Solving Particle P Collision: Angle, Momentum, Energy

[ArticMage]

Homework Statement

A particle p traveling with a speed of vpi = 3 m/s hits and scatters elastically from another particle N, initially at rest. Particle p is deflected through 90°, leaving with a speed of vpf = 2.5 m/s, and a mass mp = 2 kg.

a) What angle (in degrees) does the recoiling N make to the incident-p direction? ans = 39.8055
b) What is the magnitude of the momentum of the recoiling N? ans = 7.810
c) What is the change in the energy of the p? ans =2.75
Im stuck on the next one.
d) What is the speed of the recoiling N?

What I thought I should do is set .5*m*v^2 = 2.75 since its elastic collision kinetic energy's conserved and the initial kinetic energy was 0. Then set m*v=7.810 since the momentum is conserved then solve one for m and sub it into the m in the other equation. DOing this both ways i get .7402 which is wrong.

e) What is the mass of the N?
This will be easy to find after i get d.

 

[Fewmet]

I am not clear on what "p is deflected through 90°" means. I would expect p and n to move away with a 90° angle between them. Is that it?

 

[Ray Vickson]

Initial kinetic energy (KE) = that of p alone, before the collision. Final KE = post-collisiion KE of p + post-collision KE of N. Initial momentum = pre-collision momentum of p alone. Final momentum = post-collision momentum of p + post-collision momentum of N. Renember: KE is a scalar, but momentum is a vector.

RGV

 

[ArticMage]

http://img62.imageshack.us/img62/8481/showmepla.gif

There is a picture that should clear things up.

As for the 2nd post I've got all that I'm not seeing how it helps me with d.

 

[ideasrule]

Are you sure your answers for a-d are all correct? If so, your answer for e should definitely be correct.

 

[Fewmet]

Homework Statement

What I thought I should do is set .5*m*v^2 = 2.75 since its elastic collision kinetic energy's conserved and the initial kinetic energy was 0. Then set m*v=7.810 since the momentum is conserved then solve one for m and sub it into the m in the other equation. DOing this both ways i get .7402 which is wrong.

I got the same answers as you for a, b and c. For d I got 0.7042. Could you just be making a transcription error?

 

[ArticMage]

Nope I am plugging it into the online homework and it says its wrong. The first 3 are correct though.

 

[Fewmet]

Nope I am plugging it into the online homework and it says its wrong. The first 3 are correct though.

I agree that 0.7402 is incorrect. Is 0.7042 correct?

 

[ArticMage]

Oh yeah that did it. I guess I just miss read it a couple times there. Thanks.

 

[Fewmet]

Oh yeah that did it. I guess I just miss read it a couple times there. Thanks.

I am glad it got resolved. In the future, though, be aware that if the answer is wrong and the physics seems correct, you can save you and us time by checking for things like wrong negative signs or typos.