Why is the measured and calculated value of a projectile different?

Question

I took 5 different values from my catapult and the average distance of the projectile is 3.64 meters however, the calculated value is 3.79 meters. I understand that some deviation is inevitable but I need to understand the reason behind it and any possible sources of error.Is anyone willing to explain?

Answer 1

Probably most of the loss comes from frictional losses (air resistance, rubbing within the catapult, heat loss in the spring, separation of the catapult and projectile). Perhaps some loss from having a non-linear spring constant (the less force, the lower the k).