Help from a Physics teacher required?

Discussion in 'Science' started by ELLIS1980, Jan 1, 2012.

1. ELLIS1980

Hello, I am a trainee teacher currently doing an SKE via Brighton Uni. I have a project involving a hand held catapult. Am i correct in thinking i cannot calculate the range a projectile can be fired without having either time or initial velocity? I have Mass, acceleration and force already and have found a similar project involving a spud gun from which i can get muzzle velocity but not initial? Any help would be much appreciated.
My initial project outline was that i would calciulate the range of five different projectiles? but without initial velocity and time cant find a way of calculating the range. My next thought is to run the experiment to give range then use this to work backwards to get IV and T?
Any idea's much appreciated. Oh ive already asked in the maths forum and had some very kind pointers but require the help of a Physics expert as it is a cross curricular project, so if your happy for me to quote your advice that would be very kind.
Kind Regards

Dan

2. physics_suits_you

Happy New Year to you.
In Physics there are often a number of ways of approaching a problem (hopefully they all give the same result, but I remember one exam question where they didn't - that's another story).
The most straightforward route for projectile problems involve the equations of uniform motion and the concept that vertical and horizontal motions are independent (look up "monkey and hunter" experiments). These give you an idea that the greatest range is when the projectile is fired at 45deg (assuming from ground level to the same level - other situations can be calculated). Obviously, the greater the initial velocity, the greater the range (and the greater the time of flight, and also the maximum height).
You can, use an energy conservation concept to find the initial velocity, obtainable from the energy stored in the elastic of your catapult. If you assume Hooke's Law and measure the extension of the catapult, you should get a reasonable estimate for the velocity (I'm not sure what you mean when you say the muzzle velocity is not the initial velocity)
The initial velocity can be resolved into vertical and horizontal components, and as stated earlier, 45deg gives the greatest range, assuming no air resistance. Use vsinA and g to obtain time of flight, apply to vcosA to get horizontal distance.
One classic experiment, to find the initial velocity, is to project horizontally from a height of 1.25m ("g=10"), in which case the time of flight is 0.5s [s=ut+1/2at^2 -> 1.25=1/2 10 t^2 = 5 t^2 -> t^2=0.25]. Measure the range and you have the distance travelled in 1/2s -> double that to get the speed!
How sad to be on TES on Jan 1st morning - at least I have the Vienna Concert Orchestra for company. Happy New Year again.

3. ELLIS1980

Thank-you so much thats been a huge help out with the pens and paper now to see what i can come up with. Thanks you again and Happy New Year!