Force on a plate due to an air jet

[MspEng]

1. I am trying to come up with an explanation as to why the force on a plate as provided by a jet of air, does not vary with the distance from the plate. We set up in a lab a vertical pointing air jet giving a constant output and gradually increased the height of the plate above the jet whilst measuring the force upon it as imparted by the jet using a Newton-metre. I have done all the calcs and am happy with their accuracy but the problem is our experiment gave a constant force of 1.4 N approx and the theory gives a value of 2.96 N. My big problem is i cannot explain why this is, the equations i provide below work on the assumption that the exit velocity of the nozzle (where the jet exits) is uniform, one dimensional (basically it obeys bernoullis equation) Below is my working for the theoretical value, and the experimental value of force was found to be approximately 1.4 N as stated before... hope someone can explain why there is such a gap between the therory and the reality!



F_P= m_e v_e=(0.039)(75.56)= 2.96 N

 

[JaredJames]

I have a few thoughts on this, but I have to ask one question first:

This setup, from your description I'm assuming is an air jet blowing vertically onto a plate hanging from a force meter.

Now, what is the plate made from and more importantly, what is it's mass?

I ask, because if the plate has a bit of weight the air jet has to overcome that before it can move the force meter.

I would recommend you setup this experiment horizontally so that the weight of the plate is no longer an issue.

 

[LDransfield]

I'm doing a similar experiment, and have my theoretical values coming off around 7, even though my actual values are around 1.6...

I'm not entirely sure but isn't the force equal no matter how far away the jet is due to conservation of momentum: even though the speed of the jet may not be as high at the further away points, the pressure is higher, and there is a greater surface area of the jet hitting the plate.