Friction

The lecture’s objectives:-

At the end of this lecture, the student will be

able to :-

1- Draw the free body diagram.

2- Resolve any inclined forces if founded.

3- Choose which appropriate mechanism of

dry friction.

3- Apply formulas of Dry Friction

Mechanism.

Friction

* Friction is a force that resists the movement of two

contacting surfaces that slide relative to one another.

اإلحتكاك هي القىة التي تقاوم حركت سطحيه متصليه ينزلقان بالنسبت *

(اإلحتكاك هى القىة المتىلدة بيه سطحيه) .الى بعضها البعض

Theory of Dry Friction السطح أملس

السطح خشن

اذا السطح املس ال توجد -:مالحظة

.قوة رد فعل اذا السطح خشن توجد قوة رد -:مالحظة

.فعل عند كل النقاط على الجسم

Theory of Dry Friction

Mechanism of Dry Friction

EQUILIBRIUM.

IMPENDING MOTION.

MOTION.

𝑭𝒙 = 𝟎 𝑭𝒚 = 𝟎

𝑷 − 𝑭 = 𝟎 → 𝑷 = 𝑭

-:مالحظة عندما نؤثر بقوة فأن السطح ايضاً يؤثر بقوة مساوياً لها بالمقدار و معاكساً لها في االتجاه

. ليحافظ على اتزان الجسم فكلما تزيد ال تزيد ال

(P) (P) (F)

-:مالحظة (𝑭𝒔) هنا تصل قوة االحتكاك الى اقصى قوة احتكاك

𝜽𝒌 = 𝒕𝒂𝒏−𝟏 𝑭𝒌

𝑵 =𝐭𝐚𝐧−𝟏(

𝑴𝒌 𝑵

𝑵)

Mechanism of Dry Friction

Example (1):-

Find the magnitude of the friction force as shown in this

fig?

𝝁𝑺 = 𝟎. 𝟓 160 𝑰𝒃

P=65 𝑰𝒃

Example (2):-

A block with the weight (200 N), rests on The rough

horizontal plane is subjected to the force (P=40 N) which

inclined (25º). Determine the coefficient of friction?

200 N

25º

P=40 N

Example (3):-

A body with the weight (600 N), rests on a rough

inclined plane, inclined at angle of (40º).If coefficient of

friction is 0.58. What force is required to prevent the

body from sliding down the plane?

Example (4):-

A block of weight (981 N), is resting on inclined plane,

inclined at angle of (16º).Find the value of coefficient of

friction?

Example (5):-

Determine the rang of values which the mass (m𝟎) may

have so that the (100 kg) block shown will neither start

moving up the plane non slip down the plane. The

coefficient of static friction for the contact surfaces is

(0.3) ?

Example (6):-

Determine the magnitude and direction of the friction

force acting on the (100 kg) block shown if, first, ( P=500

N) and, second (P=100 N). The coefficient of static friction

is ( 0.20), and the coefficient of kinetic friction is (0.17).

The forces are applied with the block initially at rest ?

Example (7):-

Determine the rang of values which the mass (m𝟎) may

have so that the (120 kg) block shown will neither start

moving up the plane non slip down the plane. The

coefficient of static friction for the contact surfaces is

(0.35) ?

12𝟎 𝒌𝒈

Example (8):-

The uniform crate shown has a mass of (20 kg) if a force

(P=80 N) is applied to the crate, determine if it remains in

equilibrium. The coefficient of static friction is (𝝁𝒔 = 𝟎. 𝟑)?

Example9:- Two men are sliding a 100-kg crate up an incline. If the lower man pushes horizontally with a force of 500 N and if the coefficient of kinetic friction is 0.40, determine the tension ? which the upper man must exert in the rope to maintain motion of the crate.

Example 10:- Determine the smallest force the man must exert on the rope in order to move the 80-kg crate. Also, what is the angle (𝜽) at this moment? The coefficient of static friction between the crate and the floor is 𝝁𝑺 = 0.3.

Thank You for Your Attention