INTRODUCTION

Friction is a resistive force that occurs due to motion between two contacting surfaces. The two surfaces must either be sliding or attempting to slide over another resulting in a force that is tangent to the two surfaces and in a direction that tends to oppose the motion of each surface. In general, frictional force depends on the nature and physical properties of the surfaces in contact. It slows down the motion of an object and is directed along the two surfaces in contact in the opposite direction of the motion. Friction is always called a non-conservative force because some energy is needed to overcome it.

The following conclusions can be made of friction, Friction between two surfaces is:

Independent of the surface area of contactIndependent of the sliding speed of the surfacesProportional to the normal force acting on the surfacesActs in a direction opposite to the direction of motion of each surface.

Types of Friction

There are different types of friction. A book moving across the desk is an example of sliding friction. As the book slides cross the desk, the bottom of the book is touching the desk. The source of the friction is the contact between the surface and the book and the desk. The weight of the object and the type of surface it moves over determine the amount of sliding friction present between the two objects. A heavy object exerts more pressure on the surface is slides over, so the sliding friction will be greater.Air, water and oil are all fluids. Air resistance is a type of fluid friction. As

an object falls, air resistance pushes it up on the object.When you ride a bicycle, the contact between the wheel and the road is an example of rolling friction. When an object rolls over a surface, the force needed to overcome rolling friction is much less than that needed to overcome sliding friction.

Kinetic Friction

When you moved your book across the desk, the book experience a type of friction that acts on moving objects.This force is known as a kinetic friction force. It is exerted on one surface by another when the two surfaces rub against each other because one or both surfaces are moving. If you stack additional books on top of the first book to increase the normal force, the kinetic friction force will increase.Let's look at the formula for kinetic friction force.

There is a linear relationship between the kinetic friction force and the normal force. The coefficient of kinetic friction relates the friction force to the normal force. The kinetic friction force (F(f, kinetic)) equals the product of the coefficient of kinetic friction (µ(k)) and the normal force (F(N)). F(f, kinetic) = µ(k) * F(N)

Static Friction

Imagine trying to push a couch across the floor. You push on it with a small force, but it does not move. This is because it is not accelerating. Newton's laws tell you that the net force on the couch must be zero. There must be a second horizontal force acting on the couch, one that opposes your force and is equal in size. This force is static friction force, which is the force exerted on the surface by another when there is no motion between the two surfaces.Static friction force acts in response to a force trying to cause a stationary object to start moving. If there is no such force acting on an object, the static friction force is zero. If there is a force trying to cause motion, the

static friction force will increase up to a maximum value before it is overcome and motion starts.The maximum static friction force relates to the normal force in a similar way as the kinetic friction force. In the equation for maximum static friction force, µ(s) is the coefficient of static friction between two surfaces. The maximum static friction force that must be overcome before motion can begin is µ(s) * F(N). In the example of pushing the couch, the maximum static friction force balances the force of the person pushing on the couch the instant before the couch begins to move.

Causes of Friction

All surfaces, even those that appear to be smooth, are rough at a microscopic level. If you look at a photograph of a graphite crystal magnified by a scanning tunneling microscope, the atomic level surface irregularities of the crystal are revealed. When two surfaces touch, the high points on each are in contact and temporarily bond. This is the origin of friction.

Measuring Coefficients of FrictionOn what does a friction force depend? The materials that the surfaces are made of play a role. For example, imagine trying to play basketball while wearing socks instead of athletic shoes. You would slip and slide all over the basketball court. Shoes help provide the forces necessary to quickly change directions while running up and down the court. There is more reaction between your shoes and concrete than there is between your socks and a polished wood floor.

This table shows coefficients of static friction (µ(s)) and coefficients of kinetic friction (µ(k)) between various surfaces.

The coefficients of friction show how easily one object can slide against another.These coefficients are estimates for each combination of surfaces. Exactmeasurements of coefficients of friction are quite sensitive to the conditions of the surfaces and are determined experimentally.

Another important fact regarding the table is that all the measurements were made on dry surfaces (with exception of the oiled steel). Wet surfaces behave quite differently than dry surfaces.≤µs n.

THEORY:There are two types of frictional forces:a) Static friction ( fs)This exists when the object is at rest relative to the surface. This force must be overcome in order to make the object start moving. It is given by fs

0BJECTIVE

the nature of the frictional force between two wooden surfaces will be investigated. This will lead to the observation of the properties of friction. The following objectives will be accomplished:

1. the determination of the coefficients of static friction, µs , between two wood surfaces by sliding the block down the board.

2. Determination of the coefficient of friction by using the board as

3. Comparison of the four values of µs from the four different methods.

4. Demonstration that the coefficients of friction are dependent of the surface area of contact.

MATERIALS

1.Wooden Block2.Weight hanger3.Wooden Board4.Slotted weights5. balance6.Pulley7..String8.Paper towel9. Two sheets of Cartesian graph

paper

EXPERIMENTAL PROCEDUREFor simple surface onlyFor simple surface only

1. Weigh the wooden block on the triple beam balance and record its weight

2. Set up the block on the board with the largest surface in contact with the board surface and string attached to the block run over a pulley. Place some weights on the weight hanger. Slightly increase the load on the weight hanger until the block begins tomove slowly with a constant speed after it has been started with a very small

push. Make sure you wipe away any dust from the surfaces. Record the weight placed on the weight hanger including the mass of the weight hanger. 200 g on the surface of the block. Slowly increase the load on the hanger until the block starts slowly moving with constant speed after given a small push. Don’t forget to include the mass of the hanger.

3. Repeat Procedure 2 above placing masses of 200,400, 600, 800 and 1000 g successively on top of the wood block. Record the total weights placed on the weight hanger including the weight hanger. The coefficient of kinetic friction can be obtained from these data.

4. Turn and place the smallest side of the wood block on the board and repeat Procedure 2 above.

5. Set up the block again as in procedure 2 and place amass of 600 g on it. Place weights gently on the hanger and increase them slowly until the block just starts its motion without any push. Repeat a total of 3 independent trials and record your data. With this data you can find the coefficient of static friction

REPORT FORM

Weight of block Weight of pan

µk from graph ________

Weight on block

Weight on pan Weight of pblock +weight on block

Weight of pan+weight on pan

EXPERIMENTAL PROCEDUREFor sandy surface onlyFor sandy surface only

1. Weigh the wooden block on the triple beam balance and record its weight

2. Set up the block on the board with the largest surface in contact with the board surface and string attached to the block run over a pulley. Place some weights on the weight hanger. Slightly increase the load on the weight hanger until the block begins tomove slowly with a constant speed after it has been started with a very small

push. Make sure you wipe away any dust from the surfaces. Record the weight placed on the weight hanger including the mass of the weight hanger. 200 g on the surface of the block. Slowly increase the load on the hanger until the block starts slowly moving with constant speed after given a small push. Don’t forget to include the mass of the hanger.

3. Repeat Procedure 2 above placing masses of 200,400, 600, 800 and 1000 g successively on top of the wood block. Record the total weights placed on the weight hanger including the weight hanger. The coefficient of kinetic friction can be obtained from these data.

4. Turn and place the smallest side of the wood block on the board and repeat Procedure 2 above.

5. Set up the block again as in procedure 2 and place amass of 600 g on it. Place weights gently on the hanger and increase them slowly until the block just starts its motion without any push. Repeat a total of 3 independent trials and record your data. With this data you can find the coefficient of static friction

REPORT FORM

Weight of block Weight of pan

µk from graph ________

Weight on block

Weight on pan Weight of pblock +weight on block

Weight of pan+weight on pan

EXPERIMENTAL PROCEDUREFor oily surface onlyFor oily surface only

1. Weigh the wooden block on the triple beam balance and record its weight

2. Set up the block on the board with the largest surface in contact with the board surface and string attached to the block run over a pulley. Place some weights on the weight hanger. Slightly increase the load on the weight hanger until the block begins tomove slowly with a constant speed after it has been started with a very small

push. Make sure you wipe away any dust from the surfaces. Record the weight placed on the weight hanger including the mass of the weight hanger. 200 g on the surface of the block. Slowly increase the load on the hanger until the block starts slowly moving with constant speed after given a small push. Don’t forget to include the mass of the hanger.

3. Repeat Procedure 2 above placing masses of 200,400, 600, 800 and 1000 g successively on top of the wood block. Record the total weights placed on the weight hanger including the weight hanger. The coefficient of kinetic friction can be obtained from these data.

4. Turn and place the smallest side of the wood block on the board and repeat Procedure 2 above.

5. Set up the block again as in procedure 2 and place amass of 600 g on it. Place weights gently on the hanger and increase them slowly until the block just starts its motion without any push. Repeat a total of 3 independent trials and record your data. With this data you can find the coefficient of static friction

REPORT FORM

Weight of block Weight of pan

µk from graph ________

Weight on block

Weight on pan Weight of pblock +weight on block

Weight of pan+weight on pan

EXPERIMENTAL PROCEDUREFor greasy surface onlyFor greasy surface only

1. Weigh the wooden block on the triple beam balance and record its weight

2. Set up the block on the board with the largest surface in contact with the board surface and string attached to the block run over a pulley. Place some weights on the weight hanger. Slightly increase the load on the weight hanger until the block begins tomove slowly with a constant speed after it has been started with a very small

push. Make sure you wipe away any dust from the surfaces. Record the weight placed on the weight hanger including the mass of the weight hanger. 200 g on the surface of the block. Slowly increase the load on the hanger until the block starts slowly moving with constant speed after given a small push. Don’t forget to include the mass of the hanger.

3. Repeat Procedure 2 above placing masses of 200,400, 600, 800 and 1000 g successively on top of the wood block. Record the total weights placed on the weight hanger including the weight hanger. The coefficient of kinetic friction can be obtained from these data.

4. Turn and place the smallest side of the wood block on the board and repeat Procedure 2 above.

5. Set up the block again as in procedure 2 and place amass of 600 g on it. Place weights gently on the hanger and increase them slowly until the block just starts its motion without any push. Repeat a total of 3 independent trials and record your data. With this data you can find the coefficient of static friction

REPORT FORM

Weight of block Weight of pan

µk from graph ________

Weight on block

Weight on pan Weight of pblock +weight on block

Weight of pan+weight on pan

CALCULATIONS

1) From procedure 2, draw a graph using the data of the force of friction as the ordinate and those of the normal force as the abscissas. Draw the best straight line joining most of the points. Obtain the slope of this graph. The slope so obtained is the coefficient of kinetic friction, µk, between the wooden block and board.

2 calculate the value for µs between block and board. Find their average value and Compare the fourvalues

for µs for wood on wood,oil.san,grease by calculating their percent difference which is given

by:

Percent difference = [difference of the two values/ average] x 100 %

Name of surface

Wood and wood

Wood and oily surface

Wood and greasy surface

Woood and sandy surface

Percantagee difference