WHAT is WRONG WITH MY PROJECTILE ExPERIMENT? Can Somebody help me?

Question

I conducted an experiment, investigating how density affect a projectile with the presence of air resistance. (The object is launched horizontally from a height of 92cm)

Before the experiment, I researched and asked opinions from other people, all of which gave the same view---> greater density=greater distance. And this was the base of my hypothesis....

But the experiment turned out to be the exact opposite of my hypothesis. My results indicate that less density=higher distance. My average results are as follows:
(Objects have the same shape - 60cm^3 block)

1)Wood (density=0.467) - distance = 190cm
2)Wax (0.917) - 125cm
3)Aluminium (2.683) - 58cm
4)Iron (7.817) - 25cm

WHY is it like this? What's wrong? What seems to be the problem?

PLEASE help if you can... and try and explain cearly (my knowledge in physics is only sixth form level)

OH please i beg for your help and opinion..I'm mentally blocked right now, out of ideas... AND in need of some HELP

No, the objects have different mass

My assumption was that the force and initial velocity are constant...

Answer 1

It has been already established that the time of fall and hence the horizontal distance will be the same, if the velocity of projectile remains the same.

In your experiment since volume remains the same mass of objects thrown are different. Heavier the mass larger is the force needed to give the same velocity of projection.

It appears you have used the same force for all objects and hence initial speed of throw is different; speed is less for heavier object [iron] and is the least for less denser object [wood]

Since the height is 92 cm the time to fall for all objects
t =√ [2 x 0.92/ 9.8] = 0.3s

Velocity of each object thrown :
1) Wood; v 633.cm /s;
2) Wax; 416.6 cm /s
3) Aluminum; 193cm /s
4) Iron; = 83 cm /s

Answer 2

Good answers above as far as velocity is concerned, but all miss a point: the air resistance...
IF you vary the force for each mass, so that each object starts with the SAME initial velocity, then ALL will fall at the same distance (it is the "ballistic" formula, where the mass does NOT interveine).
However, AIR resistance will make a difference, by slowing down the object. Since they have the same SHAPE, all objects will be slowed down by the same amount! (presenting the same surface to the air).
Your experiment will not show this: the distance ran is way too short to emphesize the effect, and the small "errors" in initial velocities will be much greater than the desired effects to allow any precise measurements.
Even if you were to drop the blocks from a great height, they will arrive more or less at the same time on the ground (the Pisa tower experiment - apocryphical, probably). As they are "blocks", they will thumble during the fall, disturbing the measurement and making them unreliable. Spheres would be a solution. Even then, the slightest wind would defeat the experiement!
So, how to prove it?
- Make objects spherical and of the same size (same and constant air penetration shape)
- Drop them from a great height (no wind condition).
- Make time measurements.
During the fall, the objects are submitted to two forces:
- the gravity: Fdown = m * g. This depends of the mass, or the density.
- the air resistance: Fup = C * r * v² * A. This DOES NOT DEPEND on the mass, but only on the SHAPE.
C = a coefficient that depends of the shape of the object, 0.25 for a square, 0.1 for a pointed object like a riffle bullet. Since all objects have the same shape, it becomes irrelevant.
r = air density (1.226 at sea level): constant in all cases (no wind condition)
v² = velocity in m/s: THIS is the UNIQUE variable!
A = area presented to the air (also a constant).
At some stage during the fall, the two forces (which are in opposition) will equal each other, and the velocity at this point is called "terminal velocity": it means that the object will NOT fall any faster...
If your fall is from high enough, the air resistance will become noticeable, and objects will reach the ground at different times, proving/disproving your theory...
Now, before searching for a site to make your test, let's put some calculations:
Supposing balls of 10cm diameter.
This gives 0.5236 dm³, and 0.00785m² section
Wood: 0.245kg
Wax: 0.480kg
Alu: 1.405kg
Iron: 4.093kg
Terminal velocities (where the air resistance reaches the maximum effect):
Wood: 35.33m/s
Wax: 49.46m/s
Alu: 84.61m/s
Iron: 144.42m/s
Using h = v² / 2g, to reach the terminal velocity, we need:
Wood: 63.61m
Wax: 124.68m
Alu: 364.87m
Iron: 1063.05m
That's a tall building...
A plane or a chopper maybe, or try to find a Wind Tunnel!
Good luck!

Answer 3

You hypothesis would be correct assuming you launched each of the objects at the same velocity. As I do not know how they were launched, I would hazard a guess that they were launched with the same force, but the heavier objects would have required greater force to achieve the same velocity at time=0. Therefor, it would seem likely to me that the same force was used, but the heavier the object, the lower the initial velocity, therefor, the shorter the distance travelled.

If my assumptions are not correct, please clarify and I will look at it further.

EDIT: Your assumption is where the error lies. The inital velocity cannot be the same if the force is the same. Using Newton's law F=ma, if you increase the mass, keep the same force, the acceleration will be less, therfor the inital velocity will be less.

For you experiment to work, you must increase the force to maintain the same inital velocity.

Newton showed this by dropping objects where gravity has the same acceleration irrespective of mass. The force you use is altered by the mass which is causing the error.

Answer 4

You are probably using a spring launching system. This is most likely the source of your problem. If you compress the spring the same distance for each launch, then you are applying the same force to objects of different mass which will result in a different velocity. Try calculating the required compression of the spring for each mass, in order to deliver the exact same initial velocity for each mass ( i.e. you will have to compress more for heavier mass and less for lighter mass to achieve equal initial velocities). And yes, as long as your objects are the same size and shape, then the higher the density the farther it will go.

P.S. The equation you need for the spring is Hooke's Law:

F= -kx
or use U= .5kx^2 where U is potential energy
and K=.5mv^2 Where K is kinetic energy.
You will need to incorporate the work done by gravity as well.

Answer 5

density should have nothing to do with distance travelled...

Distance = initial velocity x times + 1/2 x acceleration x times ^2


unless you apply an angle and/or air resistance.... so therefore iron and aluminium had more drag force acting on it causing it to drop faster (due to gravity)... while the the wood and wax was gliding more... with less drag force and more air resistance... i think that's what you were lookin for... hmm..

but then again... if you apply more force... of course the object will go further.... but i'm pretty in your experiment you used the same initial velocity

Answer 6

have you considered aerodynamics? also, greater mass does mean greater distance, especially from higher altitudes and from (if all test materials were launched with greater force and speed....) . I don't remember the actual formula, but it states that as the total force and speed surpass the total mass of the object, that all objects would travel farther with that force and speed as the object has more mass, until the mass exceeds the force / speed ... try either increasing your force and or speed then restarting the whole test over?

Answer 7

Were all of the objects of the same mass ?

Answer 8

Its not density, its mass.

This is such a common mistake.