Limiting force of friction is the
Friction force acting when the body is just about to move
Understanding Friction and the Limiting Force of Friction
Friction is a force that opposes motion or the tendency of motion between two surfaces in contact. When a body is at rest relative to a surface and an external force is applied, a force of static friction acts to oppose this applied force. As the applied force increases, the static friction also increases, matching the applied force, up to a certain maximum value.
What is Static Friction?
Static friction is the friction that exists between a stationary object and the surface upon which it rests, preventing it from moving.
Defining the Limiting Force of Friction
The static friction can only increase up to a certain maximum value. Once the applied force exceeds this maximum value, the body starts to move. This maximum value of static friction, which acts when the body is just on the verge of moving, is called the limiting force of friction or simply limiting friction.
Let's analyze the given options in the context of this definition:
- Option 1: Ratio of limiting friction and normal reaction. This ratio is the coefficient of static friction ($\mu_s$), not the limiting force of friction itself.
- Option 2: Friction force acting when the body is just about to move. This perfectly matches the definition of limiting friction. It is the maximum static friction that can exist.
- Option 3: Tangent of angle between normal reaction and the resultant of normal reaction and limiting friction. This angle is the angle of friction, and its tangent is equal to the coefficient of static friction ($\tan \theta = \mu_s$). This is related to limiting friction but is not the limiting force of friction itself.
- Option 4: Friction force acting when the body is in motion. This force is called kinetic friction (or dynamic friction), which acts when there is relative motion between the surfaces. Kinetic friction is generally less than the limiting friction.
Based on the analysis, the limiting force of friction is the maximum static friction, which occurs exactly at the point where the body is just about to start moving from rest.
Conclusion on Limiting Friction
Therefore, the description that accurately defines the limiting force of friction is the friction force acting when the body is just about to move.
| Type of Friction | Description | Relation to Motion |
|---|---|---|
| Static Friction | Friction opposing applied force when body is at rest. Its value varies. | Acts when body is at rest, preventing motion. |
| Limiting Friction | Maximum value of static friction. | Acts when body is just about to move. |
| Kinetic Friction | Friction opposing motion when body is moving. Its value is relatively constant for given surfaces. | Acts when body is in motion. |
Revision Table: Key Friction Concepts
| Term | Meaning |
|---|---|
| Friction | Force opposing relative motion or tendency of motion. |
| Static Friction ($f_s$) | Friction when surfaces are at rest relative to each other; $0 \le f_s \le f_{s,max}$. |
| Limiting Friction ($f_{s,max}$) | Maximum static friction; force when motion is just about to start. $f_{s,max} = \mu_s N$. |
| Kinetic Friction ($f_k$) | Friction when surfaces are in relative motion; $f_k = \mu_k N$. |
| Normal Reaction ($N$) | Force perpendicular to the surface of contact. |
| Coefficient of Static Friction ($\mu_s$) | Ratio of limiting friction to normal reaction ($\mu_s = f_{s,max}/N$). |
| Coefficient of Kinetic Friction ($\mu_k$) | Ratio of kinetic friction to normal reaction ($\mu_k = f_k/N$). |
| Angle of Friction ($\theta$) | Angle between normal reaction and the resultant of normal reaction and limiting friction; $\tan\theta = \mu_s$. |
Additional Information on Limiting Friction
The value of limiting friction depends on two main factors:
- The nature of the two surfaces in contact (their roughness, materials, etc.). This is captured by the coefficient of static friction ($\mu_s$).
- The normal force pressing the surfaces together ($N$).
The empirical law for limiting friction states that the magnitude of the limiting friction ($f_{s,max}$) is directly proportional to the normal reaction ($N$) between the surfaces. Mathematically, this is written as:
$$f_{s,max} \propto N$$
Or, by introducing the constant of proportionality, the coefficient of static friction ($\mu_s$):
$$f_{s,max} = \mu_s N$$
This formula gives the maximum possible value of static friction before the body begins to move.
Important Questions from Friction
The maximum static frictional force that an object experiences just before it begins to slide over a surface is commonly referred to as the:
Coefficient of friction depends upon
Coulomb friction is the friction between
The minimum angle made by an inclined plane with the horizontal such that an object placed on the inclined surface just begins to slide is called-
The angle between the resultant reaction and normal to the plane on which the motion of body is impending is known as-
