# Search results

1. ### Conservation of Energy of a Spring with Weight Added

Thanks. And in this case the ‘x’ in 1/2kx^2 isn’t the distance from equilibrium? And the other statements were OK?
2. ### Conservation of Energy of a Spring with Weight Added

Thanks for your response and for outlining the energy forms. It makes sense that when the mass is released the spring stretches and gains elastic PE conceptually. But I thought the definition of the equilibrium point was when the elastic PE=0. I guess that’s only in horizontal situations, and...
3. ### Conservation of Energy of a Spring with Weight Added

The problem has a spring at equilibrium, and then a mass is added to it, so I assumed that means it‘s no longer at equilibrium (it would have a new equilibrium point determined by kx=mg, right?). Thus, if it‘s not at equilibrium, at the point of release, it would have elastic potential energy...
4. ### Conservation of Energy of a Spring with Weight Added

I'm all messed up on this problem. I see you can get the solution (74cm, as listed in the back of the book) by simply setting mgh=1/2kx^2, saying that h=x, and then adding 15 cm to that since that's the original length of the spring. This is the solved solution I was given. But now I think...
5. ### Experimental Determination of “g” using an Elevator

Thanks for your reply— it seems like it is missing something to me too! Apparently the instructor says it can be done by continuously monitoring a, v, d, and t in the elevator. But to me, if you did this experiment on the Moon, all four of those variables could be the same as the values you get...
6. ### Experimental Determination of “g” using an Elevator

I’m kind of stuck on this one. I don’t see how we can use the acceleration of an elevator to find anything about the value of g. We can of course get the acceleration of the elevator, but it feels like that would be independent of gravitational strength. I feel like we need the mass or the...
7. ### Pair Production: Which Conservation Law?

Homework Statement One of the reasons a single photon could not produce an odd number of electrons and positrons is a) energy would not be conserved b) unattainable photon energies would be needed c) matter would be created d) charge would not be conserved Homework Equations The Attempt at...
8. ### Conservation of Angular Momentum: Hopping onto a Board

Great, thank you. That last part really makes sense.
9. ### Conservation of Angular Momentum: Hopping onto a Board

So it seems the sum angular momentum is zero about any given point. That is some points of mass will be rotating clockwise, while others will be rotating counterclockwise. Is that a correct way of reconciling this? Also, from earlier, Orodruin mentioned the rotation would happen indefinitely...
10. ### Conservation of Angular Momentum: Hopping onto a Board

See above. I was answering Orodruin's question about if there is an intial angular momentum. And I thought not.
11. ### Conservation of Angular Momentum: Hopping onto a Board

So if I choose the reference frame of a point on the ice in front of the board where the skater will hit it, the board isn't rotating wrt the point before contact and neither is the skater, so I would think not, but maybe I've got something wrong.
12. ### Conservation of Angular Momentum: Hopping onto a Board

Homework Statement A long board is free to slide on a sheet of frictionless ice. A skater skates to the board (laid horizontally relative to the skater's motion) and hops onto one end, causing the board to slide an rotate. In this situation, are angular and linear momentum conserved...
13. ### Impulse and Acceleration for a Recoiling Car

Great, thanks. Honestly I was just more curious than anything. Appreciate the insight.
14. ### Impulse and Acceleration for a Recoiling Car

Thanks for all the replies. Even though the problem has flaws, I did get something from thinking about it. Without knowing how it was really intended, if I were to assume that the car acts as a spring and compresses a half of a meter and then springs back over a half a meter, then is it right to...
15. ### Impulse and Acceleration for a Recoiling Car

Right the acceleration is bigger in the 12/10 case because the difference is v^2 is 44 instead of 36. So is it just -(15^2) -(30)^2? That just seems like cheating.
16. ### Impulse and Acceleration for a Recoiling Car

OK, so they are magnitudes of 15 and 30, but recoil situations require direction. The change in velocity is 45, but somehow I don't think that is the way to do it (to just do 45^2). I am not sure how to use that equation if they are supposed to be magnitudes but they are vectors.
17. ### Impulse and Acceleration for a Recoiling Car

mv2-mv1=mat => 45=at I know I could use a system of equations, but I was told that this is not necessary for this question. Obviously I am aware that the acceleration would be non-zero in a recoil situation, so that is where my confusion begins. Does that kinematics equation not apply to...
18. ### Impulse and Acceleration for a Recoiling Car

Homework Statement A car with a mass of 1000 kg crashes into a rubber wall with an initial velocity of 30 m/s and recoils at 15 m/s. The change in momentum occurs over 1 meter. Find the force that the car experiences and time for which this force acts. Homework Equations v22 - v12=2ad F=ma...
19. ### De broglie wavelength of fast moving particle-help please!

Okay, that makes a lot more sense now! I definitely didn't realize the limits on that equation. I really appreciate you taking the time to help me with this problem-- thank you!
20. ### De broglie wavelength of fast moving particle-help please!

Thank you so much! Yes, somehow I dropped the √3/2 term and never realized. I ended up using the relation λ=h/p. Don't know why I didn't use that in the first place, but it made it much easier. I have a question though. Using the relation above, I saw that h/p=h/m0γv, so in the first case...
21. ### De broglie wavelength of fast moving particle-help please!

De broglie wavelength of fast moving particle--help please! Homework Statement A particle moving with kinetic energy equal to its rest energy has a de Broglie wavelength of 1.7898 *10^-16 m. If the kinetic energy doubles, what is the new de Broglie wavelength? Homework Equations...
22. ### Energy Dissipated in Loop (Magnetic Field)-

Oh-duh! Thank you--got it now.
23. ### Energy Dissipated in Loop (Magnetic Field)-

Energy Dissipated in Loop (Magnetic Field)-- Please Help! Homework Statement What is the energy dissipated as a function of time in a circular loop of N turns of wire having a radius of r and a resistance of R if the plane of the loop is perpendicular to a magnetic field given by...
24. ### Finding the Force between a Straight Wire and a Triangular Current Loop

Homework Statement A triangular loop of side length a carries a current I. If this loop is placed a distance d away from a very long straight wire carrying a current I', determine the force on the loop. See attachment for diagram. Homework Equations F=(\muII'/2\pir)L F=I'LB...
25. ### Calculating Magnetic Fields in Solenoids vs. Current Loops and Coils

Homework Statement There is a solenoid, with field B at its center, radius r, length L and maximum current I. Calculate the minimum number of turns the solenoid must have. Homework Equations Ampere's Law: (closed)∫B\cdotdl=\muI n=N/L Biot-Savart: B=\muIN/(2\pir The Attempt at a Solution...
26. ### Inserting Dielectric and Direction of Force on It

Homework Statement A slab of width d and dielectric constant K is inserted a distance x into square parallel capacitor plates of side L. Determine the magnitude and direction of the force on the slab (Assume V0 is constant. Homework Equations F= d/dx(U) U=1/2CV2 The Attempt at a...
27. ### Electric Field of a Straight Wire (now with the attachment)

Okay, I figured it out. I am using an online HW program and it just wanted the the answer in terms of epsilon, not a numerical value. Thanks anyway!
28. ### Electric Field of a Straight Wire (now with the attachment)

1. Homework Statement The word document with the problem is attached below. 2. Homework Equations dE=kdQ/r22 Ex=Ecos∅ 3. The Attempt at a Solution dE=kdQ/r2 E=kλ∫dy/(x2+y2) (integrating from y= -L/2 to L/2) Ex=2kλ∫xdy/(x2+y2)3/2 (change integration to y= 0 to L/2, multiplying...
29. ### Electric Field of a Straight Wire at a Particular Distance from Wire

Homework Statement See attached document Homework Equations dE=kdQ/r^2 Ex=Ecos∅ The Attempt at a Solution dE=kdQ/r^2 E=kλ∫dy/(x^2+y^2) (integrating from y= -L/2 to L/2) Ex=2kλ∫xdy/(x^2+y^2)^3/2 (change integration to y= 0 to L/2, multiplying expression by 2) Let y=xtan∅...
30. ### Static Equilibrium: Sliding and Tipping Bookcase

Homework Statement See Picture: I am only stuck on Part (c) where it asks to calculate the force as the bookcase starts to tip. Homework Equations sigma Fx=0, sigma Fy=0, sigma tau=0 The Attempt at a Solution I am setting up the problem by picking an axis that runs through the...