Types of Friction>Static Friction

Static friction is the force that prevents an object from moving when a force is applied to it. It acts between two surfaces in contact and adjusts itself to balance the applied force up to a certain limit. Static friction is essential in everyday life, allowing us to walk, drive vehicles, and keep objects stationary on surfaces.

The frictional force that balances the applied force when the body is static (or at rest). It prevents sliding motion between two surfaces in contact.

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The friction that acts on an object when it is not in motion relative to the surface and acts in the direction to cancel out the applied force F, whose magnitude varies with the applied force up to a maximum limit f_s,max = μ_s × N, is called Static Friction.

FL = μs N

Where:

• FL = Limiting force of friction (maximum static friction)
• μs = Coefficient of static friction (dimensionless constant)
• N = Normal reaction (normal force between surfaces)

μs = FL / N

Where:

• μs = Coefficient of static friction
• FL = Limiting force of friction
• N = Normal reaction

• Static friction is a self-adjusting force – it automatically adjusts its magnitude to balance the applied force
• It acts in a direction opposite to the applied force
• It can also adjust its direction when the direction of the applied force is reversed
• Static friction remains constant (equal to the applied force) as long as the object does not move
• It reaches a maximum value (limiting force) just before the object starts to slide
• Once the applied force exceeds the limiting force, the object transitions from static to kinetic friction

When a horizontal force F is applied to a wooden block on a horizontal surface:

• If F is small, static friction (fs) = F, and the block remains at rest
• As F increases, static friction also increases to balance it
• The friction continues to adjust until it reaches its maximum value (FL)
• Once F > FL, the block begins to move, and kinetic friction takes over

Law 1: The limiting force of static friction is directly proportional to the normal reaction between the two surfaces.

• FL ∝ N
• FL = μs N

Law 2: The limiting force of static friction is independent of the apparent area of contact between surfaces (as long as normal reaction remains constant).

Law 3: The limiting force of friction depends on:

• The materials in contact
• The nature and roughness of the surfaces

Conditions for Static Friction

• Before sliding (F ≤ FL): fs = F (friction balances the applied force)
• At limiting condition (F = FL): fs reaches maximum value = μs N
• After sliding starts (F > FL): Kinetic friction takes over, and the object moves

• Allows us to walk and run without slipping on the ground
• Enables vehicles to accelerate and brake safely on roads
• Keeps objects stationary on inclined surfaces
• Essential for gripping objects with our hands
• Prevents sliding in sports activities like football and basketball
• Important in mechanical systems to prevent unintended motion

The coefficient of static friction between a block of mass 0.25 kg and a horizontal surface is 0.4. Find the horizontal force required to move the block.

Given:

• Coefficient of static friction, μs = 0.4
• Mass of block, m = 0.25 kg
• Acceleration due to gravity, g = 9.8 m/s²

To Find: Maximum horizontal force (FL)

Solution:

The normal reaction, N = mg

• N = 0.25 × 9.8 = 2.45 N

Using the formula: FL = μs × N

• FL = 0.4 × 2.45
• FL = 0.98 N

Result: The horizontal force required to move the block is 0.98 N (or approximately 1.0 N).

This means any force less than 0.98 N will not move the block; the static friction will balance it. A force of 0.98 N or more is needed to overcome static friction and make the block move.

Note: Materials with higher coefficients of friction (like rubber on concrete) provide better grip, while materials with lower coefficients (like Teflon) are very slippery.

• Walking on the ground: Static friction between your shoes and the ground prevents slipping and allows you to push forward.
• Car on a road: Static friction between tires and the road surface allows the car to accelerate, decelerate, and turn safely.
• Books on a tilted table: Static friction keeps books from sliding down if the tilt is not too steep.
• Ladder against a wall: Static friction between the ladder and the ground prevents it from slipping outward.
• Holding a pen: Static friction between your fingers and the pen prevents it from slipping out of your hand.
• Braking a bicycle: Static friction between the brake pads and the wheel rim stops the bicycle.