Speed, force, velocity, (falling object)

  1. 1. The problem statement, all variables and given/known data

    (A) An egg (57g) is dropped from a balcony 4m high,calculate the speed with which the egg, when dropped from the first floor balcony will hit the ground. ( use newtons laws). ( note the time taken for the egg to hit the ground is 0.1s)

    (B) What force from the ground will be applied to the egg causing it to crack. The speed before the egg hits the ground will become the initial velocity before the egg stops.

    2. Relevant equations

    i know that i am definantly wrong but the only formula i think that i know and can truly use is V=S/T and maybe F=MA.(try make the answer simpler for me)

    3. The attempt at a solution

    (A)
    V=?
    S=4m
    T=0.1s

    V=S/t
    V=4/0.1
    V=40m/s?

    ( i know i didnt use newtons laws but this is the only thing i know)

    (B)
    F=?
    M=57grams
    A=40m/s/s???


    F=MA
    F=40X57
    F=2280N?????
    (im not sure if its right)
     
    Last edited: Nov 10, 2011
  2. jcsd
  3. The answer to question A is correct. The average speed the egg took to hit the ground is 40 m/s. The average speed is the change in distance travelled (4m) over the time taken (0.1s).

    The answer to question B is also correct depending on what your teacher is expecting. The egg has a gravitational vector pulling the egg down towards the earth. As soon as the egg hits the earth, a normal force is applied to the egg to prevent the egg from going through the ground. I believe you need to know momentum to accurately solve the problem, but if you assume the normal force and the gravitational force are equal, then yes, your answer is correct.

    I would wait for a mentor if I were you.

    Actually, I have a question for the mentor that comes to solve your problem, if you assume the gravity vector and normal force vector are equal, then doesn't that only prevent further acceleration? How does that cause the object to stop?
     
  4. Thank you but I believe I needed to use newtons laws for the first question and i'm not sure that I did.
     
  5. I don't seem to see how you could use Newton's laws to solve the question. Maybe I misunderstood the question? Is it asking for the velocity of the egg when it hit the ground? If so, you can use the kinematics equation:

    v2 = v1 +a*t
    = 0 + 9.81*0.1
    = 0.981 m/s
     
  6. PeterO

    PeterO 2,319
    Homework Helper

    When an object is dropped, it usually means it starts with zero velocity. If the balcony was 4m high and the egg was dropped, it will take more than 0.1 seconds to reach the ground

    I think the 0.1 seconds refers to the time taken for the egg to STOP having hit the ground.

    Use the usual formulas to calculate the speed just before hitting the ground.

    If you have done enough vertical motion / falling object questions you would know that if an object falls 4.9 m , it takes 1 second, and reaches 9.8 m/s.
    Since this egg falls 4 m it will take a little less than a second, and reach a slightly slower speed.
     
  7. thanks for the tips but i haven't learnt anything about that so far on kinematics equations ect.
     
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    Last edited: Nov 10, 2011
  9. PeterO

    PeterO 2,319
    Homework Helper

     
  10. Everyone knows that an egg would not really take 0.1 seconds to fall four meters. This question has been just simply used to test my knowledge about newtons laws, force, velocity.(which isn't very much).
     
  11. PeterO

    PeterO 2,319
    Homework Helper

    If you will finally get your head around the idea that the egg fell from 4m and then stopped in 0.1 seconds when it struck the ground, you may be able to increase your knowledge.
     
  12. Here i will state the question directly:
    Assume the egg is not encased in an egg holder:(Note the time taken by the egg to hit the ground is 0.1s and the hight of the balcony is 4m.)
    a). calculate the speed with which the egg, when dropped from the first floor balcony will hit the ground.(Use newtons laws.)

    This is exactly what the question asks and i do agree with you when you say it would take around 1.3 seconds or so to reach ground and would take 0.1 seconds for the egg to stop moving. so yes you are right even though we were discussing about kind of different things
     
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