AP physics question helppp!!!!!
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AP physics question helppp!!!!!

[From: ] [author: ] [Date: 11-09-30] [Hit: ]
F = (4.0 kg + 5.0 kg) * 2.0 m/s^2 = 18.......
A 4.0 kg block is put on top of a 5.0 kg block. To cause the top block to slip on the bottom one while the bottom one is held fixed, a horizontal force of at least 8 N must be applied to the top block. The assembly of blocks is now placed on a horizontal, frictionless table.

(a) Find the magnitude of the maximum horizontal force F that can be applied to the lower block so that the blocks will move together.

(b) What is the resulting acceleration of the blocks?

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Start with the stationary condition:
Sum of the forces on the top block in horizontal direction = 0 = 8N - friction
friction = 8 N

Now, when the blocks are moving
Sum of the forces on the top block in horizontal direction = m*a = friction
a = friction / m = 8 N / 4.0 kg = 2.0 m/s <--- (b)

This is the acceleration needed to produce 8N of force to break the static friction.

Sum of the forces globally on the two blocks together = (m1+m2) * a = F
F = (4.0 kg + 5.0 kg) * 2.0 m/s^2 = 18.0 N <---- (a)
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