Practical Investigation - Ballista Project - page 3
Keywords: Physics Practical Investigation Ballista Crossbow
By Jenny on 02/07/2009
Level: A Level (Year 13)
Page Number: 3 of 4 pages: 1 2 3 4
next from 0 - 30N and the last one from 0 50N. However although I checked them this does introduce the possibility of error if they werent all calibrated to measure exactly the same.
I will mark the distances for the forces I want to use on the rulers to save time and ensure I use the exact same place every time.
The gap between the rulers serves as a firing slot to keep the arrows accurate. Before the gap was too deep for the cord to touch the arrow when it was in it so I lined it with paper to make it shallower.
The width of the arrows must be constant to fit the firing slot.
It is difficult to change the shape and weight of the arrows and have them still all the same width to fit in the firing slot so they will have to be constants.
I could vary the sharpness of the arrow which would affect aerodynamics and air resistance and so would affect the results.
Diagram
Method
I set out a tape measure perpendicular to the prod to measure the distance travelled by the arrow.
Before taking each set of readings I used a spirit level to make sure the wooden block and the cord were exactly level.
I fired 5 bolts for each mark and I got an observer to mark the place where the front end of each bolt hit the ground on the tape measure (possibility of human error here as the bolts were fast and bounced, particularly for the larger forces).
I fired the bolts with forces measuring from 0 40 N in 5 Newton steps.
I shot 5 bolts on each setting to ensure a fair test.
I measured the distance along the tape measure each bolt travelled from the front of the firing slit.
I did this with the tension in the cord externally set to 10, 20 and then 30 N.
Apparatus
3 x stands
4 x stand poles
11 x bosses
1 x clamp
2 x G-clamps
1 x wooden block
2 x metre rulers
6 x screws
1 x bungee cord
10 x bawls
3 x Newton metres (ranges: 0 20N, 0 30N, and 0 50N sensitivities: +/−0.4N, +/−0.5N, +/−1N)
+/−0.4N, +/−0.5N, +/−1N)
1 x piece of paper
1 x 50 m tape measure (sensitivity: +/−1cm)
1 x marker pen
Calculations
F = Force
Es = Strain Energy stored in cord
m = mass
v = initial horizontal velocity
Ek = Kinetic Energy
u = initial vertical
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