# Homework Help: Rocket equation? verfication

1. Dec 14, 2007

### cashman91

is this equation true

Fnet=[m(Vf-Vi)]/Delta T

net force is equal to the mass times the change in velocity all over time

thanks alot

2. Dec 14, 2007

### malty

Well if it was reexpressed as

$$F_n_e_t=m(\frac{v_f-v_i}{\Delta t})$$

Does that help?

3. Dec 14, 2007

### Shooting Star

It should be the average force over time delta t in place of Fnet.

4. Dec 14, 2007

### cashman91

the formula malty gave is correct but how can you verify this formula is correct

I get where shooting star is coming from but i need to find the net force of the rocket

so far all i know is my two inclinometer reader posted scores of 53 and 54 degrees with a time of 1.03/1.07 seconds the mass of the rocket and the mass of an empty and full engine

i can solve the net force but i need to verfiy the formula is correct or not

this formula came out of a Nelson physics 11 book i don't have access to the answer manual if i did it would tell me if this formula is correct or not

5. Dec 14, 2007

### Shooting Star

malty and I have given the same formula.

Does the book define what is net force? Why don't you tell us what is meant by that? It's also a good idea to state the problem completely and show what you've done so far.

6. Dec 15, 2007

### cashman91

See attached jpeg for my attepmt

I need to find the

F(thrust)
F(net)
Acceleration
Fg
Vmax

I can find Eg and Ek easily myself but the problem is with the rest

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Last edited: Dec 15, 2007
7. Dec 15, 2007

### cashman91

Vf would be zero becasue the rocket is at the top of it height

soooo.... would Vi be 65.0m/s

If Vi is 65.0 m/s then using your formula

Fnet would equal 8372.878 Newtons**

** the (mass of the rocket full with engine)+(mass of the rocket empty)/2** is the mass of the rocket assuming the reaction is constant

am i correct?

8. Dec 15, 2007

### D H

Staff Emeritus
The equation $\bar F = m(v_f-v_i)/\Delta t$ is a direct consequence of Newton's second law and the mean value theorem for an object with constant mass. That equation is not valid for a thrusting rocket since the mass of a rocket. You need to use the Tsiolokovsky rocket equation, $\Delta v = v_e\ln(m_{init}/m_{final})$. For more details, see https://www.physicsforums.com/showthread.php?t=199087".

Last edited by a moderator: Apr 23, 2017
9. Dec 15, 2007

### cashman91

Tsiolokovsky rocket equation i have the intial mass(beofre lauch) and final mass(after launch)

we timed how long the reaction(white smoke) took before the rocket flew so how could i findout the Ve of my rocket

cashman91

Last edited: Dec 15, 2007
10. Dec 15, 2007

### Shooting Star

Now perhaps you understand why I'd asked you to state the problem completely. There was no indication in your first post that mass was changing. Let's wait for the attacment to get approved.

11. Dec 16, 2007

### cashman91

figured out my Vav is 31.71 m/s
height is 68.82m and time is 2.17s

Vav=(delta d)/(delta t)

12. Dec 16, 2007

### cashman91

still nedd help

13. Jan 2, 2008

### cashman91

anything

14. Jan 3, 2008

### Shooting Star

If you post the problem framing it properly in a single post, then people would be more inclined to help you. Maybe the whole thing is there, but nobody has the patience to gather up the pieces and fit them to find out the original problem.