# Solving 3 Block and Pulley Homework

• hjkchorong

## Homework Statement

A block of mass m1 is on top of a block of mass m2. Block 2 is connected by an ideal rope passing through a pulley to a block of unknown mass m3 as shown. The pulley is massless and frictionless. There is friction between block 1 and 2 and between the horizontal surface and block 2. Assume that the coefficient of kinetic friction between block 2 and the surface, μ, is equal to the coefficient of static friction between blocks 1 and 2.

Q1 : The mass of block 3 has been changed such that block 1 and block 2 are moving together with a given acceleration of magnitude a. What is the magnitude and the direction of the force of friction exerted by block 2 on block 1?

Q2: What is the minimum value of m3 for which block 1 will start to move relative to block 2?

## The Attempt at a Solution

For part 1, I thought that the force needed to move both block 1 and block 2 would be F= (m_1+m_2)a, but I couldn't think of how I should compute the friction force between block 1 and block 2. I know that the maximum friction force for block 1 is fs=m_1*g*mu

For part 2, I still couldn't get my head around it...

Please help! I'm generally confused with this problem.

For part 1, can you list all forces acting on block 1, and the necessary force to accelerate it?
Part 2 will follow from part 1.