#1
I searched and couldn't find a science thread so I must post here again (sorry!)

A squirrel drops an acorn from a tree branch that is 8 m from the ground.

1. How long is the acorn in the air?

2. What is the acorn's velocity when it reaches the ground?

Now I am using a simple program called the moving man, in which you input acceleration, position, velocity. Then it shows you the results.

I typed in 8 for the position and -9.8 for the acceleration. and the answers I get are:

About 1.3 for #1

About -12.3 m/s for #2

That seemed weird and I checked the program and it keeps changing the acceleration without me doing anything. So basically is this the right answer or is my program messing up and it's not right at all.
#3
Why are you making acceleration negative? The acorn will be increasing speed, not decreasing.
#4
Quote by pepsi_lovr
Why are you making acceleration negative? The acorn will be increasing speed, not decreasing.


Gravity is a downward or negative acceleration, that's why.
#5
v = inital_v + a * t

current_h = intial_h + initial_v*t + a*t^2/2
initial_v = 0
initial_h = 8m
current_h = 0m
solve for t

when you have t put it into the above formula for v
Quote by pepsi_lovr
Why are you making acceleration negative? The acorn will be increasing speed, not decreasing.


vectors biatch
Last edited by seljer at Nov 15, 2008,
#6
Be real philosophical about it and say, "technically, the acorn is always in the air...and when it reaches the ground, acceleration is zero, because it stopped."
#7
Quote by TimboSlice
Be real philosophical about it and say, "technically, the acorn is always in the air...and when it reaches the ground, acceleration is zero, because it stopped."


thats not philosophical

thats just newtonian physics and inertial/noninertial frames of reference
#8
Quote by Guitarfreak777
Gravity is a downward or negative acceleration, that's why.



Quote by seljer
vectors biatch


Exactly this.

The acorn is moving down, and so is the acceleration, hence they are in the same direction so acceleration is positive. You get a negative velocity because, just as you are with acceleration, considering the positive direction to be upwards. Remember velocity is also a vector.
#9
Quote by seljer
thats not philosophical

thats just newtonian physics and inertial/noninertial frames of reference



I thought I could pull the wool over their eyes. Your exposition of my work tells me that you are indeed a worthy adversary...

Besides, YOU'RE a newt.
#12
Try working it out by hand using SUVAT equations?

Oh gorramit, I'll do it for you.

My answers:

v=12.52
t=1.27

So yeah, seems about correct.

EDIT: Will post worked solutions, hang about.

s=8 m
u=0 m/s
v=?
a=9.81
t=?

v^2 = u^2 + 2as

v = Square root (u^2 + 2as)

v = Sq rt ( 2as)
= Sq rt (2x9.81x8)
= 12.52 m/s

v = u + at
t = (v-u)/a
=(12.52-0)/9.81
=1.27s
Last edited by LordBishek at Nov 15, 2008,
#13
Umm you guys it's not negative acceleration. It's positive.

Acceleration is only negative when it is in the opposite direction that is in a real life situation when it's deceleration. Suppose a car is reducing speed then you'd have negative acceleration. Gravity is positive acceleration cause it increases speed not reduces.

Suppose you throw a ball up and it reduces speed due to gravity, then it will be negative acceleration. Not cause it's gravity but because it's causing the object to slow down.

Sorry to break your bubble...I enjoyed it
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#15
Quote by X-Boy
Umm you guys it's not negative acceleration. It's positive.

Acceleration is only negative when it is in the opposite direction that is in a real life situation when it's deceleration. Suppose a car is reducing speed then you'd have negative acceleration. Gravity is positive acceleration cause it increases speed not reduces.

Suppose you throw a ball up and it reduces speed due to gravity, then it will be negative acceleration. Not cause it's gravity but because it's causing the object to slow down.

Sorry to break your bubble...I enjoyed it


It can be negative OR positive. It depends entirely upon your inertial frame of reference. If a car was accelerating from left to right, it would be DECELERATING from right to left with equal and opposite magnitude.
#16
Quote by X-Boy
Umm you guys it's not negative acceleration. It's positive.

Acceleration is only negative when it is in the opposite direction that is in a real life situation when it's deceleration. Suppose a car is reducing speed then you'd have negative acceleration. Gravity is positive acceleration cause it increases speed not reduces.

Suppose you throw a ball up and it reduces speed due to gravity, then it will be negative acceleration. Not cause it's gravity but because it's causing the object to slow down.

Sorry to break your bubble...I enjoyed it



The book states:

When analyzing free fall, whether you treat the acceleration as positive or negative depends upon the coordinate system that you use. If your coordinate system defines upward to be a positive direction then the acceleration due to gravity is equal to -g.
If your you decided that downward is a positive direction then acceleration due to gravity is +g

So could it not be either?
#17
Quote by Guitarfreak777
The book states:

When analyzing free fall, whether you treat the acceleration as positive or negative depends upon the coordinate system that you use. If your coordinate system defines upward to be a positive direction then the acceleration due to gravity is equal to -g.
If your you decided that downward is a positive direction then acceleration due to gravity is +g

So could it not be either?


yes.... if you really wanted to you could point your coordinate system 30° off the z axis towards the northwest or something
#18
Quote by Guitarfreak777
The book states:

When analyzing free fall, whether you treat the acceleration as positive or negative depends upon the coordinate system that you use. If your coordinate system defines upward to be a positive direction then the acceleration due to gravity is equal to -g.
If your you decided that downward is a positive direction then acceleration due to gravity is +g

So could it not be either?


It is convention with free fall to take it as positive acceleration with respect to the Earth, or whatever source of gravity, but yes, you're right, it depends. That's why you got a negative velocity. Because you effectively designated your positive direction as going UP instead of down. Since the acorn was falling DOWN, it could be said to move NEGATIVELY with respect to up. Notice that your velocity's magnitude hasn't changed, only it's sign.
#19
Yeah that's cause i like to consider my gravity as positive...

Just me guys !! You can be the rebels you are and consider it negative

And TS, no matter the sign convention the magnitudes should be the same...Your program works as far as i'm concerned
Quote by blynd_snyper
yes we all need answers to xboys questions hurry up goddam it


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Damn you X-Boy!!!


Founder And Member Of The " I Don't Masturbate Club "

TURNED 18 TODAY !!! (22/02)