# When Friction the block m2 start to move?

• navneet9431
In summary, the conversation discusses a scenario where a block m2 is placed on a surface with coefficient of friction μ2 and another block m1 is placed on top with coefficient of friction μ1. A force F=k*t is acting on block m2 and the question is when will block m2 start moving. The correct result is that the block m2 will start moving just after time "t" when the external force is greater than the sum of the frictional forces acting on block m2. This is because the maximum static friction is equal to μs*Normal force, but the static friction can take any value between zero and this maximum value.
navneet9431
Gold Member

## Homework Statement

A block m2 is placed on a surface where the coefficient of friction is μ2 and another block m1 placed over the block m2 where the coefficient of frictio is μ1.
A force F=k*t(Force as a function of time),is acting on the block m2.

Now my question is that when would the block m2 start moving?

## Homework Equations

F=ma

Static Friction=Coefficient Of Friction*Normal Force

## The Attempt at a Solution

I tried solving this by drawing the FBD of the objects.

See two forces fr1 and fr2 will be acting on the block m2.
So m2 would start to move when F>fr1+fr2.
Or simply we can say k*t=fr1+ft2.
So the block m2 would start to move just after time "t".
Is my result correct?

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Delta2
Yes it is, just be careful on how you going to equate forces fr1 and fr2 with relation to the weights ##m_1g ## and ##m_2g ## and the static friction coefficients ##\mu_1## and ##\mu_2##.

navneet9431
Delta² said:
Yes it is, just be careful on how you going to equate forces fr1 and fr2 with relation to the weights ##m_1g ## and ##m_2g ## and the static friction coefficients ##\mu_1## and ##\mu_2##.
Thanks for the reply!
So my result was correct.
See the pic I have uploaded.
Have I proceeded correctly?

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Yes your result seems correct to me. Actually a minor correction, it should be ##t_0\geq…## not just >.

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navneet9431 said:
Static Friction=Coefficient Of Friction*Normal Force
There is a common blunder in that equation, and it is causing you to get the wrong answer.
For kinetic friction it works: ##F_k=\mu_kF_N##, but for static friction there is a key difference.

haruspex said:
There is a common blunder in that equation, and it is causing you to get the wrong answer.
For kinetic friction it works: ##F_k=\mu_kF_N##, but for static friction there is a key difference.
You mean to say that my result is wrong?

navneet9431 said:
You mean to say that my result is wrong?
Yes.

haruspex said:
Yes.
And,also mention the correct result!

navneet9431 said:
And,also mention the correct result!
Consider the situation when the applied force is zero. What are the frictional forces?

Zero
haruspex said:
Consider the situation when the applied force is zero. What are the frictional forces?

navneet9431 said:
Zero
Right, so what is wrong with ##F_s=\mu_sF_N##?

Ah, I think @haruspex is right, it is the maximum static friction that is equal to ##\mu_sF_N##. Static friction can take any value between zero and this maximum value.

When the box ##m_2## is about to move then the static friction between ##m_2## and the surface is indeed as you calculate ##fr_2=\mu_2(m_1+m_2)g## but box ##m_1## doesn't move with respect to box ##m_2## so the kinetic friction between m1 and m2 is just zero and also the static friction ##fr_1## is zero at the start of movement, later it gets bigger as the external force gets bigger.

Last edited:

## 1. What is friction?

Friction is a force that opposes the motion or attempted motion of an object. It is caused by the surface roughness of two objects in contact with each other.

## 2. How does friction affect the movement of the block m2?

Friction can act in two ways on the block m2: it can prevent the block from moving if it is not strong enough to overcome the friction force, or it can slow down the movement of the block if it is already in motion.

## 3. What factors affect the amount of friction on the block m2?

The amount of friction on the block m2 can be affected by the type of surface the block is resting on, the weight of the block, and the roughness of the block's surface.

## 4. How does the coefficient of friction play a role in the movement of the block m2?

The coefficient of friction is a measure of the amount of friction between two surfaces. It determines how much force is needed to overcome the friction force and cause the block m2 to move.

## 5. Can friction be reduced or eliminated to make the block m2 move more easily?

Friction can be reduced by using lubricants or smoother surfaces, but it cannot be completely eliminated. In order to make the block m2 move more easily, a greater force must be applied to overcome the remaining friction force.

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