# Hw5solution - P1112 Fall 08 Solutions for HW 5 Problem 1:...

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P1112 Fall 08 Solutions for HW 5 Problem 1: Gravity and Blocks Place block 1 at D 1 = -10 cm, block 2 at D 2 = 0 cm, and block 3 at D 3 = 40 cm. They all have identical mass m = 500 kg. Block 1 gravitationally pulls on block 2 in the negative- x direction with a force F 12 given by Newton’s law of gravitation: F 12 = - G (m 2 /D 1 2 ) = - (6.67x10 -11 Nm 2 / kg 2 ) ( (500kg) 2 / (-0.1m) 2 ) = - 1.7x10 -3 N Similarly, block 3 pulls on block 2 in the positive-x direction with a force F 32 : F 32 = G (m 2 /D 3 2 ) = (6.67X10 -11 Nm 2 / kg 2 ) ( (500kg) 2 /(0.4 m) 2 ) = 10 -4 N The net force F net acting on block 2 is now just the sum of these two forces: F net = F 12 + F 32 = (10 -4 – 1.7x10 -3 ) N = - 1.6x10 -3 N Problem 2: Team West vs Team East a) Forces on Team West: Forces on Team East: (gravitational due to Earth) T E on W N W w W W f (tension due to East) (contact, traction due to ground) (contact, normal due to ground) N E E f (tension due to West) (gravitational due to Earth) (contact, normal due to ground) (contact, traction due to ground) T W on E w E (b) The tension forces T E on W = T W on E is a 3rd Law interaction pair. (c)

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## Hw5solution - P1112 Fall 08 Solutions for HW 5 Problem 1:...

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