Name: Work Done on a Block
Uploaded: 2019-07-19
Duration: 3 min 57 s
Description: Check out this solved illustration based on calculating work done on an object by forces acting at different angles. Just click this video to explore!

Question

State the laws of limiting friction? How would you verify them.

Accepted Answer

Laws of limiting friction

1) The force of limiting friction depends upon the nature of surfaces in contact and acts tangentially to the interface between the two surfaces.

2) The force of limiting friction between the two surfaces in contact is independent of area of contact.

3) Limiting friction is directly propotional to the magnitude of normal reaction $N$ between the two surfaces in contact, i.e,

$F \propto N \Rightarrow F = μ N$

where $μ$ is constant of proportionality known as coefficient of friction.

4) The direction of force of friction is always opposite to the direction in which the body moves or tends to move.

Verification of laws of limiting friction

Limiting friction is directly proportional to the normal reaction

Take a block A on the horizontal table. One end of the string is attached to the block and the block and the other end to the pan passing over a pulley as shown in the figure.Now the weight on the pan is kept increasing till the block A moves.If we keep block B over A, as shown in the figure, we will observe that the load on the pan must be doubled.That means if we double the weight of the block, double force is required to move it. As weight is equal to normal reaction, and applied force is equal to force of friction.

So, $F_{a} \propto R$

Similar results will be obtained if more blocks are used in the experiment.

Limiting friction is independent of area of contact.

Two blocks A and B are kept one over table as shown in figure.The lower block is connected to one end of a string blocks just start to move.Now the block A and B are kept over the table connected by a slide blocks.Through the area of contact of the blocks with the table in two cases are different, the force required to slide them are equal. Thus, limiting friction is independent of area of contact.

State the laws of limiting friction? How would you verify them.

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