Factors Affecting Frictional Force

Factors affecting frictional force

Level: Secondary 4

Project aims:

The main purposes of this project are to let students: 1. Apply the concepts of Newton’s Laws in the study of friction. 2. Investigate the relationships among different factors (e.g. mass, surface area, surface type, normal reaction etc.) and frictional force.

Learning objectives:

1. Propose hypotheses for investigation and identify dependent and independent variables in experiment. 2. Collect quantitative and qualitative data with accuracy. 3. Represent information in appropriate graphical and mathematical terms.

Students have to identify factors that affect the magnitude of frictional force and perform experiments to test their hypothesis. Conclusions should be drawn based on experimental results.

Apparatus/Materials:

Any appropriate apparatus and materials proposed by students are welcomed. Typical apparatus includes identical wooden blocks (with different dimensions on the three sides), spring balance, rope, pulley, weight, and different surfaces (e.g. sandpaper, wax paper, towel, aluminum foil, table surface etc.) and so on.

Pre-knowledge:

Friction plays a major role in our daily life. Without it, we could not drive a car, walk or even stay seated in our chairs. The force of friction actually refers to a force that resists motion when two objects are in contact. The maximum force of static friction between two surfaces can be measured from the minimum applied force acting on an object that makes it starts to move.

At the instant the object starts moving, f = F

Figure 1: Direction of the frictional force is opposite to the applied force.

The direction of the frictional force on an object is parallel to the surface with which the object is in contact and opposite to the impending motion (static friction) or the actual motion (kinetic friction) of the object relative to the surface.

Possible investigations:

1. Study the relationship between mass and frictional force: Students can carry out investigations on whether object of different masses would affect the frictional force.

2. Study the relationship between contact area and frictional force: Students can carry out investigations on whether object with different contact areas would affect the frictional force.

3. Study the relationship between different surface types and frictional force: Students can carry out investigations on whether objects with surface of different types would affect the frictional force.

Some other possible scenarios students may come up with include velocity of the moving object (involves kinetic friction), inclination of the plane, magnitude of the applied force and so on.

Teachers’ note:

The level of friction that different materials exhibit is measured by the coefficient of friction. The magnitude of the force of static friction between any two surfaces in contact has a value of f s   s N , where  S is the coefficient of static friction and N is the normal force. The higher the value of  , the stronger is the frictional force that resists the motion if two objects are going to slide past each other. The maximum static friction is obtained when f s   s N . This situation is called impending motion.

There are different types of friction: static, kinetic, deformation, rolling, etc. If one applies a small force to slide two objects passing each other, the two objects will not start to move if the maximum static frictional force is greater than the applied force. This is static friction. On the other hand, if one applies a force a little bit stronger than fs so that the objects start to slide, we enter the regime of kinetic friction where one still has to apply force to keep the object sliding, but the force does not have to be as strong as the static friction.

Figure 2: A graph of frictional force versus applied force

Theoretical expectation: The frictional force is expected to be doubled if we double the mass of the wooden block by stacking one block on the top of another. By measuring and recording the forces required to move different masses (number of blocks), we expect an increasing linear relationship between mass and frictional force.

Using different surfaces of the same block as the contact surface we can change the contact area. The forces measured should be similar. This shows that there is no direct relationship between surface area and frictional force. In principle, whether a box of a given mass has a larger or smaller surface contact area, the maximum amount of static friction should be the same since friction is proportional only on the total normal force. However, in reality, the frictional force varies between different surfaces because the coefficient of friction varies between different surfaces (see following).

For different surface types, the coefficient of friction varies. There are no simple rules to describe how the coefficient of friction changes with different types of surfaces. Generally speaking, the smoother the surface, the weaker the friction. Materials that feel rough would normally have a larger friction. Since there is no simple mathematical rules relating frictional forces between different surfaces, students may use bar chart or table to represent their findings.

Useful notes: At the time when students obtain preliminary results for analysis, teachers can remind them the importance of controlling variables: that is, while testing a particular factor, the others should be kept unchanged throughout the experiment.

If students pull the spring balance or the string attached to the mass at an angle instead of pulling horizontally, teachers can remind them that at different angles, the force acting on an object will be different and this can affect the results. While studying the factor of surface types, teachers can suggest students to take a close look at the different surfaces and draw pictures of how the surfaces actually look like so as to stimulate their thinking in how the surface texture affects friction.

Experience in pilot study:

A pilot study on this project was carried out in a secondary school for a class of around 40 Form Four students which were divided into nine groups. A rundown of the event is outlined below. The whole project was carried out in a total of 5 periods of lessons (each lesson of 50 minutes) with the briefing, planning and discussions done in a double period and the experiments were carried out in another double period. A make up class was arranged on a Saturday morning but only one student attended. Presentation (II) was carried out in another period of lesson.

Teacher Stage Students  Tried out the project himself. Preparation  Prepared handout.  Prepared a learning log as an assessment tool.  Prepared the schedule and a peer evaluation form.  Prepared a rubric for assessment.  Announced the working DAY 1  Raised some factors that may schedule of the project. (2 periods) affect friction, including the  Asked students to propose some Briefing weight, surface area, surface factors that may affect friction. (30 min) type, inclination, and velocity.  Asked students to investigate  Chose one of the factors to on one of the factors. investigate.  No groups chose inclination or velocity in the end.  Observed students’ performance Planning  Discussed the procedure, setup,  Guided student to think but & Discussion apparatus, division of labor, tried not to give them answer (30 min) precautions etc. of the project. directly.  Chose apparatus for their  Prepared apparatus after the experiments class.  Facilitated a poster presentation Presentation (I)  Drew their design of setup on a for students to present their (20 min) poster and stick on the walls for planned idea. presentation.  Chose one group to give their  One group presented their poster presentation in front of design to the whole class and let the class as a trial other groups commented. demonstration  Assessed the students. DAY 2  Implemented the project. (2 Periods)  Modified the setup or Implementation procedures whenever they found (90 min) necessary.  Asked the teacher as last resort when faced difficulty.  Launched poster presentation DAY 3  Prepared their posters including  Assessed the presentation. (1 Period) hypothesis, setup, results and  Launched peer evaluation. Presentation conclusions. (II)  Posters are posted on the walls. & Peer  Each group presented their Evaluation project at the same time with (1 period) some members being the presenters and some being audients to walk around and listen to presentations from other groups.  Graded the presentations of their peers.  Graded the in-class assessment, Assessment learning log, presentation and posters.

Role of the teacher The teacher deliberately tried to reduce his involvement to let students to get on investigating on their own.

Presentations The teacher conducted two presentations in this trial. Before the students implemented their experiments, they first presented their hypothesis to be tested, the experimental set up, different variables, precautions and predictions etc. on a poster. This presentation aimed at providing a preliminary evaluation of students’ setups. Some groups were able to locate some fatal mistakes raised out from their peers. However, the limitation of time made presentation of all posters impossible. As an alternative, the teacher asked one group to present in front of the whole class as a demonstration and allow the other groups to comment. After the student implemented their experiments, they prepared a set of posters for their oral presentation. The presentations allow them to exchange experience and knowledge with their peers. The posters which included experimental setup, results and conclusions were posted on the walls of the laboratory. Students in each group were divided into presenters and audience. They swapped their roles when the presentation went to a half. During the presentation session, the audience walked around and listened to presentations from other groups. This arrangement allowed all groups to present to some audience at the same time.

Every student was also given with them a peer assessment form to assess the presentations from other groups. The assessment categories include contents, organization, uses of visual aid, language and interaction. Students also prepared some questions that they would like to ask the presenters in the presentation session. These questions helped to facilitate the discussions among presenters and the audience which could also help the audience to assess their peers more easily.

Pictures of Students’ work Below shows some pictures of students working on the project.

Challenges faced by teacher and students Some groups of students come up with an idea that velocity of the moving object may be a factor affecting the frictional force. They were told by the teacher that this idea involves kinetic friction rather than static friction which the situation is much more complicated and out of the scope of this project and they gave up studying this factor finally.

For students who study the relationship between contact area and frictional force, they normally proposed at the beginning that the size of frictional force is directly proportional to the contact surface area. A group of students found out from their experiment that the size of friction actually does not have significant relationship with the surface area. It is neither directly proportional nor inversely proportional, so they concluded that there is in fact no relationship between contact area of an object and its frictional force. This is the only group that is brave enough to draw a conclusion that no relationship can be found.

Some students proposed that the inclination of the contact surface may affect friction. Eventually no groups chose that one to investigate.

Questions:

1. What is your hypothesis on what possible factors that would affect the size of the frictional force? 2. What are the dependent, independent and controlled variables in your investigation?