# Applyings Newton 2nd Law and Kinematics

jtwitty

## 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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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.

jtwitty
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?

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.)

jtwitty
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)

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

jtwitty
sounds good. So what's the net force?

1.18n??

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

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!

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

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

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

jtwitty
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?

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.

jtwitty
Thanks :)

ok so my acceleration would be 11.8 right?

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jtwitty
sorry i wrote it wrong, i mean 11.8

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

jtwitty
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??

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.

jtwitty
yay

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

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

Mentor
yay

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

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

jtwitty
d=vi/t?

360=vi/20

vi=7200?

(vi = initial velocity)

Mentor
d=vi/t?

360=vi/20

vi=7200?

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

jtwitty
wait is this rght

v = d/t

v = 360/20

v = 18!

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.

jtwitty
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 :)

Mentor
Good!

jtwitty
thank you so much doc al you're my hero :)