Hey there! Sign in to join this conversationNew here? Join for free
x Turn on thread page Beta

Mechanics help: watch

Announcements
    • Thread Starter
    • 1 follower
    Offline

    ReputationRep:
    • Thread Starter
    Offline

    0
    ReputationRep:
    A diver of mass 60kg dives from a diving board at a height of 4m. she hits the water travelling at 8ms^-1 and descends to a depth of 2m in the diving pool. Model the diver as a particle.

    a) find the work done against air resistance before the diver hits the water and the average magnitude of the air resistance force.

    b) find the average magnitude of the force exerted by the water on the diver as she is brought to rest in the diving pool.

    for a) i did G.P.E = 60x9.8x4 = 2352j
    K.E - when hittin water = 1/2 x 60 x 64 = 1920
    so 2352-1920 = 432j
    fxd = 432
    f=432/4 = 108

    can't do b) ?? any help? thanks..
    Offline

    0
    ReputationRep:
    (Original post by ShOcKzZ)
    A diver of mass 60kg dives from a diving board at a height of 4m. she hits the water travelling at 8ms^-1 and descends to a depth of 2m in the diving pool. Model the diver as a particle.

    a) find the work done against air resistance before the diver hits the water and the average magnitude of the air resistance force.

    b) find the average magnitude of the force exerted by the water on the diver as she is brought to rest in the diving pool.

    for a) i did G.P.E = 60x9.8x4 = 2352j
    K.E - when hittin water = 1/2 x 60 x 64 = 1920
    so 2352-1920 = 432j
    fxd = 432
    f=432/4 = 108

    can't do b) ?? any help? thanks..
    F=ma it?
    Offline

    0
    ReputationRep:
    (Original post by ShOcKzZ)
    A diver of mass 60kg dives from a diving board at a height of 4m. she hits the water travelling at 8ms^-1 and descends to a depth of 2m in the diving pool. Model the diver as a particle.

    a) find the work done against air resistance before the diver hits the water and the average magnitude of the air resistance force.

    b) find the average magnitude of the force exerted by the water on the diver as she is brought to rest in the diving pool.

    for a) i did G.P.E = 60x9.8x4 = 2352j
    K.E - when hittin water = 1/2 x 60 x 64 = 1920
    so 2352-1920 = 432j
    fxd = 432
    f=432/4 = 108

    can't do b) ?? any help? thanks..
    b) v^2 = u^2 + 2as

    a = -16m/s

    magnitude of f=ma = 16 x60 = 960N
    • Thread Starter
    Offline

    0
    ReputationRep:
    (Original post by hihihihi)
    F=ma it?
    tried it ..probably did it wrong though any suggestions?
    • Thread Starter
    Offline

    0
    ReputationRep:
    (Original post by rts)
    b) v^2 = u^2 + 2as

    a = -16m/s

    magnitude of f=ma = 16 x60 = 960N
    exactly what i did, but the answer is 1548..
    Offline

    0
    ReputationRep:
    (Original post by ShOcKzZ)
    exactly what i did, but the answer is 1548..
    ah u must 60x9.8+960=1548N
    • Thread Starter
    Offline

    0
    ReputationRep:
    (Original post by hihihihi)
    ah u must 60x9.8+960=1548N
    ahh right..thanks.
    Offline

    0
    ReputationRep:
    I get 1548 too, but you can do it by energy consideration.

    When she hits the water she has 1/2 * 60 * 8^2 = 1920 J of kinetic energy.
    In decelerating to rest, she therefore loses 1920 J KE which goes into work done against the water.

    Also, you've got a PE drop as she descends 2m. mgh = 60*9.8*2 = 1176 J. This work is also done against the water.

    Total work done = 1920 + 1176 = 3096 J
    W= Fd ==> F = W/d so F = 3096/2 = 1548 N.

    Tim
    Offline

    14
    ReputationRep:
    Remember, if you use g=9.8, never write answers to more than 2s.f. :cool:
 
 
 
Poll
Do you like carrot cake?

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.