I hate Physics (please help)

[johnjohn320]

Physics AP is eating me alive for some reason. I have 8 classes, and my first report card this year has 7 As and one C. Guess which class is the C?

Anyway, I have a test tomorrow. This problem has me stumped:

A large plate is dropped and breaks into three pieces. The pieces fly apart parallel to the floor. As the plate falls, its momentum has only a vertical component, and has no component parallel to the floor. After the collision, the component of the total momentum parallel to the floor must remain zero, since the net external force acting on the plate has no component parallel to the floor. Find the masses of pieces 1 and 2.

Here's a drawing:
http://pics.bbzzdd.com/users/johnjohn/physics21.JPG

So, all I've been able to reasonably accomplish is finding the momentum of of m3 using p = mv, for which I got 3.991. Other than that, I don't know what to do. I could use trig functions to find the components of the velocities of the other two masses, but I don't know what that accomplishes. Any help? Thanks.

 

[DrPizza]

That's not so bad....

1. Find the horizontal components of the other two velocities...
Let's call them v(h1) and v(h2)
m1*v(h1) = m2*v(h2)

(the momentum to the left and right in your diagram have to be the same.)

2. Find the vertical components of those two velocities
Let's call them v(V1) and v(V2) [I can't type subscripts]

The sum of m1*v(V1) and m2*v(V2) = m3*v3
(the momentums up and down on the page have to cancel each other out to be in equilibrium)

Thus, you have 2 equations with 2 unknowns.... solve for m1 and m2

 

[DrPizza]

note: why is it that there are never hot babes who ask for physics help, and are eternally grateful afterwards?

No, I take that back... I had an offer to help someone study for a physical chemistry test once... "I'll do anything for you if you can help me pass, and I mean anything" followed by a kiss. Unfortunately, for me, she was an idiot and was destined to fail no matter what. Further unfortunately for me, I didn't realize at the time that she was trying to hit on me regardless. *sigh* the days before I semi-emerged from geekdom.

 

[johnjohn320]

Ok, there must be a law I'm missing here. Why does the momentum have to be the same for m1 as m2? Assuming that, all the work makes sense.

Thanks DrPizza!

 

[mAdD INDIAN]

Originally posted by: johnjohn320
Ok, there must be a law I'm missing here. Why does the momentum have to be the same for m1 as m2? Assuming that, all the work makes sense.

Thanks DrPizza!

because momentum is conserved.

The momentum b4 the collision should be equal to the momentum after the collission.

Momentum questions are acytually quite easy. its when things go into polar and rotational dynamics that the sh!t hits the fan.

 

[konichiwa]

How the hell do you have eight classes in a semester?

 

[EyeMWing]

That's why I'm not taking AP Physics. Honors physics is so much more badass - all our momentum problems are linear!!!

 

[mAdD INDIAN]

Originally posted by: konichiwa
How the hell do you have eight classes in a semester?

because failure is fun.

you learn that in engineering.

 

[EyeMWing]

Originally posted by: konichiwa
How the hell do you have eight classes in a semester?

Most high schools do not use a semester system and instead have 8 classes all year long. It puts more emphasis on homework, and makes language teachers happy, but math and science teachers hate it, because they only have time for 2 or 3 problems per day. Mine has 4 classes/semester. The english teachers have adapted, the FLang teachers like to gripe about it, but they get along, and the science and math teachers LOVE it.

 

[johnjohn320]

Meanwhile, I've got another quesiton ( i know i know, i'll get it eventually):

In a football game, a receiver is standing still, having just caught a pass. Before he can move, a tackler, moving at a velocity of 4.5 m/s, grabs him. The tackler holds onto the receiver, and the two move off together with a velocity of 2.6 m/s. The mass of the tackler is 115 kg. Assuming that the momentum is conserved, find the mass of the receiver.

Alright, so I found the momentum of the tackler before hitting the recevier to be 517.5 . Now I get confused. I went for a shot in the dark here and multiplied 115 * 2.6 to say that after the hit, the tackler had momentum of 299. Thus, the remaining 218.5 must be the momentum of the receiver now. But I don't think that's right, because the 299 is the momentum of both the guys together, not just the tackler after the hit. Argh...

 

[DrPizza]

I didn't say the momentum was the same for m1 and m2..... I said the momentum in the horizontal direction was equal (but opposite in sign), because they are the only components with a horizontal momentum, and the net horizontal momentum before the breakage was 0.

On a side note, problems like this can be quite interesting.... in particle accelerators, the momentum of the system is understood before a collision. Following the collision, that momentum must be conserved. After analyzing lots of data from all sorts of sensors, physicists have in the past proclaimed... "That's weird, the momentum wasn't conserved... there must be some particle that we didn't detect" And, eureka... they discover something new.

 

[johnjohn320]

Originally posted by: konichiwa
How the hell do you have eight classes in a semester?

Don't know what to tell you, this semester I have

World Lit
Physics AP
Music Theory AP
Jazz Workshop
Trig
Wind Ensemble
PE
Government

 

[speg]

Physics rocks, as my gym teacher always said.. ."It's physics!" Im majoring in physics. Yeehaw, midterm on wednesday which I should be studying for....

 

[dighn]

Originally posted by: johnjohn320
Meanwhile, I've got another quesiton ( i know i know, i'll get it eventually):

In a football game, a receiver is standing still, having just caught a pass. Before he can move, a tackler, moving at a velocity of 4.5 m/s, grabs him. The tackler holds onto the receiver, and the two move off together with a velocity of 2.6 m/s. The mass of the tackler is 115 kg. Assuming that the momentum is conserved, find the mass of the receiver.

Alright, so I found the momentum of the tackler before hitting the recevier to be 517.5 . Now I get confused. I went for a shot in the dark here and multiplied 115 * 2.6 to say that after the hit, the tackler had momentum of 299. Thus, the remaining 218.5 must be the momentum of the receiver now. But I don't think that's right, because the 299 is the momentum of both the guys together, not just the tackler after the hit. Argh...

initial momentum = 4.5 * 115 = after = 2.6*(115 + receiver)

 

[johnjohn320]

Originally posted by: dighn

Originally posted by: johnjohn320
Meanwhile, I've got another quesiton ( i know i know, i'll get it eventually):

In a football game, a receiver is standing still, having just caught a pass. Before he can move, a tackler, moving at a velocity of 4.5 m/s, grabs him. The tackler holds onto the receiver, and the two move off together with a velocity of 2.6 m/s. The mass of the tackler is 115 kg. Assuming that the momentum is conserved, find the mass of the receiver.

Alright, so I found the momentum of the tackler before hitting the recevier to be 517.5 . Now I get confused. I went for a shot in the dark here and multiplied 115 * 2.6 to say that after the hit, the tackler had momentum of 299. Thus, the remaining 218.5 must be the momentum of the receiver now. But I don't think that's right, because the 299 is the momentum of both the guys together, not just the tackler after the hit. Argh...

initial momentum = 4.5 * 115 = after = 2.6*(115 + receiver)

smack forehead...i got it just before i read your reply...😉