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Homework Help: Physics Quiz very Urgent

  1. Oct 22, 2007 #1
    Physics Quiz....very Urgent!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

    1.As a rocket rises its acceleration increases. Why does this happen? (assume that the acceleration due to gravity and air resistance are nearly constant during the ascent)

    2. If the head of the hammer is loose, and you wish tighten it by baging it against the top of a work benchm why si it bettwee to hold it with the handle down ( as indicated in the diagram to the right) than to hold with the head down?

    3. If a four-engine jet accelerate down a runway at 2.0m/s^2 and one of its engines suddenly fails, how much acceleration will the ramaining three produce? ( asuume that all four engines are identical.)

    4. If an elephant was chasing you, its enourmous mass would be most threatning, but if made a quick turn, its mass would be to your advantage. Why?

    5. Many automobile passengers waiting at traffic lights have suffered nech injuries when their cars were struck from behind. Explain why this happens, how can head rests reduce the severity of this type of injury?

    6. Some people used to think that a spaceship could not maneuver over it left the earths's atmosphere because it would have no air to push against, obiviously, this thinking must be wrong since rocket engines have been used to steer vehicles through space since the late 1950s. how does a rocket engine manage to apply a force to a spaceship in the vacuum of outer space?

    7. If a bicycle and a truck have a head-on collision, which vehicle experiences the greater force? which undergoes the greater accleration?

    8.two closed containers look the same, but one is tightly packed with lead and the other loosely packed with feathers. how could you determine which was which if you and the containers were weightless in outer space?

    9. Two teams are involved in a tug-of-war. why is it wrong to say that the team that pulls the hardest wins? what force reallly determines who wins?

    10. The photo on the website shows a physics teacher sandwiche between two bed of nails. A second physics teacher is pulverizing a cement block on top of the upper bed with a sledgehammer. If the cement block were replaced with a smaller brick, would the demonstration be more or less safe?
  2. jcsd
  3. Oct 22, 2007 #2


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    You need to show your work before we can help you. Especially since you say this is a quiz, PF rules prevent anyone from giving you the answers, or any help, until you show some effort on your part.
  4. Oct 22, 2007 #3
    Sorry my friend. I am not permitted to help you without you posting your own solutions or maybe ask probing questions.
  5. Oct 22, 2007 #4
    oh ok... there is no acual work in these problems. they are theory and law problems...i think i figured out the first one. There is thrust and weight acting on the rocket, and the rocket releases fuel as it goes up, so it loses mass, and the difference between thrust and weight increases as time passes, so the acceleration increases accordingly. is that correct?
  6. Oct 22, 2007 #5
    Hey that makes a lot of sense! Great!
    What about the second?
  7. Oct 22, 2007 #6
    i dont get the second 1...but for the 3rd..if all the engines are the same, and together they produce an acceleration of 2m/s^2, then each one would have to produce .5 m/s^2 acc. individually, so when one fails the other three would pick up the slack and produce .666m/s^2...??
  8. Oct 22, 2007 #7
    Why would it be better to bang the head of the hammer on the table by holding the handle than to hold it by the head is how I understand it.
  9. Oct 22, 2007 #8
    the diagram shows it as you are holding the hammer, handle down and hitting the handle. rather than holding it the other way and hitting the head...am i right about the 3rd 1?
  10. Oct 22, 2007 #9
    the 4th problem deals with centripical force, but i did not understand what my teacher what saying in class about it, and i am lost
  11. Oct 22, 2007 #10
    The third also sounds correct if that is how jets work. I wouldn't know. the other scenario would be that the others would be performing the same as before and after 1 engine fails therefore each being still responsible for 0.5m/s^2 acceleration.

    The second one has to do with pressure acting in the center. I forgot from highschool what this was called. The pressure is greater when the block is acting on the stick rather than the other way round.
  12. Oct 22, 2007 #11
    in the third, it doesnt say that the accleration dec. so i presume that i remains constant, but what i dont know is what law, def,. or concept i used to describe that...i just saw that logically...

    and the 2nd seems like you mean centripical force, but once again i dont understand y it does that.
  13. Oct 22, 2007 #12
    5. the acc, of the person relative to the car is zero on impact, thrusting them backwards and snaping their neck. the head rest prevents your neck from going all the way back and reducing the amount of damage caused?...i think..
  14. Oct 22, 2007 #13
    for the second question has to do with pressure points. It is easier to exert ALOT of pressure with little effort if you take the head and apply force on the stick. It has to do with the surface area of the top of the stick. the smaller the surface area the more pressure that will be exerted.

    number 5. yea when impact happens you're body is late at keeping pace with the relative velocity of the car they are in and consequently will jerk backwards. That is exactly what happens.
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