## Frictional Force (Friction)

Learn how an object would accelerate after force is applied and a frictional force is created. See how to use the force of friction equation.

**Friction force** (**F _{f}**) is a force caused because all surfaces are rough at the microscopic level. Therefore a frictional force is created opposite to the direction motion will occur. Static frictional force is equal to the force applied until a maximum amount and an object starts moving. Once moving the object has what is called moving or kinetic friction.

:**Static Friction****frictional****force**caused by an applied force**at rest**(before motion).:**Kinetic Friction****frictional force**when an object is**in motion**.

A General Rule: __ Kinetic (moving or sliding) friction is less than static (standing) friction__.

Because moving surfaces are bouncing off each other with little less surface area interaction.

**Frictional Force Equation**

**F**_{f}=μF_{N}

**The Variables and Units:**

**F**:_{f}**force of friction**(N)**µ**:**coefficient of friction**(no unit)**F**:_{N}**normal force**(N)

__Frictional Force on a Horizontal (Flat) Surface:__

**F**: represents the_{N}**normal force**which the ground pushes up against and equal to weight- If given mass, solve for F
_{w}(F_{w}=mg) to know what F_{N}will be equal to.

__Different Coefficient of Friction (µ) Variables__

**µ**represents the coefficient of_{s}**static friction****µ**represents the coefficient of_{k}**kinetic sliding friction****µ**represents the coefficient of_{r}**kinetic rolling friction**(wheel rolling)

__More Facts About Coefficient of Friction__ (µ)

- The
**bigger the coefficient of friction (µ) the more friction** - Use the same equation but with the right situational value
**µ**_{k}almost always less than µ_{s}

Note:

- The force of static friction is always equal to the force applied until the object begins sliding.
- When you solve for the force of static friction it tells you maximum amount of force before an object would move.

**Net Force Equals Zero When in Constant Motion**

An object in **constant motion **has a **net force is zero and with no acceleration**.

This means the **applied forces** **equals** kinetic frictional force.

If applied force was ever more than the kinetic friction

- The object will have a
**net force** - Net force
**causes acceleration**, not just motion

### Example Problems:

**It takes 45N of force horizontally to move a wood block with a constant velocity across a desk.**

**1. What is the applied force?**

**2. What is the force of friction?**

**3. What is the acceleration?**

**4. What is the net force?**

**5. What force is the minimum you have to apply horizontally to get a 1.5 kg block of wood at rest to move on a wood surface?**

**6. How much force would it take to keep the 1.5 kg block of wood moving at a constant rate after its moving?**

**7a. Would a 15kg brick on a wood floor accelerate when 120N of force is applied?**

**7b. What would its acceleration be?**

**8. If you lift the side of a desk creating an incline until the moment an object first starts moving. What would happen next keeping that exact same angle and why?**

**A. The block would stop**

**B. The block would move at a constant rate down the incline**

**C. The block would accelerate down the incline**

**Why?**

Notice how frictional force uphill and parallel force downhill increase with the incline until motion occurs. Because of motion, static friction changes to a lesser kinetic friction. Therefore the force uphill (friction) is less than the force downhill. At that moment the net force is downhill. So the net force downhill creates an acceleration.

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