Homework Help: Question hel;ppp

1. Nov 29, 2003

Aiasha

question...plz hel;ppp

guys iam takin physics

nd i dont get physics attt allll!!!
pllz hlp me with this
i need to show work as well

an elvtor weighing 25,000 N is supported by a steal cable. that is the tension in the cable wehn the elavotor is begin accelerated upward at the reate of 3.0 m/s^2? (g= 9.8m/s^2)
plzzz help[

Last edited: Nov 29, 2003
2. Nov 29, 2003

jamesrc

You need to draw a free body diagram and use Newton's second law. What forces are acting on the elevator?

3. Nov 29, 2003

Aiasha

well u c
i duno how to do all that
thats wut my prblem is
can u hlp me out

4. Nov 29, 2003

jamesrc

OK, well I can't just give you the answer to a test question. If you think about the elevator, the tension is pulling it up and the weight is pulling it down. The net acceleration on the elevator is up. Newton's second law says that Fnet = ma. So if you add the forces together (using positive for up and negative for down), you can write a single equation from Newton's second law. After one step of algebra, you can use that to solve for the tension. I hope that helps you out.

5. Nov 29, 2003

arcnets

Trying a basic explanation...

First off, tension is much the same as force. It's just another name engineers invented when they're talking about a force that is transmitted by a cable, string, or something like that. So what you're looking for is a force, OK?

Now, the first force that acts on everything on Earth's surface, is the force of gravity. It makes things fall. Experiments have shown that any falling object accelerates at
$$g = 9.8 \frac{m}{s^2}$$
We know that things fall a lot slower on the Moon. Why is this? It's so because the force of gravity is smaller there (because the Moon is a smaller body than the Earth). So, ISAAC NEWTON concluded that the acceleration is a measure of force, and arrived at his famous 2nd law
$$F = ma$$
where a is the acceleration produced by a force F acting on a body of mass m.
That's why the unit of force is called one Newton:
$$1 N = 1 kg \cdot 1 \frac{m}{s^2}$$
You see it's the unit of mass *times* the unit of acceleration.

Now back to your problem: Two things are exerting forces on your elevator. One is the Earth (pulling it down), and the other is the elevator's motor (pulling it up). Both forces will add, giving the cable tension (remember: =force) Can you calculate both forces by Newton's second law? And find the sum?

Last edited: Nov 29, 2003
6. Nov 29, 2003

Aiasha

thanx guys
but if u tell the eqauitns whihc i have to solve
than i can solve it
i mean liek the free body diagram
nd wut equations i need to solve it

7. Nov 29, 2003

Aiasha

i jus dont get it [b(]

8. Nov 29, 2003

enigma

Staff Emeritus
Aiasha,

Here's a recommendation: open you book, and start reading. Setting up free body diagrams is not difficult to do.

If you try the problem and post what you've got, you may get some help.

9. Nov 29, 2003

Aiasha

ok i got
f=ma
f= 25,000(3.0m/s^2)
is that somwhat right?

10. Nov 30, 2003

enigma

Staff Emeritus
That's what the force would be if there was no gravity. You've got to sum all forces on it.

11. Nov 30, 2003

Aiasha

how do i do that?
common am soo confused

12. Nov 30, 2003

himanshu121

u should master Free Body Diagrams thoroughly.
Consult any standard book

know more about forces, types of forces, frame of reference etc

13. Nov 30, 2003

arcnets

No, it's wrong.
25,000 N is not the elevator's mass. Since it's in Newtons, it must be the force of gravity, right? (Did you read my other post?)
You got to find the mass m by using F = mg, and rearranging that. Do you know how to rearrange an equation to solve for the unknown?
Next, you use F = ma to find the elevator's force.

14. Nov 30, 2003

Aiasha

how do i rearrange that
nd wut do i subsitute in thos eletters

15. Nov 30, 2003

arcnets

You rearrange like this:
$$F = mg | :g$$
$$\frac{F}{g}=m$$
$$m= \frac{F}{g}$$
Plug in your F, and the g that I told you in my 1st post.

(You should study more. What I tell you is all very basic (grade 8), and you should know it if you're taking this course.)

Last edited: Nov 30, 2003
16. Nov 30, 2003

Aiasha

this is wut i got
f=mg
25000=m(9.8m/s^2)
2551=m
f=ma
2551*3.0
f=7653

is that righ?

17. Dec 1, 2003

arcnets

Yes. The motor force is 7653 N (you should always use units). Now that adds to the elevator's weight, giving WHAT?