Static Friction | Definition and Daily Life Examples

Static friction is the force that opposes the initiation of motion between two surfaces. It comes into play when an object is at rest and an external force is applied to move it.
The mathematical expression for static friction is given by the formula:

F_s≤μ_sN

Where:

• F_s is the force of static friction,
• μ_s is the coefficient of static friction (a dimensionless constant),
• N is the normal force between the two surfaces in contact.

Here are some daily life examples of static friction:

1. Opening a Drawer
2. Pushing a Car
3. Moving a book on a Table
4. Launching a Boat
5. Sliding a Box on the Floor
6. Pushing a Sofa
7. Moving a Fridge
8. Opening a Sealed Jar
9. Walking on Icy Pavement
10. Using a Rubber Eraser

Laws of Static Friction

1. Maximum static friction is independent of the contact area.
2. Maximum static friction is proportional to the normal force; an increase in normal force leads to an increased maximum external force that the object can withstand without moving.

Limiting Friction:

1. Limiting friction is the maximum static friction just before an object starts sliding.
2. Once motion begins, static friction is no longer relevant, and kinetic friction takes over.

Laws of Limiting friction are listed below:

1. Limiting friction acts opposite to the direction of motion.
2. It acts tangentially to the interacting surfaces.
3. The magnitude of limiting friction is proportional to the normal reaction between the surfaces.
4. The limiting friction depends on the material, nature of surfaces, and their evenness.
5. As long as the normal reaction is constant, the magnitude of limiting friction is unaffected by the shape or area of the surfaces in contact.

Case Study: Launching a Satellite into Orbit

A 500 kg satellite is positioned on a launch platform, awaiting deployment into Earth’s orbit.

Static Friction in Launch:

1. Initial Rest: The satellite is initially at rest, held by static friction (μs=0.2).
2. Applied Force: Engines apply a force, starting at 1500 N, gradually overcoming static friction.
3. Maximum Static Friction: Maximum static friction (F_static max=400 N) depends on the coefficient (μs) and normal force (N=2000 N).
4. Launch: Once applied force exceeds F_static max, e.g., at 450 N, the satellite starts moving, transitioning from static to kinetic friction.

Frequently Asked Questions

#	Questions	Answers	Examples
1	What is static friction?	Static friction is the force that tries to stop things from moving when they’re not already moving on a surface.	Trying to push a parked car.
2	What is sliding friction?	Sliding friction is the resistance that happens when things slide against each other.	Sliding on a playground slide.
3	Is frictional force a scalar or a vector?	Frictional force is like an arrow with both size and direction, so it’s a vector.	Pulling a heavy suitcase along the floor requires both force and direction.
4	How does friction produce heat?	When things rub against each other, the friction turns movement energy into heat.	Rubbing hands together generates heat.
5	What is the formula to calculate frictional force?	To figure out friction, you can use this formula: Force (F) = Coefficient of Static Friction (μs) × Normal Force (N). Here, μs shows how slippery or sticky surfaces are, and N is how hard they press against each other.	Calculating the force needed to move a book on a table based on the surface and weight.

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