by Classy Jack
Tags: college, pass, physics
 Emeritus Sci Advisor PF Gold P: 4,981 I think most people on physics forums would gladly help for free. Trying to coerse people with cash is not likely to work. Second of all you have to have a reasonable attempt at the problems yourself and post your working out. At that point then you may receive hints to which you can work it out yourself.
 Mentor P: 41,310 PLEASE Help Me Pass COLLEGE PHYSICS The way to get help is to post your attempted solution. Sure it takes time! Just like it takes time for folks to provide you help. Doesn't matter whether it's right or wrong, just show your work and you'll get help. (And, seriously, that's the only kind of help that really helps because it can pinpoint the exact spots where your understanding is off.)
 Mentor P: 40,707 -3- Hey, I did my undergrad at UC Davis! Is that where you are studying? Cool. Now, let's use this problem #3 as an example. What are your initial calculations and conclusions? We'll start out helping you with this one after you show us your work.....
 P: 290 For question one, consider the forces involved. You've gravity, and you've magnetism. What does 'your friend' think has happened? She's seen the magnet spontaneously rise up from the table (against gravity) and stick to the magnet. If you didn't know about magnetism, you'd think that little bar had somehow 'magically' defeated gravity (ie given something gravitational potential energy from nowhere). But you do know about magnetism. What can you say about the magnetic potential of the nail before and after? How does that relate to the gravitational potential?
 P: 7 Ok. Ive managed to figure out a few questions. Heres some of my work for the ones I dont understand. Most of it is guesswork. 1. Energy is always conserved according to Newtons 1st Law. If the magnet is accelerating upwards it is exhibiting Kinetic Energy and when it reaches the magnet it will transfer that kinetic energy to potential energy, conserving the total. 2. This question makes little sense to me. This is what I think is useful. Momentum equals mass times velocity. Car A would have the advantage of mass but this would be negated by the lack of velocity if Car A was indeed stopped at the red light. The momentum of the smaller car should have overwhelmed the momentum of the slightly larger car and created skid marks. 3. What am I missing for this one? All I'm given is height. 23m. I guess I know gravity acceleration 9.8 m/s2. What can I do with this? The first second the water accelerates 9.8 m/s, the second it accelerates 19.6 (bringing the total to 29.4) so somewhere between one and two seconds the water passes the given height. I dont even know what equations I am working with here. 4. Angular momentum is used with abjects rotating around a fixed axis. Angular momentum of an object changes when torque acts on the object. Angular momentum is a product of the objects mass, displacement from the center of rotation, and the component of velocity perpindicular to that displacement. Energy goes from potential to kinetic as the child leaps onto the merry go round. 5. What is this guy talking about here. I dont even know where he is going with this one. Can you give me some insight on how to convert the given height (2m) into blood pressure (terms of torr=mmHg)? Even a hint or a starting point. 6. I figured out. 7. I got it. 8. The obvious answer is the speed is greatest from the shot fired straight down because it has the combustion velocity and the gravity acceleration and the shortest distance down reducing the loss of velocity to air resistance, gravity, and other relevant forces acting on the bullets fired horizontally and vertically. The bullet fired straight up should only have gravity as its speed on the way down. I dont know if any of this is right or not. 9. Acceleration or deceleration is a function of change in velocity over time elapsed. I increase speed by 150%, and i need a formula using v and v' to find deceleration. Or is it something else? What should I look into here? I'll do some more in a moment. Please help me in the right direction.
 P: 7 10. Momentum equals mass times velocity. In elastic collisisons KE is conserved. If KE is not conserved the collision is inelastic. A completely inelastic collision is one in which the colliding objects stick together after the collision. For the momentum of the smaller car to equal the momentum of the larger car it would have to make up for the difference in mass with a greater velocity. 11. Given mass (20kg) and in part A, the height of the hill (4m), how do I calculate speed? Do I use gravity again for acceleration (9.8 m/s2)? When given the stretch of the net (.6m) and stiffness constant (k=4000N/m) how do I calculate mechanical energy lost? Whats the conversion of one can of Sprite (155 kcal = 657 kJ) to energy for hill climbing? This is way over my head. 12. Seems I am given mass of object 1 (m) and object 2 (20m). I am also given speed of object 1 (0m/s) and object 2 (v)Assuming an elastic collision happens leads to the formula .5Ma(Va)2+.5Mb(Vb)2=.5Ma(V'a)2+.5Mb(V'b)2 but i couldnt make this give me anything useful. What am I doing wrong or missing?
 HW Helper P: 2,944 For the first question, major misconception : Newton's Laws don't say anything about Conservation of Energy. They're quite distinct. Don't forget Magnetic Potential Energy (and heat due to air resistance as the nail flies, and heat and sound energy dissipation from the nail hitting the magnet). You have to consider *everything* (all forms of energy) in a closed system if you want to show conservation of energy.
 P: 4 For 3 have you ever seen the equation x=v0t+1/2at^2? You have x (distance), and a (gravity), and x0 and v0 both = 0 so solving for time is just manipulating the equation to solve for T. Since you have the time it takes to go that distance... well that should be enough to figure velocity. the second part is using the velocity found in that same equation to solve for x... which is now the variable you are missing. Hope i helped. For 8, your thinking is flawed, slightly... Remeber that the bullet shot straight up had the same velocity leaving the gun as the one pointed down and also straight out. When the bullet moving up turns back and falls back to earth it will pass the gun at the same exact speed as when it left (which is the same for the bullet shot down). So my thinking is they both (up and down) hit at same velocity. Correct me if im wrong.
P: 4
 Quote by Curious3141 For the first question, major misconception : Newton's Laws don't say anything about Conservation of Energy. They're quite distinct. Don't forget Magnetic Potential Energy (and heat due to air resistance as the nail flies, and heat and sound energy dissipation from the nail hitting the magnet). You have to consider *everything* (all forms of energy) in a closed system if you want to show conservation of energy.
I was going to say something similar, but you put it in better words than i had.
 P: 7 thank you so much. I got the fire hose problem to work all the way through. That was a great help. Could I bother you for some more? I really do appreciate this. Where do I start when figuring out the giraffes blood pressure when only given the 2m length of its neck?
 P: 7 I've done 1-4, 6-8, 10-11. I just need a little more help to get through these last couple. Please just lend a hand and give me a clue if you got any. How do I start problems 5, 9, and 12???????? How do I convert a 2m neck height into blood pressure? How do I turn an increase of 150% speed into an increase in brake factor? How do I turn two masses (m and 20m) in one moving and one unmoving object into 2 post collision speeds?
 P: 7 dont forget to help me. Please....
 Emeritus Sci Advisor PF Gold P: 4,981 5) Consider the work done in moving a particular mass of liquid 2 meters vertically up and down. 9) Braking is the dissipation of ENERGY so consider the kinetic energy increase. 12) Momentum is conserved, where momentum is (mass x velocity)