Why is water more efficient than air in absorbing kinetic energy?

  • Context: Undergrad 
  • Thread starter Thread starter Da Apprentice
  • Start date Start date
  • Tags Tags
    Air Water
Join the discussion
Ask a follow-up here, or get your own question answered by working scientists, mathematicians and engineers — people, not an autocomplete.
Real named experts · corrections over time · the nuance an AI answer skips
Sign up Log in
4 replies · 6K views
Da Apprentice
Messages
57
Reaction score
0
For a project of mine I tested crashing model cars into walls with medical syringes attached filled with varying ratios of air to water. from the results of this it was found that the syringe filled entirely with water was actually the syringe that was most efficient (absorbed the most force). I figured this to be due to this syringe absorbing more kinetic energy becasue it lost the most weight (Ek = 1/2mv^2). But I was just thinking that the differnce in the mass of air and water isn't that much, is it? I figured the dfference would be only the mass of the two additional hydrogen ions, but this is only 2. so the relative atomic mass of oxygen is 16 and the atomic mass of H2O would be 18. the diffence between these two is really small, whereas in the tests the syringe involving air compressed 7 of a possible 12cm and in the test involving water the syringe compressed only 1 cm a relatively large differnce(for aproximately the same velocities)

Is this correct how diffence in the mass of the air and water was calculated or should this be done in another way? would anyone have anyother ideas why the syringe filled entirely with ar would've been the most successful?

Thanks of the help
 
Physics news on Phys.org
What about the densities of air and water, the water would be much more dense than the air, regardless of how similar the molecular masses are. A better approximation for your molar mass of air would be considering a mixture of 80% molecular Nitrogen (N2) and 20% molecular Oxygen (O2)
 
Try this simple experiment in the privacy of your home.

Take two identical containers. Fill one container with air. Fill the other with water. See which container is heavier. You may use a scale or any other weighing device to find out which container is heavier.
 
Something that you must factor in is that water is incompressible whereas air is highly compressible.

In the extreme case, if your syringes are blocked (i.e. piston goes in but no air/water escapes), then, when force is applied, the water syringe will not budge, whereas the air syringe will be highly ... budgeable.