Charles Sanchez, Geminesse Sianghio, Ferguie Solis
Department of Chemistry
College of Science, University of Santo Tomas
España Street, Manila Philippines
In this experiment, a block of wood is used to observe friction on different surfaces such that an extra weight is also added to the block of wood to measure the same units under the different surfaces. With also the use of lubircant a member of the group was asked to observe the effect adding a lubricant on friction. And with the use of a motion detector did the group observed air resistance.
Friction is the force resisting the relative motion may it be unto solid surfaces, fluid layers, and even material elements that might cause a moving object to slow down when it is touching another object. Friction itself is not a fundamental force but arises from interatomic and intermolecular forces between the two contacting surfaces. In this experiment, the researchers are tasked to do five activities regarding friction which corresponds to the five laws of friction. The five laws states that it is dependent among the surface in contact, but within larger units friction is independent of velocity and the area of contact as long as there is an area of contact. it is also proportional and perpendicular to the normal force. These theories will be further verified as the experiment proceeds. And also to observe the effect of air resistance on falling coffee filters
Friction is the action of a surface or object rubbing against another. It can also be defined as the motion between two bodies in contact. Its Velocity is opposite to the velocity of an object or its relative velocity. Force is an interaction between two physical bodies which can be pull or push depending on the interaction performed in the object and it exists when physical bodies interact which is then proportional to the acceleration. Two objects that are physically interacting has force called contact force. On the other hand, action-at-a-distance of two objects does not perform physical interaction but still results a force. Moreover it can be exert a force like push or pull even without physical interaction.
Newton’s have three laws of motion. The first law of motion is a body acted on by no net force moves with constant velocity (which is probably zero) and zero acceleration.
Net force = 0
The second law of motion states that if an external force acts on a body, the body accelerates. With the direction of the acceleration is proportional to the direction to the net force. The mass of the body multiply by the acceleration is equal to the net force vector.
Net force vector = m.a
Newton’s third law states that if body A exerts a force on a body B (action), then body B exerts a force on body A (reaction). These two forces have the same magnitude but are opposite in direction. These two forces act on different bodies.
A force producing the acceleration is the weight. With:
In equation one, F is force and is equal to mass (m) multiplied to acceleration (a). For equation two weight (W) is equal to mass (m) multiplied to gravity (g).
Static friction is the friction between two object in contact that are not moving. Static friction is greater that kinetic frictional force, two object surfaces are moving with respect to one another.The formula for the coefficient of kinetic friction is: (Eq. 3)
In equation three, is the value for the coefficeint and is equal to is the value of the maximum static friction divided by normal force which value of the object in newton measure. The formula for the coefficient of static friction is:
In equation four is defined to be the coefficient of kinetic friction and is equal to kinetic friction ) divided by normal force.
For the first activity the group place a wood block in top of the wood board and was coonected to a spring scale to the wood block. The group then determined the minimum force needed to start the motion of the block by pulling the spring balance or force sensor. The force is numerically equal to the maximum static friction and reading is recorded as the spring balance fsmax. The block then is set into motion by pulling the spring balance. The force needed to move the block with constant velocity is also determined. The force is numerically equal to the kinetic friction and recorded as fK. The steps are repeated using the other sides of the block.
For the second activity, the block is weighed and was added a 100g. the normal force is said to be equal to the weigh of the block plus the 100g added. The maximum static friciton and inetic friciton is determined as it was in the first activity. With succession for each step, an addition of a 100g was added for each trial. The results are then tabulated; as such the coefficient of static friction and coefficient of kinetic friction is calculated.
For the third acitivity, an aluminum foil is placed on the wood board with the block on top of it. The maximum static friction and kinetic friction is detemined with the same steps done on the first activity. The group replaced the aluminum foil is replaced with plastic and the first step is repeated. The same is also done with sand paper and cartolina.
For the fourth activity, one of the group rubbed his hands for a minute and was placed in his cheeks and the feeling is observed. The same step is repeated
For the last activity the group connected the motion detector to a DIG/SONIC channel of the interface and then openned the file “13 Air Resistance” from the Physics Computer Folder. One of the members held a coffee filter or a paper cup 1.0m above the motion detector and was released as soon as the data collection began. From the graph of position versus time, the value of terminal velocity was determined which is the slope of position versus time graph and by clicking on Linear Fit button. The steps were repeated for 2, 3, 4 and 5 filters. The values are determined and was graphed as follows A) terminal velocity versus mass of filters and B) square of terminal velocity versus mass of filters, as such the relation that exist between terminal velocty and mass of filters was looked upon.
4. Results and Discussion
In this experiment, the group underwent five activities that tackled different ideas of friction in which static friction and kinetic friction were calculated, With also the use of a motion detector the group was also able to determine the terminal velocity of a falling object.
Table 1. Friction and Area of Contact
With the use of block of wood, the group dragged on two of its sides with the use of a spring balance to be able to determine its static and kinetic friction. It is shown that there is a variation in its values determined for the block assigned to the group was rectangular in shaped and it might be a possibility that due to its shape different values were attained.
Table 2. Friction and Normal force
Wieght of Block = 2.4 Newtons
Ww + 100g
Ww + 200g
Ww + 300g
Ww + 400g
In this part of the experiment the group was asked to add additional wieghts on top of the block and determine is maximum static friction and kinetic friction where (Ww) stands for the wieght of block in newtons. The coeficients for static friction and kinetic friction was also determined. The coeficients were also found to be in equal values, although the values presented are not equal, but when rounded off can be of equal values.
Table 3. Friction and Nature of Surfaces in Contact.
Surfaces in Contact
Wood and wood
Wood and sand paper
Wood and plastic cover
In this experiment the group observed friction of surfaces in contact and it is seen that some surfaces has less values than the others. It could be because some surfaces causes lees friction or force to the block to be able to achieves kinetic friction
For the fouth activity a member of the group rubbed his/her hands against each other and observed its effect on his cheeks. As for the next step a lubricant was added during the rubbing of the hands and it is observed that with lubricant present less heat was felt upon the trial on which the lubricant was added.
Graph 1. Terminal Velocity Versus Number of Filters
In graph one it is seen that as the number of papers used is terminal velocity decreases, this shows that as the weight increases the faster it approaches the motion detector.
Graph 2. Square of Terminal Velocity versus Number of Filters
In graph two the group asked to look for the square of the terminal velocity and plot it versus the number of filters. The grpah shows respectively that as the number of filters increases the square of the terminal velocity also increases showing a directly proportional relationship.
In this experiment the group concludes that different surfaces can have greater or lesser quantities of both static and kinetic friction. Not only did the group examined such behaviour on the block of wood but also it was examined on the palm of tha hands and it is found that by the addition of a lubricant can there be less heat. Normal force was also observed in the second acitvity and from the units given, it is seen that for the value of its coefficients is at a constant value and as for air resistance it is found that as its mass increases the faster its square velocity and as for its terminal velocity, it decreases showing indirect relationship towards the number of filters.
1. Which will require a greater force: to start the motion of a heavy box or to keep the box moving at a constant speed?
The action that requires a greater is force is to start a motion of a heavy box, since the maximum static friction is always of greater value than kinetic friction which is found when the box is kept moving at constant speed
2. Why is it difficult to walk with new leather shoes on a polished floor?
Because a polished floor has less friction and as for the situation using new leather shoes, the shoes itself will maximize contact and therefore a large amount of friction will be dealt upon and using an old pair will greatly minimize fricition due to a decrease in normal friction
3. How can an adjustable inclined plane be used to measure the coefficeint static friction and coefficient of kinetic friction?
With the use of an adjustable inclined plane would just be an aid in to determine the value of friction that moves the motion involved.
4. The coefficient of static friction and kinetic friction between a refrigerator and the floor are 0.45 and 0.38 respectively. The refrigerator weighs 1000N, find the horizontal force needed to a) start the motion of the refrigerator b) to keep the refrigerator from moving at constant speed once it started moving. If a horizontal force of 400N is applied, what will happen to the refrigerator? In this situation, what is the value of friction?
0.45 x 1000N = 450N
0.38 x 1000N = 380N
If a force that is equal to 400 N is applied to the object which is the refrigerator it will not budge since the force applied is less than the maximum static friction. However the static friction in the given situation is valued at 450 N.
5. Explain the usedfulness of a parachute for safe landing.
The parachute itself acts the object that negates the mass of paratrooper by projecting a surface area the obstructs the wind, therefore air resistance is achieved and as for the idea of safe landing, it sometimes not possible due to the place of contact with the ground but aside from that fact it can be done due to the decelaration of the paratrooper with the help of the parachute.