Friction

Friction is the force that opposes the relative motion between two surfaces in contact. It slowly brings a rolling ball to rest. In this chapter you will learn that friction is caused by the interlocking of irregularities on the two surfaces, so it depends on the nature of the surfaces and on how hard they are pressed together. Using a spring balance, you will compare static friction, which must be overcome to start moving an object, sliding friction, and rolling friction, the smallest of the three, which is why wheels and ball bearings make work easier. You will see that friction is a necessary evil. It wears out shoe soles and tyres and produces heat, but without it we could not write, walk, or stop a vehicle. You will study ways of increasing friction, such as treaded tyres and brake pads, and of reducing it with lubricants, powder and ball bearings. Finally, you will learn about fluid friction, or drag, and why birds, fish, boats and aeroplanes have streamlined shapes.

Detailed Worksheet: Friction Section A - Definitions (10 marks) 1. Define friction. In which direction does the force of friction act on a moving object? (2 marks) 2. What causes friction between two surfaces? On what two factors does it depend? (2 marks) 3. Distinguish between static friction, sliding friction and rolling friction. Arrange them in increasing order of magnitude. (2 marks) 4. What is a lubricant? Give two examples and explain how a lubricant reduces friction. (2 marks) 5. What is fluid friction (drag)? On what factors does it depend? (2 marks) Section B - Calculations and Applications (15 marks) 6. A brick tied with a string is pulled along a table with a spring balance. The reading is 6 N just as the brick starts to move and 4.5 N while it moves steadily. (i) What is the static friction just before motion? (ii) What is the sliding friction? (iii) Why is sliding friction smaller? (3 marks) 7. The same brick is pulled on three surfaces. The spring balance readings for steady motion are: glass 2.5 N, wooden table 4.5 N, towel 6.0 N. Arrange the surfaces in increasing order of friction and explain the result. (3 marks) 8. A box needs a force of 120 N to be dragged across the floor. When it is placed on rollers, only 20 N is needed. (i) By how many newtons is the friction reduced? (ii) What percentage of the original force is now needed? (iii) Name the type of friction in each case. (3 marks) 9. A child pushes a toy car with a force of 15 N, and the force of friction on the car is 4 N. Find the net force on the car and its direction. What happens to the car when the child stops pushing? (3 marks) 10. A pile of 4 books is pulled along a table with a spring balance, and the reading is 3 N. When 4 more identical books are added, the reading becomes 6 N. What does this show about friction? Predict the reading with 12 books. (3 marks) Section C - Diagrams (10 marks) 11. Draw a labelled diagram of the activity in which a brick is pulled along a table with a spring balance. Mark the direction of the applied force and of the force of friction. (4 marks) 12. Draw a magnified diagram of two surfaces in contact, showing the irregularities that interlock and cause friction. Show how a lubricant layer reduces this interlocking. (3 marks) 13. Draw a labelled diagram of ball bearings between the hub and the axle of a bicycle wheel, and explain how they reduce friction. (3 marks) Section D - Analysis and Higher-order Thinking (15 marks) 14. "Friction is a necessary evil." Justify this statement with three advantages and three disadvantages of friction from daily life. (5 marks) 15. Explain the following: (i) Tyres have treads and shoe soles are grooved. (ii) Gymnasts apply a coarse powder on their hands. (iii) A little powder is sprinkled on a carrom board. (iv) Brake pads press against the wheel rim to stop a bicycle. (v) Sportsmen use shoes with spikes. (5 marks) 16. Explain why the bodies of fish and birds and the shapes of boats and aeroplanes are streamlined. Describe how fluid friction depends on the speed and shape of an object, and explain why a parachute is designed to have large drag. (5 marks) Instructions: Time allowed 2 hours. Attempt all sections. Show the directions of all forces with arrows in your diagrams, and write units in every answer.