Investigating Static and Kinetic Friction Force

1. Investigating Static and Kinetic Friction Force

2. Static and Kinetic Friction Force • Frictional forces play an important role in our daily lives. If we do not counteract them, they would stop every moving object and bring to a halt every rotating shaft. About 20% of the gasoline used in an automobile is needed to counteract friction in the engine and in the drive train. • On the other hand, if friction were totally absent, we could not get an automobile to go anywhere, and we could not walk or ride a bicycle. • We could not hold a pencil and if we could, it would not write. • Nails and screws would be useless, woven cloth would fall apart and knots would untie.

3. An Introduction to Forces • There are two factors that influence the friction between two surfaces: • The normal force • The type of surface • In this experiment we are going to determine the relationship between the frictional force and the normal force. • Frictional force: • Frictional force is a resistance force that exists between two surfaces that are in contact with each other. • If we use a microscope to observe the two surfaces in contact, we would see that they are not smooth. • The uneven sides move against each other, causing a frictional force in a direction along the surface, that apposes the movement. • There are two types of friction namely static friction and kinetic friction.

4. An Introduction to Forces continued…. • Static and kinetic friction: • A small force is applied on a stationary block. • The block remains stationary, which indicates that a force of the same magnitude as Fapplied acts to the left - frictional force. • The block also remains stationary in the vertical direction, because N = Fg. • The applied force can be increased to a point where the block just starts moving from rest. • The maximum static friction is reached just before the block starts moving. • In other words, the static friction is not constant, but increases with the applied force up to a maximum value, where the object will be dislodged and begin moving. • The symbol for this maximum value of static friction is fsmax. • As soon as the block starts moving, the frictional force is called kinetic friction. • Kinetic friction is smaller than the maximum static friction. • fk < fsmax • After the block starts moving, we can maintain movement by exerting a smaller force than the force needed to cause motion.

5. An Introduction to Frictional Coefficients • Static and kinetic friction is highly dependent on the nature of the surfaces that are in contact with each other. • Friction is also directly proportional to the normal force acting on the object. • Normal force is the force exerted by the surface, vertically upwards on the object. • fs ∝ N and fk ∝ N • fs = μsN and fk = μkN • μs is the coefficient of static friction and μk is the coefficient of kinetic friction. • Coefficients of friction are proportionality constants that depend on the types of surfaces that are in contact with each other. • 𝜇_𝑘<𝜇_𝑠 (for a specific object). • Lubrication of the surfaces will cause a decrease in frictional coefficients. • The table below provides the frictional coefficients of a few surfaces.

6. An Introduction to Force Diagrams Description Diagram 1: Example of a Force diagram • If the applied force on an object, acts in a horizontal direction (parallel to the surface), the normal force = force of gravity (N = Fg = mg). • If the object is pulled with a force that acts at an angle with the surface, the y-component of the force, will be upwards and the normal force will decrease. • N + Fy = Fg • N = Fg – Fy • Fy = Fsin𝜃, where 𝜃 is the angle between the applied force and the surface. • If the object is pushed with a force that acts at an angle with the surface, the y - component of the force, will be downwards and the normal force will increase. • N = Fy + Fg • Fy = Fsin𝜃, where 𝜃 is the angle between the applied force and the surface.

7. Diagram of: Relationship between fs, fk and applied force

8. Advantages to Friction Discussed Diagram 2: Photographs of people making fire • Friction between the soles of our shoes and the floor enables us to have traction when we walk. • Friction between tyresand the surface of the road allows vehicles to move. • A car can be stopped by the brakes because of friction. • Friction allows motion in gear systems. • Friction between surfaces allows us to unscrew a lid with our fingers. • From the earliest times, people used friction to make fire. • The San people still have the skill to make fire in this way

9. Disadvantages to Friction Discussed Diagram 3: Photograph of a second degree friction burn • It is difficult to run or walk on loose sand because of friction. • Falling on tar roads causes severe injuries. • Friction prevents high speed during car or cycle races. • Carbon brushes in motors and generators have to be replaced often because of friction. • Tyres of a car have to be replaced because of friction.

10. The Experiment Diagram 4: Photograph of a Spring balance Discussed • A spring balance is used to determine: • The maximum static friction (fsmax) and • The kinetic friction (fk) of blocks made of the same material but with different masses on the same horizontal surface. • The normal force on each weightis determined with the formula N = Fg = mg (g = 9,8 m∙s-2). • The results obtained are used to draw graphs of • Static friction vs normal force • Kinetic friction vs normal force • From the shape of the graphs (straight lines) it can be concluded that: fs ∝ N and fk ∝ N • The coefficients of static and kinetic friction can be determined by calculating the gradients of the graphs. • μs = 𝑓_𝑠/𝑁 • μk = 𝑓_𝑘/𝑁

11. Key words

12. References • Physical Sciences; Grade 11 ; Shuters; > pg. 16 - 20 • Physical Sciences text- and workbook (DocScientia); Grade 11; > pg 28 - 54 • Study & Master Karin H. Kelder; Grade 11 > pg 36 – 41 • Fundamentals of Physics Jearl Walker, Haliday and Resnick; > pg 117 - 121 Links https://youtu.be/CTLXubXOTUQ https://youtu.be/CTLXubXOTUQ https://youtu.be/qy-EJRDyt-A