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Old Oct 29th 2015, 08:21 PM   #1
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determine the force involving static coefficient

A force P acts as shown on a 200kg block placed on an inclined plane. The static coefficient of friction between the block and plane are Us= 0.5 . Determine the force of P to ensure he block is in the verge of moving down. The ans given is P = 560.4 N
i have done in 2 ways , but my ans is incorrect .... why am i wrong ?
1. ) P cos 20 = Fs

Pcos20 =0.5x9.81x200xcos10

P= 1028N

2. ) Wsin10 + P cos20 = 0.5x9.81x200xcos10

Pcos20 = 0.5x9.81x200xcos10 - Wsin10

Pcos20 = 0.5x9.81x200xcos10 200x9.81 sin10

P= 665.5N
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Old Oct 30th 2015, 02:01 AM   #2
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Originally Posted by ling233 View Post
A force P acts as shown on a 200kg block placed on an inclined plane. The static coefficient of friction between the block and plane are Us= 0.5 . Determine the force of P to ensure he block is in the verge of moving down. The ans given is P = 560.4 N
i have done in 2 ways , but my ans is incorrect .... why am i wrong ?
1. ) P cos 20 = Fs

Pcos20 =0.5x9.81x200xcos10

P= 1028N

2. ) Wsin10 + P cos20 = 0.5x9.81x200xcos10

Pcos20 = 0.5x9.81x200xcos10 - Wsin10

Pcos20 = 0.5x9.81x200xcos10 200x9.81 sin10

P= 665.5N
i didnt mean to spam , anybody can help ?
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Old Oct 30th 2015, 08:11 AM   #3
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Your attempt #1 ignores the pull of gravity trying to pull the block down the incline, so is clearly incomplete.

Your attempt #2 is correct, given the data provided. Are you sure you have correct value for the coefficient of static friction?

Last edited by ChipB; Oct 30th 2015 at 08:16 AM.
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Old Oct 30th 2015, 09:41 AM   #4
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Originally Posted by ChipB View Post
Your attempt #1 ignores the pull of gravity trying to pull the block down the incline, so is clearly incomplete.

Your attempt #2 is correct, given the data provided. Are you sure you have correct value for the coefficient of static friction?
Yes, anything else wrong with my working?
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