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    I think the book is wrong (yet again) but can't be sure so thought i'd better check...

    A basic air riflr fires slugs os mass 0.5 g at a velocity 160 m/s. The air rifle of mass 0.8kg is free to move when a slug is fired.
    Using conservation of momentum the air rifle recoils at 0.1 m/s.

    It then asks to calculate the mean force the person experiences to prevent the rifle recoilling more than 0.5 mm.

    The momentum of the air rifle is 0.1 x 0.8 = 0.08 kg m/s

    As the velocity of the air rifle is 0.1 m/s and it cant move more than 0.5 mm this is a time of (0.5 x 10^-3) / (0.1) = 5 x 10^-3 s

    so the force needed is (0.08) / (5 x 10^-3) = 16 N


    However the answer in the book is only 8 N so someone must be wrong...
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    (Original post by Chris2505)
    I think the book is wrong (yet again) but can't be sure so thought i'd better check...

    A basic air riflr fires slugs os mass 0.5 g at a velocity 160 m/s. The air rifle of mass 0.8kg is free to move when a slug is fired.

    Using conservation of momentum the air rifle recoils at 0.1 m/s.

    It then asks to calculate the mean force the person experiences to prevent the rifle recoilling more than 0.5 mm.

    The momentum of the air rifle is 0.1 x 0.8 = 0.08 kg m/s

    As the velocity of the air rifle is 0.1 m/s and it cant move more than 0.5 mm this is a time of (0.5 x 10^-3) / (0.1) = 5 x 10^-3 s

    so the force needed is (0.08) / (5 x 10^-3) = 16 N


    However the answer in the book is only 8 N so someone must be wrong...
    I get 8N as well. :yep:

    You can't use speed = distance x time for a non-constant velocity situation. Use v^2 = u^2 + 2as. This should give you the answer.

    (Often, this is the case when you get double the answer given in the solutions - it's a hint that you've omitted the possibility of non-constant velocity in your calculations. :yes:)
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    (Original post by james.h)
    I get 8N as well. :yep:

    You can't use speed = distance x time for a non-constant velocity situation. Use v^2 = u^2 + 2as. This should give you the answer.

    (Often, this is the case when you get double the answer given in the solutions - it's a hint that you've omitted the possibility of non-constant velocity in your calculations. :yes:)
    ahhh of course!! though s = [(u + v) / 2] x t is probably better to use to calculate time.
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    Use force times distance is work done.
    Work done is the loss in the KE of the rifle. Use 1/2 m v^2 for it.
    This equals average force times distance (0.5mm)
    This gives the answer 8N
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    (Original post by Chris2505)
    ahhh of course!! though s = [(u + v) / 2] x t is probably better to use to calculate time.
    Yes, sorry, I was thinking F = ma (constant mass), but finding the time also works. :yes:

    Stonebridge's suggestion along the lines of energy considerations appears a simpler route, though.
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    (Original post by james.h)
    Yes, sorry, I was thinking F = ma (constant mass), but finding the time also works. :yes:

    Stonebridge's suggestion along the lines of energy considerations appears a simpler route, though.
    Yes quite a bt Thanks
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    very good question!!!!!
 
 
 
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