Lesson (1 - 1) Friction What is friction? Activity: The motion on the floor we can observe that the ball stops moving after a little time in the case on the playground, but it keeps moving for a longer time in case on the class room floor. Activity: A bike motion Of this activity, we infer that: The effect of friction force exists between the bicycle tires and the ground where this force has an effect in the opposite direction of the movement. Therefore, the bicycle slows down.

Activity: Friction force Friction occurs between two surfaces when one of them touches the other. The friction force is always in the opposite direction of the moving object. There are two types of friction force: 1. The friction force is between the surfaces of two objects that one of them is relatively constant to the other. This force prevents the object from moving in a sliding way on the wooden board while going up. 2. When the cube slides down on the wooden board, it is affected by a less friction force. When the cube moves down, the direction of the friction force is upwards.

Friction force: It is the amount of force that exists between two surfaces and affects the movement in the opposite direction.

Friction disadvantages The internal parts of machines are often damaged as a result of the friction force between their moving parts. This friction causes a rise in temperature of these parts. If the temperature rises up more than a certain extent, it causes the damage of machines and accordingly a lot of money is wasted.

Ways of decreasing the friction force: 1. Using lubricants and oil form a thin layer between the two surfaces; and this will decrease the effect of the friction force. 2.Technicians use which is called ball bearings (Fig. 9) to be put between the surfaces of the machines internal moving parts. *Ball bearing are formed of a group of small metal balls which have smooth surfaces. Therefore, the friction force between them is almost non-existent. *Ball bearing are used, for example, at the axis that transmits the motion from the car engine into the wheels.

Lesson (1 - 1) Exercises Tick ( ✔ ) in front of true sentence, and if the sentence is wrong carrect the underlined wards: 1.The friction force is always in the same direction of the object’s movement. (x) The friction force is always in the opposite direction of the object’s movement. 2.The friction force between two surfaces is greater while moving than stopping. ( ✔ ) Write the scientific terms that expresses the following sentence: - A force that slows down the moving object and has its effect in the opposite direction of the object’s movement. (friction force)

Answer the following questions: 1. Mention the disadvantages of the friction force. 2. Mention the most important ways that can be used to decrease the disadvantages of friction force. Complete the following sentences: 1. Lubricants & oil is used to decrease the effect of friction force between the internal moving parts of machines. 2. Friction force has its effect on the opposite direction of the object’s movement. 3. The force that slows down the objects motion is called friction force.