# Applyings Newton 2nd Law and Kinematics

## Homework Statement

A 10,000 kg rocket is acted upon by a upward thrust of 118,000N

How fast will the rocket be moving after 20 seconds? How far will it have moved in this time?

Also, how do i determine the net force

## Homework Equations

a = $$\Sigma$$F/m

d=1/2at2+vit

## The Attempt at a Solution

a = $$\Sigma$$Force/mass

a = 118,000/10,000

a = 11.8?

------

to find how far

d=1/2at2+vit

d=1/2(11.9)(202)
d=2360

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## Answers and Replies

Doc Al
Mentor
Also, how do i determine the net force
What forces act on the rocket? (Hint: There are two.)

Finding the net force is the first step before you can calculate the acceleration.

What forces act on the rocket? (Hint: There are two.)

Finding the net force is the first step before you can calculate the acceleration.

gravity and the mass of the object?

Doc Al
Mentor
gravity and the mass of the object?
The force of gravity is one force. You're given the other. (Mass is not a force.)

The force of gravity is one force. You're given the other. (Mass is not a force.)

oh ya the upward thrust.

so its 118,000N is one force and 100,000N is the force of the earth due to the mass of the object multiplied by 10 (in class we always round gravity to ten)

Doc Al
Mentor
Sounds good. So what's the net force?

sounds good. So what's the net force?

1.18n??

edit: wait no, that's the accelaration. the net force is 18? idk :(

Doc Al
Mentor
1.18n??

edit: wait no, that's the accelaration. the net force is 18? idk :(
Careful--You're missing a factor of a thousand!

Careful--You're missing a factor of a thousand!

idk what to do?????????? :'(

Doc Al
Mentor
Try this: If you had 20 million dollars and you spent 10 million, how much would you have left? 10 dollars?

Try this: If you had 20 million dollars and you spent 10 million, how much would you have left? 10 dollars?

10 million.

so would i have 18,000 as my netforce?
but don't i divide, not subtract?

Doc Al
Mentor
so would i have 18,000 as my netforce?
Exactly.
but don't i divide, not subtract?
Not to find the net force. But yes to find the acceleration. That's the next step.

Thanks :)

ok so my acceleration would be 11.8 right?

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sorry i wrote it wrong, i mean 11.8

Doc Al
Mentor
sorry i wrote it wrong, i mean 11.8
How did you get 11.8?

How did you get 11.8?

a=$$\Sigma$$f/m

a=

---- ok mid post i just realized i used the wrong number!

a=18,000/10,000

a=1.8??

Doc Al
Mentor
a=$$\Sigma$$f/m

a=

---- ok mid post i just realized i used the wrong number!

a=18,000/10,000

a=1.8??
Now you've got it.

yay

ok so i am using the d=1/2at2 formula

d=1/2(1.8)(202)
D=360???

Doc Al
Mentor
yay

ok so i am using the d=1/2at2 formula

d=1/2(1.8)(202)
D=360???
Good! (What about the speed?)

d=vi/t?

360=vi/20

vi=7200?

(vi = initial velocity)

Doc Al
Mentor
d=vi/t?

360=vi/20

vi=7200?

(vi = initial velocity)
No. Instead, consider the definition of acceleration.

wait is this rght

v = d/t

v = 360/20

v = 18!

Doc Al
Mentor
wait is this rght

v = d/t

v = 360/20

v = 18!
That would give you the average velocity as the rocket rises, not its velocity after 20 seconds.

That would give you the average velocity as the rocket rises, not its velocity after 20 seconds.

it's accelerating isn't it?

a = vf -vi / t

1.8 = vf -0 / 20

1.8 = vf /20

36 = vf????

i think that's it :)

Doc Al
Mentor
Good!