SPEAK OUR LANGUAGE

• CELL – Core Experience Learning Lab
• SDR – Scientist Data Record

ASK WHY

Great scientists question the world around them. We encourage our LabLearner students to do the same. In anticipation of this, we explain the importance of learning the concepts in the Ask Why section within the CELL. Our hope is that these explanations help students understand why science matters.

BRANCH OUT

Each Investigation introduces students to a different branch of science or STEM (Science, Technology, Engineering, Mathematics) career that utilizes the scientific concepts of the CELL. These real-world connections will help students see the relevance of what they are learning. STEM connections are also integrated into each Performance Assessment.

GET FOCUSED

The Focus Questions in each Investigation are designed to help teachers and students focus on the important concepts. By the end of the CELL, students should be able to answer the following questions:

Investigation 1:

• What is the relationship between speed, velocity and acceleration? A change in velocity occurs when an object in motion either changes speed or changes direction. Acceleration denotes a change in velocity in a period of time.

• How does frictional force affect motion? Frictional force decreases the velocity of an object. That is, it decreases the speed of an object.

Investigation 2:

• How does velocity of an object affect the frictional force between it and the surface with which is comes in contact? Frictional force is independent of velocity.

Investigation 3:

• What is the relationship between weight and frictional force? As the weight of a load increased, the frictional force between the load and the surface it moves on increased.

• What is the relationship between the surface area of an object in contact with another surface and the frictional force between two surfaces? As the surface area of a load increased, the frictional force between the load and the surface remained constant.

Investigation 4:

• How does the smoothness of the two surfaces in contact affect the frictional force between the two surfaces? The smoother the two surfaces, the less frictional force. The less smooth the two surfaces, the more frictional force.

Note: These are succinct responses to the Focus Questions and are placed here for easy reference. Fully developed responses to the Focus Questions can be found on each PostLab page.

Note: Some questions may be revisited as the CELL progresses. As students acquire additional knowledge, their responses should reflect this.

LEARN THE LabLearner LINGO

The following list includes Key Terms that are introduced within the Backgrounds of the CELL. These terms should be used, as appropriate, by teachers and students during everyday classroom discourse.

Note: Additional words may be bolded within the Background(s). These words are not Key Terms and are strictly emphasized for exposure at this time.

Investigation 1:

• Force: a push or pull on an object
• Acceleration: the rate an object changes velocity
• Velocity: the rate an object changes its direction
• Applied force: force that is applied to the object from an outside force

Investigation 2:

Note: No new terms are introduced in Investigaton 2.

Investigation 3:

• Frictional force: the force that prevent two objects from easily moving against one another

Investigation 4:

• Coefficient of friction: a number that describes the degree of mechanical and molecular interaction between two surfaces
• Normal reaction force: the force of gravity on that object when pulling an object across the table
• ΣF = ma: the sum of forces are equal to mass multiplied by acceleration

BE PREPARED

An overview of the materials for each lab is placed here for easy reference. Specific teacher preparation for the labs is placed at the beginning of each Lab page.

EXTEND YOUR THINKING

The following information is included so that teachers have additional background knowledge pertaining to the concepts introduced in the CELL. Teachers may choose to use this information to enrich students during instruction by integrating it into individual Concept presentations.

This CELL is designed to help students understand frictional force, Newton’s three laws of Motion and the process of scientific experimentation. Friction is the force that opposes motion between two surfaces. Frictional force is due to the mechanical and molecular interaction between the two surfaces. Friction occurs due to the movement of one surface over another (dynamic friction) or a force acts to move one of the surfaces but they remain stationary (static friction) (Figure 1). If you push harder on an object it fails to move because the static friction force is equal and opposite to the force you apply. The maximum static friction force is the maximum force that can be applied without the object moving. Once the maximum static frictional force is exceeded the object will move, and the friction force is dynamic, and is less than the maximum static frictional force.

In the first activity students push a penny with the aim of making the penny travel and stop on the end line. This task is an example of the role of friction and can be described using Newton’s three laws. With the penny at rest the student must apply an external force to the penny to accelerate it (speed it up in the forward direction). This is an example of Newton’s 1st law which states that an object remains at rest or moves at a constant velocity (constant speed in a straight line) unless acted upon by an external force. According to Newton’s 2nd law the acceleration of the penny is directly proportional to the force applied and inversely proportional to the penny’s mass (see equations below).

Once the student lets go of the penny it experiences a negative acceleration (slows down in the forward direction) until it comes to a stop. This negative acceleration indicates that a force must be present in the opposite direction to the motion of the coin. Once the coin comes to a stop no frictional force is present.

We have mentioned Newton’s first and second laws, but have failed to mention Newton’s third law, which states that for every action there is an equal and opposite reaction. How does Newton’s third law apply to the penny situation? When the student pushes on the coin the coin applies an equal and opposite force back on the person. The reason this is not observed is because the large mass of the person relative to the penny means the person only experiences a very small acceleration. The equal and opposite force to the friction force on the coin is the frictional force of the coin on the table. Again, the acceleration of the table is small due to its mass and hence not observed.

In order to measure frictional force students will pull on a spring scale attached to a wooden block. If someone pulls on the spring scale carefully increasing the force it is possible to observe the block remain stationary while the force increases up to the maximum static frictional force. Then the block accelerates (starts to move forwards). Now if the block is pulled so that it moves at a constant velocity the friction force equals the pulling force, and the spring scale indicates the dynamic friction force (when the forward force is equal and opposite to the backward force the block does not accelerate). In order to stop the block the pull force is reduced below the friction force so that the friction force slows down the block’s motion. For activities two, three and four the students will measure the static friction force of the wooden block by pulling the block at a constant velocity between the two points and recording the force on the spring scale.

As scientists we will make hypotheses about what possible factors could influence frictional force and will test them in Investigations 2, 3 and 4. The four properties that we will consider are movement velocity (speed in particular direction), weight, contact surface area and surface type. The beliefs that either movement velocity or surface area influence friction are common misconceptions. We have included these two variables because it is equally important in the scientific process to find out what does not influence friction as what does. The students will examine all four properties by running experiments and analyzing the data. The results will reveal that friction force is the same for different surface areas, indicating surface area does not affect friction. Also, for different velocities the friction force is constant (some small noise in measurement are likely, but the values should be essentially the same). In contrast, increasing weight will lead to a linear increase in friction. Also, surface type will lead to different friction forces. These results suggest that friction is dependent on weight and surface type.

Students should understand Newton’s three laws as they apply to this situation. Students should also have specific knowledge of friction and variables that influence friction. In addition, the usefulness of equations in describing relationships between variables and the importance of what is not included in the equation should be stressed. Finally, students will have experienced the scientific process of examining variables to determine their influence.

In summary, through investigation and the use of mathematical formulas, students will begin to define the relationship between speed, velocity, and acceleration. Students will begin to understand that velocity is a measure of speed and direction, and students will find that an object with no change in velocity has no acceleration.

Students will perform investigations that involve pulling a wood block along a table at a constant velocity. Using the formula   ΣF = ma, students will discover that the sum of the forces on an object with constant velocity is zero. Through use of the formula ΣF = F_friction + F_applied, students will determine if the sum of the forces is zero, frictional force is equal and opposite to the applied force. Using this knowledge, students will determine the frictional force on a wood block and a wood box in a variety of scenarios.

Through investigations, students will discover that frictional force and applied forces affect the motion of an object. Further investigating frictional force, students will find that it is dependent on the weight of the object being moved and on the smoothness of the surfaces in contact with one another. Students will also find that frictional force is independent of the surface area of the object being moved and the velocity at which the object moves. Through these Investigations, students will begin to draw conclusions about the way forces affect motion.