physics help

by **lord twiggy1** on Mon Oct 01, 2007 9:09 pm

does any one here know how to solve fore time when you have the distance(78.4 m) and the acceleration(free-fall so 9.8 m/s squared). so in other words: How long will it take for an object dropped from a window to fall a distance of 78.4 m?(free fall is 9.8m/s/s i think is another way of writting it as opposed to the formerly written one). i only need the formula but an answer would be good too. thanks.

by **misterman10** on Mon Oct 01, 2007 9:13 pm

ya, I can help you out, are you only working with gravity and no air resistance?

by **misterman10** on Mon Oct 01, 2007 9:17 pm

Xf = .5at^2+Velocity initial (time) + Xo

Velocity initial is zero because you are dropping it, not throwing it.
X final will be 0, and X initial will be 78.4.

by **misterman10** on Mon Oct 01, 2007 9:17 pm

On second thought, is gravity going to be negative or is it positive?

by **misterman10** on Mon Oct 01, 2007 9:19 pm

I got 8.57 seconds

by **misterman10** on Mon Oct 01, 2007 9:22 pm

is twiggy still here?

by **lord twiggy1** on Mon Oct 01, 2007 9:23 pm

gravity as stated before is 9.8, free fall, so if that implies no air resistance then ya no air resistance.

by **misterman10** on Mon Oct 01, 2007 9:24 pm

lord twiggy1 wrote:gravity as stated before is 9.8, free fall, so if that implies no air resistance then ya no air resistance.

well, I gave you the answer already and the formula

but the formula may be confusing since I cant do all the subscripts and squared and stuff.

by **lord twiggy1** on Mon Oct 01, 2007 9:25 pm

ok so thats it then thanks

by **misterman10** on Mon Oct 01, 2007 9:26 pm

lord twiggy1 wrote:ok so thats it then thanks

no problem, what physics class are you taking right now?

by **Koronna** on Mon Oct 01, 2007 9:45 pm

misterman10 if you're not a student, physicist, or currently taking physics in school, then I have to say that you are awesome...I just took physcists in summer school 3 months ago and got an 88, and these things are way over me already...

by **F1fth** on Mon Oct 01, 2007 10:17 pm

I got 4.0 seconds.

I used
Vf² = Vi² + 2da
to solve for Vf.

Then I used
a = ( Vf - Vi ) / t or rearranged, t = ( Vf - Vi ) / a
to solve for a.

Edited for scientific notation.

by **Gold Knight** on Mon Oct 01, 2007 10:24 pm

I got 7.

Thought of an appletree (gravity :wink:

) and a number one through ten and thought which number goes most smoothly with apples. When you buy apples, you dont want to buy too few that you'll have to get more later, but 10 is just way to much for one trip...

by **misterman10** on Mon Oct 01, 2007 10:35 pm

F1fth wrote:I got 4.0 seconds.

I used
Vf² = Vi² + 2da
to solve for Vf.

Then I used
a = ( Vf - Vi ) / t or rearranged, t = ( Vf - Vi ) / a
to solve for a.

Edited for scientific notation.

No, why would you solve for Vf when you can directly solve for Time using the given info.

Theres already an equation that can be used with this problem which is:
Xf = .5at²+ Vot + Xo

Xf = final position = -78.4
a= acceleration = =9.8
t= time = ?
Vo= velocity initial = 0
Xo= initial position = 0

Since you know gravity (accel) is -9.8 (going down so its negative ) and assuming your starting position is (0,0) and your final position is (0, -78.4)
all you have to do is plug and chug to get the answer.

by **F1fth** on Mon Oct 01, 2007 10:41 pm

Yeah, sorry it's been a few years since I've taken physics :lol:

.

by **misterman10** on Mon Oct 01, 2007 10:42 pm

F1fth wrote:Yeah, sorry it's been a few years since I've taken physics .

No probs

I'm taking AP physics right now so this stuff is easy for me :lol:

by **OnlyAmbrose** on Mon Oct 01, 2007 11:35 pm

There are 4 main kinematics equations you'll need to be worrying about in physics... it wouldn't be a bad idea to tattoo them on your forearm if you plan to stay in the subject

Where X = displacement, V = final velocity, Vo = initial velocity, a = acceleration, and t = time...

X = Vo(t) + (1/2)(a)(t^2)

Taking the derivative of the above equation, you get the one for velocity:

V = Vo + (a)(t)

The others can be derived as well, but how you do that isn't too important. Here they are:

X = (1/2)(V + Vo)(t)

and

V^2 = Vo^2 + (2)(a)(X)

Using these four equations, you'll be able to solve any kinematics problem in which you have at least 3 of the 5 basic variables (X, V, etc).

Something to keep in mind that everyone trips up on...

Remember that velocity and acceleration are vectors. That means it matters if you make them positive or negative. If you throw a ball up in the air, it has a POSITIVE initial velocity, but a NEGATIVE acceleration. This is probably the most common kinematics mistake ever made.

I hated Physics. My teacher was an absolute idiot (though he wasn't so bad that I had to come to the forums for help

)

Anyways, hope this helps for future problems

by **misterman10** on Mon Oct 01, 2007 11:36 pm

Also to go on what Ambrose was saying, those 4 equations can be used in the x or y direction. Sometimes you will have to solve for t using the y components, and then plug the time in to the x equations to solve for x components :wink:

by **lord twiggy1** on Tue Oct 02, 2007 6:39 am

misterman10 wrote:
lord twiggy1 wrote:ok so thats it then thanks
no problem, what physics class are you taking right now?

IB physical science 1

by **lord twiggy1** on Tue Oct 02, 2007 7:38 pm

alright, another question. here goes: You hold an apple over your head. (A)Identify all the forces acting on the apple and their reaction forces. (B)When you drop the apple, identify all the forces acting on it as it falls and the corresponding reaction forces. Neglect air drag

by **misterman10** on Tue Oct 02, 2007 8:27 pm

lord twiggy1 wrote:alright, another question. here goes: You hold an apple over your head. (A)Identify all the forces acting on the apple and their reaction forces. (B)When you drop the apple, identify all the forces acting on it as it falls and the corresponding reaction forces. Neglect air drag

are you talking about free body diagrams?

by **lord twiggy1** on Tue Oct 02, 2007 9:11 pm

?i dont think so, we are just finishing newtons 3 laws

by **lord twiggy1** on Tue Oct 02, 2007 9:29 pm

OnlyAmbrose wrote:I hated Physics. My teacher was an absolute idiot (though he wasn't so bad that I had to come to the forums for help )

its not that my teachers bad it just that i was over-thinking it. now that you and mm posted that stuff i noticed my mistake.

by **freezie** on Tue Oct 02, 2007 9:37 pm

lord twiggy1 wrote:alright, another question. here goes: You hold an apple over your head. (A)Identify all the forces acting on the apple and their reaction forces. (B)When you drop the apple, identify all the forces acting on it as it falls and the corresponding reaction forces. Neglect air drag

A: Gravity. ALWAYS. Then there is the force your head is doing against the apple, to counter gravity.

I think that's it, it's not moving, so..

B: Gravity. I really feel this is imcomplete..

Someone else can answer better than me, I took physic quite a while ago...I don't know if I forgot something

by **misterman10** on Tue Oct 02, 2007 9:38 pm

lord twiggy1 wrote:alright, another question. here goes: You hold an apple over your head. (A)Identify all the forces acting on the apple and their reaction forces. (B)When you drop the apple, identify all the forces acting on it as it falls and the corresponding reaction forces. Neglect air drag

Wait, are you holding the apple above your head with your hand, or is your head supporting it?

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