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Homework Help: Motion questions and a mass one.

  1. Mar 8, 2012 #1
    1. The problem statement, all variables and given/known data

    A hot-air balloon is ascending at the rate of 10 m/s and is 95 m above the ground when a package is dropped over the side. (a) How long does the package take to reach the ground? (b) With what speed does it hit the ground?

    2. Relevant equations


    3. The attempt at a solution

    using 0 for vi^2, -9.8 for a, and 95 for d. I got -43.25 m/s. I put this into Wiley Plus and it says it's wrong. I only have 1 more attempt. So I'm thinking, maybe the initial velocity of the object 10 m/s, rather than 0. This has me a little confused.

    And for the time taken , i got 3.259s using (v-u)/d


    On a dry road, a car with good tires may be able to brake with a constant deceleration of 3.03 m/s2. (a) How long does such a car, initially traveling at 25.0 m/s, take to stop? (b) How far does it travel in this time?


    t= (x-x0)/(v0+1/2a)


    I used 103.125 as x, 0 for x0, 25 for v0, and -3.03 as a. But it says my answer is wrong. I got 4.391secs.

    I got part b right.


    Iron has a mass of 7.87 g per cubic centimeter of volume, and the mass of an iron atom is 9.27 × 10-26 kg. If you simplify and treat each atom as a cube, (a) what is the average volume (in cubic meters) required for each iron atom and (b) what is the distance (in meters) between the centers of adjacent atoms?


    For part a, I divided 7.87*10 to the 3 by 9.27*10 to the negative 26. I got the answer right. I don't know where to start with b.

    Thanks very much. :)

    PS: Also, it's due in around 6 hours. I'm an idiot for leaving it too late.
    Last edited: Mar 8, 2012
  2. jcsd
  3. Mar 9, 2012 #2

    for the first problem, you are correct in guessing that initial velocity should be 10 m/s. You have to use the convention that upward distance is positive and the downward is negative. So
    in your equation, d should be taken negative. also acceleration , a , being downward, should be taken negative. that will give you answer to part b. for part a, you have used the wrong formula. here you have acceleration going, so you can't assume that the velocity is constant. you need to use the formula which connects initial and final velocities and the time and the acceleration. again follow the sign conventions..

    for the second problem, i don't understand what you are doing. try to understand the underlying physics of the formulas. this plug and chug approach will not help you in physics.
    you are given initial velocity. since the car comes to a stop, the final velocity is known. and acceleration is known. and t is asked. so which formulas should work here ?

    for the last question, know that the volume of a cube is [itex]b^3[/itex] if the side of the
    cube is [itex]b[/itex]. You found the volume, that means you can find [itex]b[/itex]. Now since atom is approximated as a cube, the distance between the centers of two atoms is
    distance between the centers of two cube, which is [itex]b[/itex]
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