Unit 1 lesson1

1. Unit 1lesson1 Friction force

2. When you ride your bike, what makes the bike move forward?What happens when you stop pedaling the bike? • First, the bike needs a force to move, this happens when you start pedaling. • Then when you remove your legs away from the pedals, the bike slows down until it stops. • As the bike needs a force to move in a certain direction, it needs a force to stop as well. • The force which caused the bike to slow down then stop is known as the friction force.

3. As the tires of the bike touch the ground, there is friction force acting in the opposite direction of the movement of the bike causing the bike to slow down and stop. • Friction force: The force that exists between two surfaces touching each other, and this force affects the movement in the opposite direction.

4. Activities

5. Roll a small ball or a glass marble on a ceramic surface Roll the same ball on the carpet with the same pushing force. Which one stops first?

6. The ball rolling on the carpet will stop first, because the surface of the carpet is rough while the surface of the ceramic is smooth. The friction force is affected by the type of surface.The friction force increases on the rough surfaces making the moving object to stop first.

7. We can walk on the ground without slipping due to the friction force between our shoes and the ground.When the ground is wet, water on the ground makes it smoother so the friction force decreases and we fall.

8. When you travel by car and the car moves in a high speed and you put your hand outside the window. When the car slows down and you put your hand out. In which case you feel the air moving your hand backwards more?

9. The friction force is affected by the speed of the moving object.By increasing the speed of the moving object the friction force increases.

10. Try to move both boxes forward. In which case you can move the object easier?

11. Moving the box on its wide side is more difficult than moving it on its narrow side.By increasing the surface area that touches the ground, friction force increases.

12. In fact there are some factors that affect the friction force which are: 1- The type of the touching surfaces: Smooth surfaces decrease the friction force, while rough surfaces increase the friction force. 2- The speed of the moving object: Friction force increases by increasing the speed of the object. 3- The surface area of the moving object: Friction force increases by increasing the surface area of the object.

13. What do you do to make your hands feel warm when the weather is cold ?

14. So, when you rub your hands together, friction force is produced, which increases the temperature of your hands and you feel warm.

15. What about the internal moving parts of the machines?What will happen when a machine is kept working several hours without stopping?Of course the temperature of the internal moving parts, that are touching each other, will increase and this will damage the machine and a lot of money will be wasted.

16. So, the disadvantage of the friction force is the increase of the temperature of the moving parts of the machines. How can we overcome this disadvantage ?* Rub your hands together but before doing this put few drops of oil on them.* This time try to put a glass marble between your hands before rubbing them.Do you feel the same temperature as that was produced on rubbing the hands without using oil or glass marble?What about the movement of your hands? Is it easier ?

17. To decrease the friction force between the internal moving parts of the machines we may use: • Oil and lubricants They decrease the friction force by forming a thin layer between the internal moving parts of the machine. • Ball bearings: Formed of a group of small metallic balls with smooth surfaces to make the friction force almost non-existent.

18. Now, which boy slides easier ? Explain your answer. • To answer this question, compare between the two surfaces on which the boys slide. • Then explain how these surfaces affect the friction force.