Question

In our experiment we used a block with different materials on its sides. One side was wood, the other was felt, the last was cork. We then used attached a string to the block and put that over a levey. At the end of it was a hanging mass. The mass of the block mentioned in the questions below is in reference to the mass of the block on the horizontal surface (not the hanging mass) Please answer the questions conceptually. If you feel the need to mark this question as "needs more information" then please do not attempt to answer this question. I appreciate the help.

1. How does the coefficient of kinetic friction vary with the mass of the block? Please explain the "why" behind these answers. I heard that number 1 has no change in kinetic friction, but why? I thought that as the mass of the block increased, the change in acceleration would cause a change in friction? Please explain in detail.

2. How does the coefficient of static friction compare to the coefficient of kinetic friction for the same surface material. (woode felt, and cork). For 2, why is one greater than the other? Is it according to some law or theory? Please explain why.]

Answer 1

Solution -

Friction -

Friction force depends on the interlocking between irregularities present on the two surfaces interacting.

More rough the surfaces are more the Interlocking will be and more will be the friction.

Static friction is self adjustable in nature till the applied force exceeds the maximum value of it and block start moving this varies with normal reaction between the block and surface and the nature of the surfaces in contact . In contrast the cofficient of static friction depends only on the nature of surfaces in contact only .

When the force increase the static friction force also increases till maximum values because the interlocking offers more and more resistance and once the applied force increases more than maximum value these interlocking between irregularities breaks and block starts sliding and the friction decreases and comes to a constant value known as Kinetic friction.

Ans 1 -

Cofficient of kinetic friction do not depend on the mass of the block as it's the property of the material depends on the nature of surfaces in contact .

Kinetic friction comes into play when block starts sliding and all the interlocking between irregularities breaks then the surfaces starts rubbing with each other and offers a constant force opposing motion that is known as Kinetic friction.

On increasing the mass of the block the surfaces gets pressed tightly with each other and hence the kinetic friction increases .

Kinetic friction depends on the nature of surfaces in contact and the normal reaction between the surfaces . So on increasing the mass kinetic friction increases ( as explained above) but this has to be because of increase in the normal reaction between the surfaces.

The cofficient of kinetic friction will still remains same because it depends on the nature of surfaces in contact.

F = u_{​​​​​​k}​​​N.  

Increase on mass friction increased but it's increase clearly adjusted by the increase in the normal reaction and cofficient of kinetic friction will remains the same unless the material of surfaces has been changed.

Ans 2 - As explained above the cofficient of static friction is self adjustable in nature until the applied force exceeds the maximum static friction value.

This happens between the interlocking between irregularities offers greater and greater resistance to the applied force till they gets breakdown .

So we can say the the cofficient of static friction is variable in nature and keep on increasing till it attains it's maximum and once the irregularities breaks the friction value comes down and become constant and this is known as Kinetic friction and the cofficient of kinetic friction remains constant irrespective of the surface area of the contact and the velocity of sliding.

Thanks you

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