Friction (Frictional force)
*the force that opposes motion
*always acting in a direction opposite that of the motion of the body
Situations involving friction:
1. walking, running, stopping is possible
2. rubbing hands makes them warm
3. making fire by rubbing stones or match and the matchbox
4. braking of car
5. wearing out of belts and gears …..and many more!
In horizontal surfaces, the normal force equals the weight. The greater the normal force the greater is the friction.
Ff α Fn
Coefficient of friction -a number without unit, ranging from 0.01 to 0.99, which measures the degree of how a surface affects the motion of objects in contact with it.
Symbol: µ (Greek letter mu)
Formula: Ff = µ Fn
Take note:
A low value indicates a smooth contact between the object and the surface, meaning less friction, while large value assigned for rougher surfaces.
Types of Friction
1. static friction - the friction between surfaces that are at rest with respect to each other.
*starting friction- maximum static friction
2. kinetic (sliding) friction – friction between sliding or moving surfaces.
3. rolling friction- friction between bodies ,mounted on wheels
Static Friction Example:
Case 1: A mass is resting on a flat surface which has a normal force of 98N, with a coefficient of static friction of 0.35. What force would it take to move the object?
N = 98N, μs = 0.35
Step 1: Substitute the values in the below static friction formula:
This example will guide you to calculate the static friction manually.
Case 2: A 15kg piece of wood is placed on top of another piece of wood. There is 35N of static friction measured between them. Determine the coefficient of static friction between the two pieces of wood.
N = 15kg x 9.81 m/s2 = 147.15N, Fs = 35N
Step 1: Substitute the values in the below Static Friction Coefficient formula:
This example will guide you to calculate the Static Friction Coefficient manually.
(http://easycalculation.com/physics/classical-physics/learn-static-friction.php)
| Kinetic Friction Example: Case 1: A steel box is lying on a ground with a coefficient of kinetic friction of 0.55. It is pushed with the force of 100N? N = 100N, μk = 0.55 Step 1: Substitute the values in the below kinetic friction formula:  This example will guide you to calculate the kinetic friction manually. Case 2: A truck is travelling at a contant speed up a hill with a normal force of 1360N. The Force of the Kinetic friction acting on the truck is 266N. What is the coefficient of kinetic friction between the truck and the road? N = 1360N, Fk = 266N Step 1: Substitute the values in the below Kinetic Friction Coefficient formula:  This example will guide you to calculate the Kinetic Friction Coefficient manually. |
Answer this in your notebook.
1. A 9.75-kg lead brick rests on a level wooden table. If a force of 46.4 N is required to slide the brick across the table at a constant speed, what is the coefficient of friction?
2. A cup of coffee is sitting on a table in an airplane that is flying at a constant altitude and a constant velocity. The coefficient of static friction between the cup and the table is 0.30. Suddenly, the plane accelerates, its altitude remaining constant. What is the maximum acceleration that the plane can have without the cup sliding backward on the table?
Challenge Problem:
A sled is travelling at 4.00 m/s along a horizontal stretch of snow going to the right direction. The coefficient of kinetic friction is 0.0500. How far does the sled go before stopping?
Walang komento:
Mag-post ng isang Komento