Spring mass system and Simple harmonic motion

[physicsnewb12]

Homework Statement

A block of mass 300 gm is attached to a spring with spring constant 1000 N/m. A smaller block of mass 100 gm is pushed against the first block, compressing the spring 5 cm. The coefficient of static friction between the two blocks is u = 0.2. The blocks are then released from rest.

How far from its unstretched, equilibrium position is the second block located before it just starts to slip against the first?

Homework Equations

Ff= u*N (N= normal force; u=coefficient of friction)
Fx=N=-k*x
Since x=-0.5 Fx is in positive direction or Fx=N=k*x

The Attempt at a Solution

Ff = u*N = u*k*x
Mass 2 (m_2) will start to slip when Ff = (m_2)*g.
Ff =u*k*x = (m_2)* g
or when x= m_2*g/(u*k) = 0.004905 m or 0.49 cm

Can someone tell me if i did this right?

 

[SammyS]

Draw a free body diagram for each block.

The normal force will NOT be -kx .

 

[xiaoB]

Homework Statement

A block of mass 300 gm is attached to a spring with spring constant 1000 N/m. A smaller block of mass 100 gm is pushed against the first block, compressing the spring 5 cm. The coefficient of static friction between the two blocks is u = 0.2. The blocks are then released from rest.

How far from its unstretched, equilibrium position is the second block located before it just starts to slip against the first?

Homework Equations

Ff= u*N (N= normal force; u=coefficient of friction)
Fx=N=-k*x
Since x=-0.5 Fx is in positive direction or Fx=N=k*x

The Attempt at a Solution

Ff = u*N = u*k*x
Mass 2 (m_2) will start to slip when Ff = (m_2)*g.
Ff =u*k*x = (m_2)* g
or when x= m_2*g/(u*k) = 0.004905 m or 0.49 cm

Can someone tell me if i did this right?

For me if not going wrong:

Yup ! your concepts is right.

 

[physicsnewb12]

Draw a free body diagram for each block.

The normal force will NOT be -kx .

What will the normal force be then?

 

[SammyS]

What will the normal force be then?

-kx is the force the spring exerts on m₁. It's also the net force exerted on the combination of the two blocks if they are in contact.

What is the acceleration of the blocks when they are in contact?

Have you drawn the free body diagrams?