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Need help with some HW Problems!

  1. Oct 15, 2003 #1

    Just wondered if any of you guys could help me out with these problems. Anything is appreaciated.

    1)Can a rapidly moving object have the same acceleration as a slowly moving one? Why?

    2) When you throw a ball straight up in the air, ignroing air resistance, what acceleration is it undergoing at the beginning? At the top of its motion? ( I think It is undergoing 9.8m/s2 in the begining and at the end its not undergoing anything?)

    3) Three Forces, each 10N, act on the same object. What is the maximum total force they exert on te object? The minimum total force?
    (I think 30N total, and 10N minimum)

    4) If you drop your wallet from the top of a building, how long would it take to reach the ground.? Ignoring air resistance. The height of the building is 450m. ( I was thinking of finding the average velocity and then just 450m divided by it, the meters cancel out and u get the seconds?)

    5) A bullet traveling at 500 m/s hits a tree and penetrates 0.170m before coming to a rest. Assuming constant decceleration, what is its average acceleration? How long does it take to stop? (NO IDEA!!)

    Please list info on how to solve with solution. I am trying to understand this stuff. Exam is Friday!! Need halp badly. Thanks
  2. jcsd
  3. Oct 16, 2003 #2


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    Gold Member

    Hi physicszman!

    You've got some conceptual problems here, so I apologize for answering your questions with questions. If I simply give you the answers, you're not going to understand it any better. It's also forum rules that you post your attempt before we help you out, for the same reason.

    You should really look in your book and read up on the definition of an acceleration, and study it... you're going to be using it for at least the rest of the semester.

    Acceleration is a change in velocity over time. If you've got an object which starts at rest, and at the end of 1 second is going 2 m/s, then the acceleration was 2 m/s^2. If that object continues to accelerate, at the end of the 2nd second, it will be going at 4 m/s. An object at rest can have the same acceleration as an object going 200 m/s, it just won't stay at rest for long.

    Considering what I just told you, what do you think the answers are for the first two problems?

    You're half right. The maximum is 30N. Grab a cup. Can you pick the cup up with three fingers, each pushing straight into the cup? Where are you placing those fingers? Is the cup accelerating? What does that tell you about the force?

    An object in free-fall is undergoing an acceleration due to gravity. That means it's speeding up (er... down). Take a look in your notes for the formulas you use for constant acceleration problems. This is a plug-and-chug problem.

    This one is another twist on the same formulas you'll need for problem 4. Take a look in your book, find and post the relevant formulas, and I'll give you some more help if you still need it.

    PS. Always, always, always draw a picture.
    Last edited: Oct 16, 2003
  4. Oct 16, 2003 #3
    1> What do you think about this one?

    2> Please rephrase your answer. What do you mean by "at the end"?
    Answer these: What is the RATE of acceleration
    (a) the instant after it is thrown
    (b) halfway up
    (c) at its highest point
    (d) halfway down
    (e) 1/100 of a second before it lands

    (this is a trick question)

    3> Think about this: What if the 3 forces are in three directions,
    120o apart (picture a wheel with three evenly-spaced
    spokes, one force along each spoke)?

    4 & 5> Have you looked at your book? Seen any equations that
    relate distance, velocity, acceleration and time?
  5. Oct 16, 2003 #4
    Hi, Im going to start off by answering enigmas questions first.

    1) Yes, a rapidly moveing object can have the same acceleration as a slowly moveing one because acceleration is just the rate by how much the velocity increases as time goes by.

    2) When the object is thrown up it will deccelerate at -g. Once it hits the top of its motion it will start accelerating at g towards the earth.

    3) The maximum force is 30N. The minimum would be 0N I am guessing because they cancell each other out if you position the fingers correctly ona cup.

    4) I get this problem. There is a equation in the book I found which solves for the final velocity. It asks for inital velocity, acceleration, distance, and doesn't require time. So I can mix it up to solve for the Initial velocity since we know that the final will be 0 once it hits the floor. Solving it i get Initial Velocity is equal to 94m/s. Then do 450m divided by 94m/s which gives me 4.7 seconds. Does that sound about right?

    5) I'm still stuck as to where to start with this problem. I am thinking that the time needed to stop will be 0.170m divided by 500m/s?



    2) I think it will be -9.8m/s squared for all except when it starts moving down, then the acceleration will be positive.

    3) The forces will cancell each other out equaling 0N?

    4+5) I listed them in the first portion.

    THanks alot!! Please correct or advise any mistakes.
    Last edited: Oct 16, 2003
  6. Oct 16, 2003 #5
    1> acceleration is just the rate by how much the velocity increases or decreases

    2> the minus sign indicates the direction of the acceleration. It is ALWAYS -9.8 m/s^2 except when it's in contact with your hand (or the ground). Meaning, the acceleration is always 9.8 m/s^ downward even when the ball is moving up; even when it momentarily stops at the top of its flight.

    3> correct

    4> If you drop your wallet (DROP, not throw) the initial velocity is 0.
    x = vi*t + (1/2)a*t2
    I get 9.58 sec.

    5> There's another handy equation that relates velocity to position and acceleration:
    vf2 = vi2 + 2*a*(xf-xi)
    Can you figure out how to use that?
  7. Oct 16, 2003 #6
    heya im really stuck on some radioactivity questions
    can anybody help me?

    a geiger-muller tube has a window area of 1.8cm2. when placed 4cm from a beta-emitter, 6200 counts are recorded in one minute (after correction for background count rate). Estimate the activity of the source in counts min-1. Explain why the true activity of the source will be higher than your estimate.

    thanks in advance for any help

  8. Oct 16, 2003 #7

    quick question about number 4. Do I need to use the quadtratic formula to solve it?

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