3 Nisan 2013 Çarşamba

FRICTION FORCE

PURPOSE OF THE EXPERIMENT:
The purpose of this experiment is to investigate static friction and kinetic friction. Determine what happens to the coefficient of friction when the normal force is changed. Compare the static and kinetic friction of three different surfaces.
PROCEDURE:
This experiment has three parts.

In first part we taken a wooden block and placed a 500g mass on it. Then we measured the mass of the system as 574 ± 1g. Before carried out the experiment we thinked about the two situations mentioned below and drawed their free body diagrams as shown below again.

A)      ‘Consider you start pulling the block horizontally by applying very small force without making it move.’ According to this statement given, we drawed the diagram as shown in below.

B)      ‘Consider you apply a slightly larger force than previous situation but still without making the block move.’ According to this statement given, we drawed the diagram as shown in below.

C)      ‘How are the friction forces in a and b compared? Explain your reasoning.’ We answered this question like that the comparison between a and b can be shown like the relation below.

Fs1 < Fs

We thought this relation because in this case the block did not move. Because we apply a force on the block, there is another force related to friction. However, due to the the stagnation of the system, this force is equal to the force that we exerted on the system. Aditionally, when the conditions given in the part a and be provided, Fs1 < Fs2 relation is corrected.

After all, we acquired the graph shown in below after doing the experiment.

Later on the first part, we actually did the experiment to see our predictions were true or not and why.  We took datas while pulling the block horizontally very very slowly until the block starts moving and we kept pulling the block with constant speed for 3-4 seconds. We reached a graph shown below.

Then we done the data analysis and we obtained the maximum value of the force measured by the sensor before the block starts moving and noted this value as 5,94 N.  Then we noted down the average value as 3,4 N for the moveable part. Then we completed the first part. The questions related to first part is mentioned in the responses to the questions part.

In second part, we keeped the equipment setup the same as in the part1. In this part we investigated the effect of mass on friction. Before carrying out the experiment we predicted what we observe when we change the mass of the system that we apply force. We also drawed a graph to clarify our answer. Our prediction:

Our prediction drawing shown below. Then we followed the same procedures in part 1. We assembled three different systems by using different surfaces and recorded our measurements. We also tabulated the data.

Tables are also shown below.

3)      Compare the values measured by the force sensor just before and after the block starts moving. Are these values the same or different? How do you interpret the result?
As long as the body is stationary, the frictional force (static friction) is a self-adjusting force and once it begins to move, the frictional force (kinetic force) is a constant force, but has a value less than the value of limiting frictional force. We interpret this result from graphs like shown below.

As more force is applied, the friction force increase. The friction force will continue to increase until the instant immediately prior to the initiation of movement. At this moment:
–Motion≠0
–Ffriction= Flim
–μ= μs
Therefore, at this moment,μs= Flim/ Rn

4)      Compare the magnitudes of the friction force exerted on the block when the block is stationary and when it is moving.
When block is stationary, friction force’s value is measured as 5,94N and named as static friction force. When the block is moving the friction force’s value is measured as 3,4N and named as kinetic friction force. Generally kinetic friction force is not greater than static friction force.

We know that   and we thought if mass increase then the normal force increase.  is constant for each material so if normal force increase then F will also increase. So we predicted that if mass increase then both static and kİnetic friction forces increase. Our prediction was true. However, Coefficient of friction does not depend on the mass applied. it depends only on the area and nature of two surfaces in contact. However, the frictional force exerted does depend on the the normal force, which in turn may depend on the mass.

For cardboard, the graph shown above and the magnitudes of the kinetic and static friction force mentioned below.

Fs=7,40N
Fk=6,11N

For glass, the graph shown above and the magnitudes of the kinetic and static friction force mentioned below.
Fs=11,4N
Fk=10,1N
Therefore we can say that friction depends partly on the smoothness of the contacting surfaces, a greater force being needed to move two surfaces past one another if they are rough than if they are smooth. However, ‘friction decreases with smoothness only to a degree; friction actually increases between two extremely smooth surfaces because of increased attractive electrostatic forces between their atoms. Friction does not depend on the amount of surface area in contact between the moving bodies or (within certain limits) on the relative speed of the bodies. It does, however, depend on the magnitude of the forces holding the bodies together. When a body is moving over a horizontal surface, it presses down against the surface with a force equal to its weight, i.e., to the pull of gravity upon it; an increase in the weight of the body causes an increase in the amount of resistance offered to the relative motion of the surfaces in contact.’ (http://www.infoplease.com)

We predicted that the friction force of cardboard is greater than marble. This prediction is true, we saw that. However, we predicted the friction force of glass is smaller than both marble and cardboard. We are mistaken in this point because we think that if the surface is smooth then friction must be decrease. This statement is not false but its accuracy exists to a specific axtend. In other words, friction decreases with smoothness only to a degree. In summary;

1. The frictional force is independent of area of contact
2. The frictional force is independent of the velocity of motion
3. The frictional force is proportional to the normal force.

CONCLUSION:
‘Friction is a key concept when you are attempting to understand car accidents. The force of friction is a force that resists motion when two objects are in contact. If you look at the surfaces of all objects, there are tiny bumps and ridges. Those microscopic peaks and valleys catch on one another when two objects are moving past each other.

The level of friction that different materials exhibit is measured by the coefficient of friction. The formula is µ = f / N, where µ is the coefficient of friction, f is the amount of force that resists motion, and N is the normal force. Normal force is the force at which one surface is being pushed into another. If a rock that weighs 50 newtons is lying on the ground, then the normal force is that 50 newtons of force. The higher µ is, the more force resists motion if two objects are sliding past each other.’ (http://ffden-2.phys.uaf.edu)

‘If you try to slide a heavy box resting on the floor, you may find it difficult to get the box moving. Static friction is the force that is counters your force on the box. If you apply a light horizontal push that does not move the box, the static friction force is also small and directly opposite to your push. If you push harder, the friction force increases to match the magnitude of your push. There is a limit to the magnitude of static friction, so eventually you may be able to apply a force larger than the maximum static force, and the box will move. The maximum static friction force is sometimes referred to as starting friction.

In second part we analyse the relationship between mass and frictionforce.We found that  "FRICTION VARIES DIRECTLY WITH MASS"
According to the laws of mechanics friction is defined as the force resisting motion. Whenever there is motion there is friction opposing it. The frictional resistance of a surface is given by an unitless index called coefficient of friction. This depends upon the roughness of the surface.
Friction = coefficient * normal reaction

Thus, from the formula it is seen that friction is directly proportional to normal reaction and coefficient of friction. Normal reaction is nothing but the the perpendicular reaction provided to a body by a surface as a result of all the perpendicular forces acting on the surface due to the body.
For example, consider a block of wood on a table. Weight of the body acts downwards perpendicular to the surface of the table. Thus here the table will apply an equal and opposite force on the block(Newton's third law). This force will be perpendicular to the both the block and the table surface. Thus it is known as the normal reaction.
As is seen that every object exerts it weight upon the surface it is placed on.
Weight = mass* g.
Where g = acceleration due to gravity(=9.81 for earth surface)
Thus increase in mass increases normal reaction. Hence increasing friction.

In third part, we analyze the depending between frictionn force and surface. we said that friction depends partly on the smoothness of the contacting surfaces, a greater force being needed to move two surfaces past one another if they are rough than if they are smooth. However, friction decreases with smoothness only to a degree; friction actually increases between two extremely smooth surfaces because of increased attractive electrostatic forces between their atoms. Friction does not depend on the amount of surface area in contact between the moving bodies or (within certain limits) on the relative speed of the bodies. It does, however, depend on the magnitude of the forces holding the bodies together. When a body is moving over a horizontal surface, it presses down against the surface with a force equal to its weight, i.e., to the pull of gravity upon it; an increase in the weight of the body causes an increase in the amount of resistance offered to the relative motion of the surfaces in contact. Our evidence was tabulated like that.