1. Friction
Static and Dynamic

2. What do you know about Friction?
Please write down one or two points from your previous knowledge of friction.
Pair up with the person to you left and share your points.

3. Nature of Friction
Parallel to the common surfaces between the two objects
To determine the direction of friction you need to imagine what direction the object would move if there was no friction.
Friction is present in all real life situations and present for all types of mediums. Yes, even air!

4. Predict, Observe, Explain
How does the properties of the materials affect friction? Make a prediction of which surfaces will produce the greatest frictional force.

5. Static Friction
Is the force preventing the object at rest from starting to move.
It’s magnitude does not have a fixed value, it varies from zero to a maximum value.
The maximum value of static friction is reached when the object starts to move.
The amount of force that is required to start an object in motion is called the starting friction.

6. Kinetic Friction
Kinetic friction is present anytime one object is sliding on another.
Kinetic friction has a fixed value for two particular objects. (regardless of the speed)
Kinetic friction is the force that causes a cart to stop after you push it along the floor. If there was no kinetic friction (in a vacuum) the cart would continue on with the same velocity.

7. Predict, Observe, Explain
What frictional force do you think will have a higher value? The static friction of the kinetic friction?

8. Magnitude of the Force of Friction
The magnitude of the kinetic friction force is never greater than the maximum static friction force.

9. Magnitude of the Force of Friction

10. Predict, Observe, Explain.
What will the effect of changing surface area be for an object’s kinetic friction?

11. A Little Bit of History
Leonardo da Vinci was the first person to discover experimentally the first two observations of friction:
The force of kinetic friction acting on an object is dependant on the normal force acting on the object.
The change surface area of an object does not affect the force of kinetic friction because the normal for will remain the same.

12. Co-efficient of Static Friction
The maximum magnitude of the static friction force is directly proportional to the normal forcing acting on the object.
The relationship can be written as:
As an equation this can be written as:
where is the coefficient of static friction.
Since the force of friction can be anywhere from zero to the max:

13. Coefficient of Kinetic Friction
The magnitude of the coefficient of kinetic friction is proportional to the magnitude of the normal force.
A constant can be added, the formula rewritten as:
The coefficient for kinetic friction has only one value.

14. Characteristics of Friction Coefficients
Determined experimentally
The formulas for static friction and kinetic friction cannot be written as vectors because the frictional force and normal force are perpendicular.
FN is equal in magnitude to mg and in the opposite direction.
μS and μK have no units.

15. Example#1
A sled with waxed hickory runners rests on a horizontal surface of dry snow. Calculate the mass of the sled if the maximum force that can be applied to the sled before it starts moving is 46 N [forward] . The static friction coefficient is 0.06 and the kinetic friction coefficient is 0.04.

16. Example #2
A 1640-kg lift truck with rubber tires is skidding on wet concrete with all four wheels locked. The static friction coefficient is 0.7 and the kinetic friction coefficient is Calculate the acceleration of the truck.

17. Inquiry! Can we calculate the μ?
Each table is a group.
Each group receives a spring scale.
Choose an object
Determine, experimentally, the static and kinetic friction coefficient.