Physics 11 Net force and Acceleration

[anOldMan]

Homework Statement

g=10m/s^2
A fully loaded Saturn rocket has a mass of 2.92x10^6kg. Its engines have a thrust of 3.34x10^7N.

a) How much does the rocket weigh. answer 2.92x10^7 N
b) What is the net force on the rocket when it blasts off? answer 4.2x10^6N
c) What is the acceleration as it leaves the launch pad? 144m/s^2
d) What is its speed after 60 seconds? answer 86.4m/s
e) As the rocket moves away, the engine thrust is constant but the mass decreases - why? What effect does this have on acceleration. answer Fuel is consumed; m decreases, a increases.

Homework Equations

F=ma P.E. =mgh

The Attempt at a Solution

I have no idea how the answer for b) becomes 4.2x10^6N
Any help would be appreciated.

 

[anOldMan]

Part c

Based on the question I got the mass as 2.92 and the thrust as 33.4. 33.4 to match the exponent for both numbers. The answer I got when F/m=a is 33.4kg by m/s^2/2.92Kg=11.4m/s^2m/s^2. This is obviously wrong. What am I doing wrong?

 

[Moetasim]

The answer for b) is found by using the equation F=ma. We know the mass of the rocket (2.92x10^6kg) and the acceleration (g=10m/s^2), so we can plug those values in to find the net force. The calculation is 2.92x10^6kg x 10m/s^2 = 2.92x10^7N. This is the weight of the rocket, which is equal to the net force when the rocket is at rest on the launch pad. However, when the engines are blasting off, there is an additional thrust force of 3.34x10^7N. Therefore, the net force on the rocket when it blasts off is the sum of the weight and thrust forces, which is 2.92x10^7N + 3.34x10^7N = 4.2x10^7N. This is the answer for b).

I hope this clarifies the calculation for you. Feel free to reach out if you have any further questions.