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

 

[Doc Al]

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?

 

[Doc Al]

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 you 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]

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 :(

 

[Doc Al]

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? :'(

 

[Doc Al]

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?

 

[Doc Al]

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?

 

[jtwitty]

sorry i wrote it wrong, i mean 11.8

 

[Doc Al]

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??

 

[Doc Al]

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?

 

[Doc Al]

yay

ok so i am using the d=1/2at² formula

d=1/2(1.8)(20²)
D=360?

Good! (What about the speed?)

 

[jtwitty]

d=vi/t?

360=vi/20

vi=7200?

(vi = initial velocity)

 

[Doc Al]

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!

 

[Doc Al]

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 :)

 

[Doc Al]

Good!

 

[jtwitty]

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