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Homework Help: PLEASE Help Me Pass COLLEGE PHYSICS

  1. Mar 31, 2006 #1

    1. Your friend is convinced that she's found a situation where the total energy in the universe is not
    being conserved. To demonstrate, she takes a magnet out of her pocket and uses it to pick up a nail.
    Analyze this situation when the nail is accelerating upward (i.e. using energy concepts). How would
    your friend argue that energy isn't conserved? Try and give an argument to your friend proving that
    the total energy is conserved.

    2. You are called to investigate a traffic collision (involving two very expensive cars) where the facts are
    in dispute. Driver A claims that Driver B rammed into him from behind while he was stopped at a
    red light. The damage is very light, but the repairs will be expensive. You investigate the scene and find no skid marks on the road. Driver A explains this by saying that his car (car A) weighs a bit more
    than the other car (car B) which is why you don't see any skid marks. Use momentum and energy
    concepts to reason about what actually happened in the collision.

    3. You hook a fire hose directly up to a hydrant that sits beneath a water tower at UC Davis. If the
    water in the tank is at a maximum height of 23 m, with what velocity does the water exit the hose on
    the ground (assuming no loss to thermal energy systems)? Using this velocity, calculate the maximum
    height to which the water will rise if you point the hose straight up. How would this height change if
    you pointed the hose at an angle?

    4. A merry-go-round is fixed to the ground at its pivot point (it is free to rotate about this point).
    A child runs with a speed v and leaps onto the merry-go-round (she holds on tight), setting it in
    motion. Consider the system of merry-go-round and child together and analyze the Energy, Momentum,
    and Angular Momentum of this system before and after the child leaps onto the merry-go-round.
    (Which are conserved? Which change?) If any of the above quantities change, account for this change

    5. A large adult giraffe has a neck that is about 2 m long. These animals can move thier heads from being
    vertically upward, to nearly vertically downward.

    (a) Calculate the change in blood pressure in the giraffe's head as it lowers its head from being
    vertically upward to the ground. Express this pressure in units of torr = mmHg which are the
    usual units for blood pressure.
    (b) How does this pressure relate to normal blood pressure ranges for healthy animals (the same as
    humans, 70 mmHg-140 mmHg). Comment on the plausibility of your results from (a).

    6. At the scene of a highway collision, skid marks stretch out for 90m away from the collision site.
    Assuming the deceleration of the vehicle was at least 6.00 m/ s 2 ; estimate the speed of the car before
    the incident.

    7. A book flies past your dorm room window (which is 1 m tall), taking 0.25 s to pass. Assuming it
    was dropped, from what height (above your window) was the book dropped?

    8. You fire three shots from your gun (let's label the shots A, B, and C). One straight up (A), one straight
    down (B) and one horizontally (C). Assuming that you shoot from the hip (i.e. your hand is in the
    same place each time, right by your holster) and you ignore air resistance,
    (a) (30%) which of the three bullets (A, B, or C) will have the greatest speed as it reaches the ground
    and why?
    (b) 10% Now include air resistance, how does your answer change and why?

    9. If the speed of a car is increased by 150% (i.e. v' = 2.5v), by what factor will its minimum braking
    distance be increased, assuming all else is the same? Ignore the driver's reaction time.

    10. You are driving in a large truck when a small car crashes into it. Is is possible that all kinetic
    energy is lost immediately after the collision? If not, then explain why it is not possible to lose all
    kinetic energy in this collision. If so, describe how it is possible.

    11. A child on a sled (total mass = 20 kg) slides (from rest) down a hill, through a snow-bank, and into a
    (a) If the hill is 4 m high, calculate the speed of the child before she hits the snow-bank.
    (b) If the child stretches the net 0.6 m and it acts like a spring with stiffness constant k = 4000 N/ m, how much mechanical energy was lost in the snow-bank?
    (c) If the child drank one can of Sprite soda (155 kcal = 657 kJ), how many times could the
    child run back up the hill (carring the sled) if she could somehow channel this energy with 100%

    12. A light bowling ball (mass = m) is sitting at rest. A much heavier ball (mass = 20m) is moving
    with a speed v and collides headson with the first ball. Assume an elastic collision happens and find
    the speeds of each ball after the collison.

    13. We claim that momentum and angular momentum are conserved. Yet most moving or rotating
    objects eventually slow down and stop. Explain.

    14. I have a wrench that is 30 cm long and 3 cm wide, the maximum amount of force I can exert on
    the end of the wrench is 400 N. Suppose I cannot turn a certain bolt that is stuck. Two friends offer
    to loan me their wrenches to do the job. One is 25 cm long and 5 cm wide, the other is 35 cm long but
    only 2.1 cm wide. Which wrench should I use to do the job and how much extra torque will it provide

    15. (a) Is it possible to have a system where the net force is zero, but the net torque is not zero? If not,
    explain why not. If yes, then give an example.

    (b) Is it possible to have a system where the net torque is zero, but the net force is not zero? If not,
    explain why not. If yes, then give an example.

    (c) Is it possible to have a system where neither the net torque or the net force is zero? If not, explain
    why not. If yes, then give an example.
  2. jcsd
  3. Mar 31, 2006 #2


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    Staff Emeritus
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    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.
  4. Mar 31, 2006 #3
    It was pointed out that my attempt at reimbusement was a bit out of line and in bad taste.

    I did not mean for it to come off that way. I was only looking for help and wanting to get it even if I had to offer someone compensation for their help. Like tutoring. I work tons better with solutions in hand to work back from.

    And as for trying them myself, I have tried every problem and have five plus answers for each without knowing what avenues are correct and which are not. It would take me hours to type in my work thus far. Im sorry if I came across as rude or materialistic or unworthy of assistance.

    I have tried many different courses of action already to this point but am basically screwed if I cannot get some answers to the posted questions within the time frame allowed so as to better my chances of passing the last and deadliest test of the quarter, my final final.

    I have written over 75 pages of term paper and technical essay in the last three weeks. Ive taken 3 finals and two non cumulative exams in the same span. Ive barely been sleeping and I work full time.

    I'll take any lecture you feel appropriate about the etiquette of my post or otherwise if you will assist me in working through the problems. Which are not all of the ones for my study guide by a long shot. Ive answered at least as many on my own and worked with other students to answer again as many. Please help me. I am going to have a nervous breakdown if you dont.
  5. Mar 31, 2006 #4

    Doc Al

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    Staff: Mentor

    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.)
  6. Mar 31, 2006 #5


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    Staff: Mentor

    -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.....
  7. Mar 31, 2006 #6
    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?
  8. Apr 1, 2006 #7
    heres some of my work, what do you think?

    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.
  9. Apr 1, 2006 #8
    heres 10-12

    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
    but i couldnt make this give me anything useful. What am I doing wrong or missing?
  10. Apr 1, 2006 #9


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    Homework Helper

    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.
  11. Apr 1, 2006 #10
    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.
  12. Apr 1, 2006 #11
    I was going to say something similar, but you put it in better words than i had.
  13. Apr 2, 2006 #12
    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?
  14. Apr 2, 2006 #13
    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?
  15. Apr 3, 2006 #14
    dont forget to help me. Please....
  16. Apr 3, 2006 #15


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    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)
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