same as the answers in the back of the book. If anyone can post their solution it will be appreciated.

 

[DamTameNaken]

a) its initial acceleration is dependent on its initial mass, thrust and force of gravity.

First we should work out how much weight it has (force of gravity)

F=ma
F= 0.0873 x 9.8
F= 0.86 N downwards

total force = 6.1 - 0.86 = 5.24 N

What we've done is just made our gravity and thrust into one vector, i.e. 5.24 N upwards. Now that we have a single vector we can use F=ma
F=ma
5.24 = 0.0873 x a
a = 60.0 m/s^2 upwards

The rocket is experiencing an upwards acceleration of 60.0m/s^2 AND gravity. Total G-force for something moving directly upwards = 1 + a/9.8 = 1 + 60/9.8 = 7.12 g's

b) for b. we do the same process except our mass is 76.8 g as we subtract the mass of the propellant from the total mass

 

[Scheldor]

Oh. I see my mistake, I made the initial mass 97.8 g (i added the fuel). Thanks! That was explained very well.