How Do You Calculate Rocket Propulsion and Acceleration?

[redred7]

Homework Statement

A small 120kg rocket is fired vertically, propelled by an engine with a thrust of 6500N. The engine ejects gas at a rate of 4.5kg/s.
a) Calculate the change in velocity of the gases relative to the engine
b) after 20 s what will be the rockets acceleration.

Homework Equations

F * delta T = m * delta V

The Attempt at a Solution

(6500-1176)(1) = (120-4.5)delta V
5324= 115.5V
V= 46 m/s


I am not sure if I acceleration the correct force (because I am not sure if I should subtract Ff), correct mass (because I believe that mass is contant in this formula as there isn't delta M)

 

[alphysicist]

Hi redred7,

Homework Statement

A small 120kg rocket is fired vertically, propelled by an engine with a thrust of 6500N. The engine ejects gas at a rate of 4.5kg/s.
a) Calculate the change in velocity of the gases relative to the engine
b) after 20 s what will be the rockets acceleration.

Homework Equations

F * delta T = m * delta V

The Attempt at a Solution

(6500-1176)(1) = (120-4.5)delta V
5324= 115.5V
V= 46 m/s


I am not sure if I acceleration the correct force (because I am not sure if I should subtract Ff), correct mass (because I believe that mass is contant in this formula as there isn't delta M)

Yes, the mass is changing. The problem states that every second the mass changes by 4.5kg. So in your equation keep the term that has the change in mass.