Turn on thread page Beta
    • Thread Starter
    Offline

    8
    ReputationRep:
    Hey Guys, Kinda need help on this question,

    Al and Betty are wearing identical mass-less parachute. Al has 3 times the mass of betty. They descend to the ground with constant velocity.

    How does the force of air resistance on Al and his parachute compare with Betty and her parachute.

    A. Stay the same
    B. Three times greater
    C. 1/3 as Great

    I thought it was B, since at terminal veloctiy, mg=Air Resistance. Since Al has more mass surely he would have more air resistance. However, the answer is A, could someone explain this?

    Thanks
    • Community Assistant
    Offline

    12
    ReputationRep:
    Community Assistant
    (Original post by ChemBoy1)
    Hey Guys, Kinda need help on this question,

    Al and Betty are wearing identical mass-less parachute. Al has 3 times the mass of betty. They descend to the ground with constant velocity.

    How does the force of air resistance on Al and his parachute compare with Betty and her parachute.

    A. Stay the same
    B. Three times greater
    C. 1/3 as Great

    I thought it was B, since at terminal velocity, mg=Air Resistance. Since Al has more mass surely he would have more air resistance. However, the answer is A, could someone explain this?

    Thanks

    I would suggest you re-checked with the answer again. I believe B is the answer.
    • Community Assistant
    Offline

    12
    ReputationRep:
    Community Assistant
    (Original post by BDunlop)
    Draw out a diagram that includes all the forces acting. You'll have mg down and Resistive forces upwards. When terminal velocity you're right mg=air resistance. Now put it into F=ma and you'll see the acceleration is the same regardless of the mass because masses cancel. Remember on the moon a feather will drop at the same rate as a 1kg mass. Air resistance is not affected any mass

    I am not sure what you are trying to explain. You seem to self-contradict yourself by stating that

    (Original post by BDunlop)
    ….When terminal velocity you're right mg=air resistance. …
    first and then conclude that

    (Original post by BDunlop)
    …. Air resistance is not affected any mass
    In between the conclusion, you are using an irrelevant example to explain why mass is cancelled.
    Offline

    16
    ReputationRep:
    (Original post by Eimmanuel)
    I am not sure what you are trying to explain. You seem to self-contradict yourself by stating that



    first and then conclude that



    In between the conclusion, you are using an irrelevant example to explain why mass is cancelled.
    Agreed, i meant to say acceleration not air resistance. This seems like quite a hostile comment from you, was it needed? :angelblush::angelblush:
    Offline

    21
    ReputationRep:
    (Original post by BDunlop)
    Draw out a diagram that includes all the forces acting. You'll have mg down and Resistive forces upwards. When at terminal velocity you're right mg=air resistance. So if you increase the mass, the resistive forces must increase when at terminal velocity.
    Yeahh but they are not on the moon!
    Offline

    16
    ReputationRep:
    (Original post by Rohit_Rocks10)
    Yeahh but they are not on the moon!
    Mass definitely doesn't affect air resistance, i'm not sure why OP said it does.

    acceleration will be g regardless of the mass.

    The more air molecules which the parachute collides with, the greater the air resistance force. Subsequently, the amount of air resistance is dependent upon the speed of the falling object and the surface area of the falling object. Based on surface area alone, it is safe to assume that for the same speed the two masses would encounter the same air resistance.
    • Community Assistant
    Offline

    12
    ReputationRep:
    Community Assistant
    (Original post by BDunlop)
    Mass definitely doesn't affect air resistance, i'm not sure why OP said it does.

    acceleration will be g regardless of the mass.

    The more air molecules which the parachute collides with, the greater the air resistance force. Subsequently, the amount of air resistance is dependent upon the speed of the falling object and the surface area of the falling object. Based on surface area alone, it is safe to assume that for the same speed the two masses would encounter the same air resistance.
    It should be easy to see why air resistance can be dependent on mass in this case. When the object is falling at terminal velocity, based Newton's 2nd law,
    Net force = ma
    mg - air resistance = 0
    mg = air resistance

    So air resistance should depend on mass in this scenario.
    • Thread Starter
    Offline

    8
    ReputationRep:
    This was my thinking, and the answer is said to be A not B I checked.
    (Original post by Eimmanuel)
    It should be easy to see why air resistance can be dependent on mass in this case. When the object is falling at terminal velocity, based Newton's 2nd law,
    Net force = ma
    mg - air resistance = 0
    mg = air resistance

    So air resistance should depend on mass in this scenario.
    • Community Assistant
    Offline

    12
    ReputationRep:
    Community Assistant
    (Original post by ChemBoy1)
    This was my thinking, and the answer is said to be A not B I checked.
    This is actually a SAT question. I just check with the source and the answer say it is B.
 
 
 
Reply
Submit reply
Turn on thread page Beta
Updated: November 1, 2017

University open days

  1. University of Bradford
    University-wide Postgraduate
    Wed, 25 Jul '18
  2. University of Buckingham
    Psychology Taster Tutorial Undergraduate
    Wed, 25 Jul '18
  3. Bournemouth University
    Clearing Campus Visit Undergraduate
    Wed, 1 Aug '18
Poll
How are you feeling in the run-up to Results Day 2018?

The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE

Write a reply...
Reply
Hide
Reputation gems: You get these gems as you gain rep from other members for making good contributions and giving helpful advice.