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 (Ff) 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
The Variables and Units:
- Ff : force of friction (N)
- µ : coefficient of friction (no unit)
- FN : normal force (N)
Frictional Force on a Horizontal (Flat) Surface:
- FN : represents the normal force which the ground pushes up against and equal to weight
- If given mass, solve for Fw (Fw=mg) to know what FN will be equal to.
Different Coefficient of Friction (µ) Variables
- µs represents the coefficient of static friction
- µk represents the coefficient of kinetic sliding friction
- µr represents the coefficient of 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
- µk almost always less than µs
- 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
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
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.