Data analysis and interpretation 1. Define the force of limiting friction. 2. Define the term normal force. 3. How can you vary the normal force in this experiment? 4. What is the purpose of adjusting the spring scale to read zero when no force is applied? 5. How do you determine when the wooden block just begins to move? 6. Describe the setup to ensure a purely vertical force is applied. 7. What do you record as the force of limiting friction in this experiment? 8. How does the force of limiting friction change as the normal force increases? 9. What happens if the normal force is reduced to zero in this experiment? 10. How would you calculate the coefficient of friction from the data collected in this experiment? 11. Why is it important to repeat the experiment for different normal force values? 12. Can you identify any potential sources of error in this experiment? 13. How could you improve the accuracy of this experiment? (1) (2) (1) (1) (1) (1) (1) (1)

Below are answers to each question based on a typical experiment investigating limiting friction: 1. Define the force of limiting friction.  The force of limiting friction is the maximum frictional force that must be overcome to initiate motion between two surfaces in contact. It is the peak friction force experienced just before an object begins to move. 2. Define the term normal force.  The normal force is the contact force exerted by a surface perpendicular to the object resting on it. It is essentially the force with which the surface pushes back against the object's weight. 3. How can you vary the normal force in this experiment?  You can vary the normal force by changing the weight acting on the block. For example, adding or removing masses on top of the block while it remains in contact with the horizontal surface directly alters the normal force, since N equals the weight (mass × gravity) in these conditions. 4. What is the purpose of adjusting the spring scale to read zero when no force is applied?  Zeroing the spring scale (also known as “taring”) ensures that any pre‐existing offsets or errors in the instrument are corrected. This helps ensure that subsequent measurements of force are accurate and are relative only to the applied force. 5. How do you determine when the wooden block just begins to move?  You determine this by gradually increasing the applied force and carefully watching the block. The point at which there is a slight and consistent movement—indicating that the static friction has been overcome—is when the block "just begins to move." Observing very subtle slips or a change in the force reading can also be clues. 6. Describe the setup to ensure a purely vertical force is applied.  To ensure a purely vertical force, attach the spring scale and weight system directly above the block so that the pulling string is perpendicular to the surface. Using a pulley or guide that forces the string to change direction without additional horizontal components can help guarantee that the force applied is strictly vertical. 7. What do you record as the force of limiting friction in this experiment?  You record the reading on the spring scale at the exact moment the block starts moving. This reading represents the force of limiting friction. 8. How does the force of limiting friction change as the normal force increases?  Generally, the limiting friction force increases proportionally with the normal force, since they are related by the equation f_friction = μN. An increase in the normal force, if the coefficient of friction remains the same, leads to a higher frictional force. 9. What happens if the normal force is reduced to zero in this experiment?  If the normal force is reduced to zero (for example, if the block is lifted off the surface), there would be no contact between the block and the surface, and therefore no frictional force would be present. 10. How would you calculate the coefficient of friction from the data collected in this experiment?  The coefficient of friction (μ) can be calculated using the equation μ = (force of limiting friction) / (normal force). For each set of measurements, divide the measured limiting friction force by the corresponding normal force. 11. Why is it important to repeat the experiment for different normal force values?  Repeating the experiment for different normal forces allows you to verify the proportionality between normal force and limiting friction. It also increases the data set, enabling you to check for consistency and to average out any random errors, thereby confirming that the relationship f_friction = μN holds over a range of conditions. 12. Can you identify any potential sources of error in this experiment?  Potential sources of error include:   • Inaccuracies in the spring scale calibration or reading.   • Friction in the pulley (if one is used) which might add extra resistance.   • Misalignment of the pulling force, causing it not to be exactly vertical.   • Variations in the surface roughness or imperfections on the contact surfaces.   • Human error in determining the exact point at which motion begins. 13. How could you improve the accuracy of this experiment?  To improve accuracy, you could:   • Calibrate the spring scale and any other measuring devices before the experiment.   • Use a low-friction pulley system or a guide that minimizes additional friction.   • Ensure the pulling force is as close to vertical as possible by carefully aligning the setup.   • Take several measurements for each normal force value and use the average to minimize random errors.   • Improve the method for detecting the onset of motion, possibly using high-speed video or sensors to more objectively determine when the block starts to slide. These answers encapsulate the main ideas and considerations you need when analyzing and interpreting an experiment focused on limiting friction.