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Unformatted text preview: homework 09 FIERRO, JEFFREY Due: Feb 18 2008, 11:00 pm 1 Question 1, chap 5, sect 6. part 1 of 1 10 points The horizontal surface on which the objects slide is frictionless. The acceleration of gravity is 9 . 8 m / s 2 . 3 kg 9 kg 6 kg F F r = 0 If F = 26 N and F r = 5 N, what is the magnitude of the force exerted on the block with mass 9 kg by the block with mass 6 kg? Correct answer: 12 N (tolerance 1 %). Explanation: m 1 m 2 m 3 F F r = 0 Given : vector F = +26 N , vector F r = 5 N , = 0 , m 1 = 3 kg , m 2 = 9 kg , m 3 = 6 kg , and g = 9 . 8 m / s 2 . Note: F is acting on the combined mass of the three blocks, resulting in a common acceleration after accounting for friction. m 1 F 21 F m 2 F 32 F 12 m 3 F r F 23 Let F , F r , F 32 represent the force exerted on the system from the right, from the left, and the force exerted on m 2 by m 3 , respec tively. Note: vector F 21 = vector F 12 and vector F 32 = vector F 23 , and bardbl vector F 21 bardbl = bardbl vector F 12 bardbl and bardbl vector F 32 bardbl = bardbl vector F 23 bardbl , where bardbl vector F bardbl F is the magnitude of vector F . The sum of the forces acting on each block separately are m 1 a = + F F 21 = + F F 12 (1) m 2 a = + F 12 F 32 = + F 12 F 23 (2) m 3 a = + F 23 F r (3) To find the acceleration we can treat the three masses as a single object or add the forces acting on each component of the sys tem, Eqs. 1, 2, and 3. F F r = ( m 1 + m 2 + m 3 ) a Solving for a, we have a = F F r m 1 + m 2 + m 3 (4) = 26 N 5 N 3 kg + 9 kg + 6 kg = 1 . 16667 m / s 2 . We can solve for F 23 using Eq. 3 and sub stituting a from Eq. 4. The result is F 23 = m 3 a + F r (3) = m 3 ( F F r ) m 1 + m 2 + m 3 + F r ( m 1 + m 2 + m 3 ) m 1 + m 2 + m 3 = m 3 F ( m 1 + m 2 ) F r m 1 + m 2 + m 3 (5) = (6 kg) (26 N) 3 kg + 9 kg + 6 kg + (3 kg + 9 kg) (5 N) 3 kg + 9 kg + 6 kg = 12 N . Alternative Solution: Using Eq. 3 and the numerical result of Eq. 4, we have F 23 = m 3 a + F r (3) = (6 kg) (1 . 16667 m / s 2 ) + 5 N = 12 N . Question 2, chap 5, sect 6. part 1 of 1 10 points homework 09 FIERRO, JEFFREY Due: Feb 18 2008, 11:00 pm 2 The surfaces between a 8 kg block and the 15 kg wedge and between the 15 kg wedge and the horizontal plane is smooth (without friction). The acceleration of gravity is 9 . 8 m / s 2 . A block is released on the inclined plane (top side of the wedge). 8 k g = 25 15 kg = 0 F 9 . 8m / s 2 What is the force F which must be exerted on the 15 kg block in order that the 8 kg block does not move up or down the plane? Correct answer: 105 . 106 N (tolerance 1 %)....
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 Spring '08
 Turner
 Acceleration, Friction, Gravity, Work

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