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| Static Friction |
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| Static friction is the friction experienced when we try to move a stationary body on a surface, without actually causing any relative motion between the body and the surface which it is on. |
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| It can be defined as the force of friction which exactly balances the applied force during the stationary state of the body. |
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| Static frictional force is a self-adjusting force. i.e., the static friction will always be equal and opposite to the force applied. |
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| Consider the figure, R is the reaction force due to the weight W. fr is the friction and F the external force. F = -fr when no motion occurs. |
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 The maximum force of static friction is independent of the area of contact. |
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 The maximum force of static friction is proportional to the normal force i.e., if the normal force increases, the maximum external force that the body can withstand without moving, also increases. |
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| It is the maximum value of static friction which comes into play when a body is just about to slide over the surface of another body. For an applied external force greater than the limiting friction, the body begins to move. Once motion has begun, static friction cannot be considered. A new type of friction called kinetic friction comes into play. |
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 The direction of limiting frictional force is always opposite the direction of motion. |
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 Limiting friction acts tangential to the two surfaces in contact. |
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 The magnitude of limiting friction is directly proportional to the normal reaction between the two surfaces. |
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 The limiting friction depends upon the material, the nature of the surfaces in contact and their smoothness. |
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 For any two given surfaces, the magnitude of limiting friction is independent of the shape or the area of the surfaces in contact so long as the normal reaction remains the same. |
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| Take a block of wood of specific mass, a thread, pulley, a pan and a few weights and arrange them as shown in the figure. |
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| Now add a few weights in the empty pan. The block does not move. This shows that even though the string pulls the block to the right, the frictional force pulls it to the left. Hence, 1st law is verified. |
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| Since the frictional force f acts horizontal to the surface, it is tangential to the surface of contact. Hence, 2nd law is verified. |
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| Keep adding weights in the pan and on the block so that the block just begins to move. Now add some additional weight on the block and adjust the weight on the pan so that the block just begins to move again. Note the weight of the block + the weight on it and the weight on the pan. You will notice that they increase or decrease proportionally. Hence, law three is verified. |
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| Replace the wooden block with a glass block, stone block and note the weight of the pan. This observation will verify the law of static friction i.e. the fourth law. Now consider any one of the blocks. Change the face of the surface of contact and position of the block. You will notice that the weight in the pan will be the same for all cases (when the block just begins to move). This verifies the law of limiting friction. This verifies the fifth law. |
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