- #1

AN630078

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- Homework Statement
- Hello, I have a question below about elastic and inelastic collisions and momentum. I have answered the first question but I am struggling a little with the second question and would be very appreciative of any help. Although I have answered the first question I am uncertain whether my method would be correct anyhow.

A stream of water is pushed out through an opening of area 40 cm2 at a speed of 30 m/s and hits a wall (incident normally).

The density of water is 1000 kg/m^3

Question 1: What force is exerted on the wall if the water does not rebound?

Question 2: What would be the force if the water rebounds elastically?

- Relevant Equations
- Force= change in momentum

Question 1:

Since Force=Change in momentum = ∆P / ∆t

= mv / t

Momentum of the water coming out=mv

mv=ρVv=ρAv

Force d/dt (ρAv2t)=ρAv^2

Force = 1000*40*30ms^2

Force = 3.6*10^7 N

Question 2:

This is where I am confused because I understand in an elastic collision total kinetic energy and momentum are both conserved.

Therefore, Ek before =Ek after

However, the force previously calculated (equal to the change in momentum) was found when the water does not rebound.

Sorry, I think I am overthinking this and cannot formulate a sound conclusion as a result.

I know how to find the kinetic energy using Ek=1/2mv^2 (if that is at all useful here) but how would I find the force when the water rebounds elastically, since this would be different to the force calculated earlier? Sorry I am rather confused here!

Since Force=Change in momentum = ∆P / ∆t

= mv / t

Momentum of the water coming out=mv

mv=ρVv=ρAv

Force d/dt (ρAv2t)=ρAv^2

Force = 1000*40*30ms^2

Force = 3.6*10^7 N

Question 2:

This is where I am confused because I understand in an elastic collision total kinetic energy and momentum are both conserved.

Therefore, Ek before =Ek after

However, the force previously calculated (equal to the change in momentum) was found when the water does not rebound.

Sorry, I think I am overthinking this and cannot formulate a sound conclusion as a result.

I know how to find the kinetic energy using Ek=1/2mv^2 (if that is at all useful here) but how would I find the force when the water rebounds elastically, since this would be different to the force calculated earlier? Sorry I am rather confused here!