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Homework Help: A few physics questions. Please help me!

  1. Jun 3, 2009 #1
    1. The problem statement, all variables and given/known data

    Some of these questions are easy and I just want to make sure i"m right. Others are pretty hard for me. Please only do a few problems because there are a lot and i don't want to overload anyone. If several people could help me by each doing a few problems it wouldn't be a hassle for you guys and would be a huge help for me. Thanks in advance. You can show me how you got the answer if you would like, but again there are a lot of problems so if you don't have time dont worry about showing the work. Thanks.

    1. A 1000 kilogram sports car accelerates from rest to 25.0 m/s in 7.5 seconds. What is the average power output of the automobile engine. Choices: A. 20.8 KW, B. 30 KW, C. 41 KW, or D. 52.4 KW I believe the answer is C but I'm not sure.

    2.The more powerful the motor is A. The longer the time interval for doing the work is. B. the shorter the time interval for doing the work is. C. the greater the ability to do work is. or D. the shorter the workload is. I believe the answer is C to this, but again not 100% sure.

    3. A hill is 100 meters long and makes an angle of 12 degrees with the horizontal. As a 50 kilogram jogger runs up the hill how much work does gravity do on the jogger? A. 50,000 J, B. 10,000 J, C. -10,000 J, or D. 0 J I believe the answer is a.

    4. A flight attendant pulls a 50.0 N flight bad a distance of 250 meters along a level airport floor at a constant speed. A 30 N force is exerted on the bag at an angle of 50 degrees above the horizontal. How much work is done on the flight bag? A. 12500 J, B. 7510J, C. 4821 J, D 8040 J. I believe it is C.

    5. Which of the following energy forms is involved in winding a pocket watch. A. electical energy, b. nonmechanical energy, c. gravitational potential energy, or d. elastic potential energy. I think it is d.

    6. which of the following energy forms is involved in a pencil falling from a desk. a. kinetic energy. b. nonmechanical energy, c. gravitational energy, or d. elastic potential energy and kinetic energy. I think it is A. but im not sure.

    7. If the only force acting on an object is friction during a given physical process, which of the following assumptions must be made with regard to the objects kinetic energy?
    A. the kinetic energy decreases, B. the kinetic energy increases. C. the kinetic energy remainds constant, or D. the kinetic enery decreases and then increases. I think it is A.

    8. WHich of the following energy forms is associated with an object in motion. A. potential energy, B. elastic potential energy. C. nonmechanical energy, or D. kinetic energy. I think it is D.

    9. which of the folowwing parameters does not express how resistant a spring is to being compressed or stretched? a. compression distance, b. relaxed length, c. spring constant, or d. stretching distance?

    10. A 16 KG child on roller skates initially at rest rolls 2.0 m down an incline at an angle of 20 degrees with the horizontal. If ther eis no friction between incline and skates what is the kinetic energy of the child at the bottom of the incline? A. 210 J. B> 610 J. C. 11 J. D. 107 J

    11.. A pole vaulter clears 6 meters. With what velocity does the vaulter strike the mat in the landing area? (I think this is assuming the vaulter clears exactly 6 meters and no higher). A> 2.7 M/s B. 5.4 M/s C. 45 M/s D. 54 M/s.

    12.A bobsled zips down an ice track starting at 150 m vertical distance up the hill. Disregarding friction, what is the velocity of the bobsled at the bottom of the hill?

    13. A 40 N crate starting at rest slides down a rough 6 meter long ramp inclined at 30 degrees with the horizontal. The force of friction between the crate and the ramp is 6 Newtons Using the work-kinetic energy theorem find the velocity of the crate at the bottom of the incline. A. 8.7 m/s b. 3.3 m/s c. 45 m/s d 54 m/s

    14. A parachutist with a mass of 50 kg jumps out of an airplane at an altiude of 1000 meters. After the parachute deploys the parachutist lands with a velocity of 5 meters per second. Using the work-kinetic energy theorem, find the energy that was lost to air resistance during the jump. A> 49300 J. B. 98800 J. C. 198000 J. D> 489000 J.

    15. A child riding a vicyle has a total mass of 40 kg. the child approaches the top of a hill that is 10 meters high and 100 meters long. at 5 meters per second. if the force of friction between the bicycle and the hill is 20 newtons, what is the child's velocity at the bottom of the hill (disregard air resistance). A. 5 m/s B. 10 m/s. C. 11 m/s D. the child stops before reaching the bottom.

    16. Water flows over a section of niagra falls at a rate of 1.2 x 10^6 kg/s and falls 50 meters. What is the power of the waterfall? (mega = 10^6). A> 590 MW. B. 294 MW. C. 147 MW. D. 60 MW

    17. Explain how energy, time, and power are related.

    18. a tired squirrel with a mass of 1 kg does pushups by applying a force to elevate its center of mass by 5 cm determine the number of pushups which a tired squirrel must do in order to do a mere 1 joule of work. If the tired squirrel does all this work in 4 seconds then determine its power.

    2. Relevant equations

    I would give you them, but if I knew them I wouldn't be asking for help. :(

    3. The attempt at a solution

    I put my attempts in the questions.
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Jun 3, 2009 #2
    I suggest you ask your teacher for extra help. If you're having trouble with all of these questions, it means that you're having difficulty understanding the concepts behind what you're learning.
  4. Jun 3, 2009 #3
    Can anyone please help me. These are due tomorrow. If you can just help with a few problems that will still be good.
  5. Jun 3, 2009 #4


    User Avatar
    Homework Helper

    You will need these formulas.


    v=final velocity,u=initial velocity, t=time, a=acceleration

    [tex]Power=\frac{Work \ done}{time} [/tex]

    [tex]Gravitational \ Potential \ Enegy= mgh [/tex]

    [tex]Work \ done = Force \times distance [/tex]

    Work-Energy Theorem which is basically
    Inital potential energy+initial kinetic energy = final potential energy +final kinetic energy.
  6. Jun 3, 2009 #5
    12. There isn't enough information to answer the question. Is it a 2-man or 4-man sled? Is the sled at its maximum weight? What track you are on? How steep is it? According to the question, the sled is already moving. How far down the track is this? After the first curve you could be going 30mph, after the 10th curve its more like 80mph. There is no way to determine the velocity with the given information.

    Steven Holcomb
    2009 4-man Bobsled World Champion
    2006 Olympian 2-man & 4-man Bobsled
  7. Jun 4, 2009 #6
    It tells you v=0 at the 150m point. You should be able to use conservation of energy since friction is neglected.
  8. Jun 4, 2009 #7
    At 150m, what bobsled is moving at v=0? It has to be moving at some sort of speed in a certain direction (the bobsled track). Friction or no friction, you can't determine the velocity at the bottom of the hill.

    I say this as openly as possible. If you have an answer, please, and I mean PLEASE, contact me. I am looking for any advantage going in to the 2010 Olympic Winter Games. Seriously. We could use your help, and you will get full credit.

    Steven Holcomb
    2009 4-man Bobsled World Champion
    2006 Olympian 2-man & 4-man Bobsled
  9. Jun 4, 2009 #8


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    v=0 at a height of 150 m above the ground, not at 150 m after the start. Apparently the start is 150 m above the ground.

    If you don't know them, look in your textbook and/or class notes. You're supposed to attempt the problems on your own, and show what you attempted, before receiving help.
  10. Jun 4, 2009 #9

    This one's wrong. You've got to understand what "powerful" means - it's not about how much work it can do (this depends on how much fuel it has, substantially), it's about how fast it can do that work.

    It's a mistake. Consider the hill's inclination - it's not like the runner is going 100 meters vertically. You have to multiply for the angle sinus to see how high is he getting. Plus, the gravity is just resisting the runner, not doing actual work. So, it's a negative work. This alone does the trick.

    It's C. Of course the pen has kinetic energy when it reaches the floor, but it already had it, in the beginning, as gravitational potential energy. At least, that's what I think the question means.

    At first I thought we were missing one piece of information, i.e. how steep the hill is. Actually we don't need to know that. It's simply energy conservation - no friction, so the bob's initial potential energy equals its final kinetic energy. You don't need to know its mass, either, 'cos it figures in both sides of the equation and can be deleted.

    I hope I helped you a bit. But I quote what PhysicsJunkie said - it's useless to just do some homework and have the same problem again in a few days, you'd better try to understand everything more clearly. Ask your teacher or find somebody (a friend, a relative) who can explain it to you directly. Face-to-face learning it's always more effective :wink:.
  11. Jun 4, 2009 #10
    Sometimes people forget they're on the internet. If you type that question into the Google search box, I guarantee the answer to that question will immediately pop up on the screen.
  12. Jun 4, 2009 #11
    It has some kinetic energy, and that is also a strictly correct name for it. It started out with gravitational potential energy, but the word "potential" is missing from the question.
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