Practical Investigation - Ballista Project - page 2
Keywords: Physics Practical Investigation Ballista Crossbow
By Jenny on 02/07/2009
Level: A Level (Year 13)
Page Number: 2 of 4 pages: 1 2 3 4
for the ballista to fire well and strong so that it can stand high tensions with out breaking.
• Elastic bands can stretch a lot but break easily when under tension and also undergo a large amount of plastic deformation which could affect my results.
• I will use bungee cord as it is very elastic and strong, and so can be stretched a large amount without breaking or undergoing large amounts of plastic deformation, so it could be used to fire the arrow with a large force.
• I could also measure the Young’s modulus of the cord to see whether it undergoes any plastic deformation at the tensions I am using and if so how much, as this could have a bearing on my results.
• The rulers must be securely attached to the stock as any wobbling could affect my results.
• I could attach them with screws, nails, or glue. Screws are the strongest and most secure (they’re also removable) but have the disadvantage of needing to dismantle the structure to attach them.
• I will just be pulling the cord back with my fingers to fire it rather than building a firing mechanism as it would be quite complex and I only have a limited amount of time.
• After I reattached the main block (I used the screws for maximum strength) I noticed a problem – the stands weren’t flat on the surface causing the structure to wobble.
• This is a problem as it will affect the accuracy of my results. The block and the cord must be exactly level (and perpendicular to each other) and not move as the angle of the cross bow is a constant in the investigation. I’m making it horizontal so that it will only have acceleration in one direction.
• To solve this I rebuilt and clamped my structure using a spirit level to make sure each part was exactly level. It is now reasonably stable as long as it isn’t moved, (possible future problem), another factor may be that the table isn’t exactly level.
• To measure the force used to pull back the cord I am going to use a newtonmeter to measure how far back along the block I need to pull the cord to give it a certain force.
• I am using 3 Newton metres with different ranges to measure out the marks to get the maximum accuracy, the first one measures from 0 - 20N, the
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