# Friction

#### Students` Learning Outcomes

·    Describe friction and its causes
·      Suggest methods to reduce friction.
·      Identify what cyclist, swimmers and parachutists do to reduce friction.

### Information for Teachers:

 The force which opposes the motion of one object over another is called friction. ·         It is caused by the roughness of the two surfaces.
·
·         Friction can be a help or a hindrance for motion. It prevents objects from starting to move.
·         It can be reduced by making surfaces smooth e.g. by making surface wet or by oily.
·         The friction of rolling objects is less than the sliding objects.
·         To minimize air or water friction, the aeroplanes, cars, ships, boats etc. are shaped pointed or wedge like from the front.
·         The cyclists, swimmers and parachutists wear tight to dresses and cover their heads with tight caps to reduce the friction.

#### Material / Resources:

A book, a ball , wooden block, glass sheet, oil, textbook.
Worm up:
·         Ask a student to push a book lying on the table.
·         Then ask the students what happened to book? (Student’s response: The book moved on the table. Eventually it slowed down and stopped. )
·         Why do bodies stop? (Students’ response: There must be a force acting opposite to the motion of objects)
·         What is that force which stops the movement of objects? (Students’ response: When an object moves,  it rubs against the surface on which it moves . Rubbing provides an opposing force. This force is called friction.)
·         Ask the students what does cyclist do to stop a bicycle? ( Students’s  response: He applies brakes.)
·         Why does brake stop a moving bicycle? (Students’ response: The brakes rubs against the rim of the wheel and hence opposes the motion of bicycle.)
·         Draw out the conclusion that the force which opposes the motion of the objects is called friction.
##### Development:
Activity1 :
·         Place a wooden block on the table and tell a student to push it to slide over the surface of the table.
·         Ask a student what opposes the motion of the wooden block on the table? (students’ response: The roughness of surfaces.)
·         Then place glass sheet on the table and ask the same student to push the wooden block over the surface of the glass sheet.
·         Ask the students what difference do they notice when the block moves over two different surfaces (students; response: The block comes to rest after covering greater distance on the glass sheet)?
·         Inform them that the surface of table opposes the motion more than the glass sheet.
·         Ask the students why is sliding of the wooden block over the glass sheet easier than over the table? (students’ response: The surface of the glass sheet is smooth whereas the surface of the table is rough).
·         Inform them that the roughness of surfaces causes friction.
Activity 2:
·         Take a glass sheet.
·         Ask the student to rub the palm of her/his hand  on it.
·         Now pour a little oil on the sheet.
·         Ask the student to rub the hand again on oily sheet.
·         Then ask what difference do you feel on rubbing the dry glass sheet and the oily sheet? (students’ response: The oily   surface has less friction.)
·         Ask them how can we decrease the friction? (students’ response: By applying oil or grease between the surfaces which slide over each other.)
Activity 3:
·         Take a trolley school bag with wheels attached to its bottom.
·         Ask a student to drag the bag on a smooth floor over the side without wheels.
·         Then ask him/her to drag it over the wheels.
·         Ask the students, is it easier to move the bag or wheels? (students’ response: yes , the wheels reduce friction.)
·         Inform the students that friction can also be reduced by using wheels attached to the heavy objects to make the movement easier.
##### Sum Up / Conclusion:
The teacher should ask the students: What have you learnt today and the write the conclusions on the board:
·         The forces which opposes the motion of the objects over another is called friction.
·         Friction is sometime desirable and some other times a hindrance.
·         The roughness of surface causes friction.
·         Friction can be reduced by making surfaces smooth, by oiling the surface or by using wheels.
##### Assessment:
·         Make a table on the board and ask the students to copy on their note books.
·         Ask them to make a list of objects from their surroundings, offering more friction and less friction.
·         Involve the students in solving the questions given at the end of chapter/ unit in textbook.