A Failed Experiment in the Acceleration of Gravity

Purpose

We don’t know the speed of the pellets that are fired from a spring-loaded ‘airsoft’ gun. They fly too fast to measure with a stopwatch.

Hypothesis

I think we can compute how fast the pellets fly by finding how far they fall, and using an equation with the acceleration of gravity (which we already know is 9.8 meters per second).

Velocity = RangeAcceleration of Gravity

2 x Height

Materials

bubble level

Planned Procedure

• Secure gun in a vice, 8 meters from wall.• Cock and re-level gun before each shot.• Fire gun and record height of strike.• Repeat for a total of 20 times.• Measure amount of drop.• Plug numbers into equation to compute

speed of pellets.

Procedure (Illustration)

Range = 8 meters

Height

Velocity = RangeAcceleration of Gravity

2 x Height

Height in cm:95.083.0

110.589.594.099.0

101.593.098.0

106.0

Height in cm:

105.0

111.0

122.0

111.5

128.5

113.5

118.5

133.5

119.0

124.0

Data Collection

Both models of airsoft guns were held and leveled in a vice, and fired from a height of 83.5 cm.

Measurements were taken to find how much the pellets fell over disance…

Data Analysis

• We expected that the pellets would hit the wall lower than the height of the gun.

• They weren’t – almost all strikes were HIGHER than the gun. (OOPS!)

Discussion

• Since the strikes were higher than the height of the gun, we needed to research what else (other than gravity) might have impacted our results.

• Possibilities:– Wind resistance?– Helium-filled pellets?– Angry Invisible Rabbits?

• “Hop-up is also the theory of backspin, in which spin, usually up-spin, is placed on the pellet as it grinds along the top of the inside of the barrel. So, the friction creates the spin, and then that spin offsets things such as gravity and air resistance, allowing the BB to travel further.” (Justin Kander)

Research

• “Precession can be described as the tendency of the spin axis of a rotating body (a pellet in flight) to twist at right angles to any external force.” (B.B. Pelletier)

Conclusion

• We didn’t learn what we expected; but we learned something else!

• We have learned in our research that the pellet travels along the top of the barrel of the gun. This gives the pellet ‘backspin’, just like a basketball player does at the foul line. Backspin uses air resistance to create lift for projectiles.

Bibliography

• “Trajectories” by Carl Rod Nave. Georgia State University, Department of Physics and of Astronomy– http://hyperphysics.phy-astr.gsu.edu/hbase/traj.html#tra11

• “Interesting Ballistics” by B.B. Pelletier. Air guns - Pyramid Air Report – http://www.pyramydair.com/blog/2006/06/interesting-ballistics.html

• “Hop-Up in Airsoft” By Justin Kander. Ezine Articles.– http://ezinearticles.com/?Hop-Up-in-Airsoft&id=589601