# Homework Help: Kinematic Equations Rearrangment

1. Nov 21, 2011

### Malelia123

1. The problem statement, all variables and given/known data
A= Acceleration
Vf = Final Velocity
Vi = Initial Velocity
D= Displacement
T = Time

2. Relevant equations

D=Vf*T - 1/2*a*(t)2

Solve for T

3. The attempt at a solution

I need to solve for T: This is what I got as my attempt but it is so very wrong still
(T)2/T = Vf - 1/2*a -D

Please break down step by step exactly how to do this step by step... I'm seriously struggling here :(

2. Nov 21, 2011

### TaxOnFear

Well, the velocity in the equation is wrong to start with, that's probably why you're struggling.

D = ViT = 0.5AT2

Rearranging should give you a quadratic.

3. Nov 21, 2011

### Staff: Mentor

This is really an algebra question rather than a physics question, so I've moved this to a more appropriate forum.

How about instead, you show us step by step how you got to your attempted answer? Then we can show you where you went wrong.

TaxOnFear is correct that you should have a quadratic equation for t. Are you familiar with the "quadratic formula" for solving quadratic equations?

4. Nov 21, 2011

### Malelia123

No I'm not exactly sure what you mean by that.

I just checked the text book and that is the right equation.

Here is the question I'm stuck on:

If the arresting device on an aircraft
carrier stops a plane in 150 m with
an acceleration of 15 m/s2, find
the time the plane takes to stop.

Here is the equation the text book says to use:
D=Vf*T - 1/2a(T)2
or
D=1/2(Vf-A*T+Vf)T

This is what I did:

D=Vf*T - 1/2a(T)2
+1/2a

D+1/2a = Vf*T-(T2)
+Vf
D+1/2a+Vf = T-(T)2

and this is where I get stuck... I don't know what to do now?

5. Nov 21, 2011

### Staff: Mentor

No, adding (1/2)a to both sides gives

$$D + \frac{1}{2}a = v_f t - \frac{1}{2}at^2 + \frac{1}{2}a$$
$$D + \frac{1}{2}a = v_f t - \frac{1}{2}a (t^2 - 1)$$

6. Nov 21, 2011

### Malelia123

okay - so that is the wrong step? Where do I go from here. I'm sorry I just need a bit more information. I'm missing the basics and a quick reminder will make it click!

If you could even give me a step by step on it, or even a website with some basics too.

Last edited: Nov 21, 2011
7. Nov 21, 2011

### Staff: Mentor

As TaxOnFear and I have noted, you really need the quadratic formula to do this. There's no way to rearrange this equation to solve for t "directly." Look up the quadratic formula if you need to refresh your memory.

Then, as a first step, rearrange your equation so it looks like this:

(something)*t2 + (something else)*t + (yet something else) = 0

8. Nov 21, 2011

### Malelia123

Would you be able to tell me how you would go about figure out this question:
If the arresting device on an aircraft
carrier stops a plane in 150 m with
an acceleration of 15 m/s2, find
the time the plane takes to stop

Maybe I need to start somewhere? I do not understand the quadric equation business

9. Nov 21, 2011

### Mentallic

I had a hunch you probably wouldn't need to use the quadratic formula in your specific question. You should've given us this question in your first post :tongue:

Think about what vf is in this question.

10. Nov 21, 2011

### Malelia123

Vf = 0 m/s becuase the plane is stopped.

Should I be finding out what Vi is before find out the time? Ahhh you seem like you know what I need to do... A little hint here

11. Nov 21, 2011

### Ray Vickson

Hint: what is meant by the word "acceleration"? (Yes, I'm serious.) You need to work these things through for yourself; there will be no outside help available on the final exam.

RGV

12. Nov 21, 2011

### Malelia123

Okay people I think I got it:

T=√2*d/A

I've worked through the problem a couple time so I think I have got this.

When Vf or Vi = 0 you use the Scalar form of the equation.

Am I right here?

PS. Seriously thank you, you will probably see me back here. I'm taking Physics 30 & Math 30 applied next semester YIKES.

13. Nov 21, 2011

### Mentallic

Well, think about it. They don't tell us how fast the plane is travelling initially, but they do tell us that it decelerates to a complete stop in 150m at a rate of 15 m/s2. Doesn't it seem as though we have enough information to find out what the initial velocity of the plane is? Or even the time it was decelerating for as the question asked.

Using $$D=v_ft-\frac{1}{2}at^2$$

and plugging in $v_f=0$, solving for t isn't quite what you had there. Try again and take it slowly, if you're still not sure, show us each step you take and we'll point at your error.