Static friction

In this experiment you will measure the relationship between static friction of an object and its mass.

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 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.

Once the box starts to slide, you must continue to exert a force to keep the object moving, or friction will slow it to a stop. The friction acting on the box while it is moving is called kinetic friction.

In this experiment we will only be investigating static friction and starting friction.

Consider the static situation below. Draw all the forces involved.

What do you know about friction? What can increase or decrease the friction on an object?

In this experiment you will measure the applied force required to make an object just start moving. This applied force is expected to be equal but opposite in direction to the retarding force of the friction.

Draw in the static frictional force on the diagram when the block is pulled.

MATERIALS

computer

string

Vernier computer interface

block of wood with hook

Logger Pro

balance or scale

mass set

Vernier Dual-Range Force Sensor

PRELIMINARY QUESTIONS

1. In everyday life, you often experience one object sliding against another. Sometimes they slip easily and other times they do not. List some things that seem to affect how easily objects slide.

2. Consider a box sitting on a table. It takes a large force to move it at constant speed. List at least two ways you could reduce the force needed to move the box at constant speed.

3. In pushing a heavy box across the floor, is the force you need to apply to start the box moving greater than, less than, or the same as the force needed to keep the box moving? On what are you basing your answer?

METHOD

PART 1 - Starting friction

1. Measure the mass of the block and record it in the data table.

2. Set the range switch on the Dual-Range Force Sensor to 10 N. Connect the Force Sensor to Channel 1 of the interface.

3. Open the file “12a Static Kinetic Frict” from the Physics with Vernier folder.

4. Tie one end of a string to the hook on the Force Sensor and the other end to the hook on the wooden block. Place a total of 1 kg mass on top of the block, fastened so the masses cannot shift. Before you collect data, practice pulling the block and masses with the Force Sensor using a straight-line motion: Slowly and gently pull horizontally with a small force. Very gradually, taking one full second, increase the force until the block starts to slide, and then keep the block moving at a constant speed for another second. 5. Hold the Force Sensor in position, ready to pull the block, but with no tension in the string.

Click to set the Force Sensor to zero.

6. Click to begin collecting data. Pull the block as before, taking care to increase the force gradually. Repeat the process as needed until you have a graph that reflects the desired motion, including pulling the block at constant speed once it begins moving. Print or copy the graph for use in the Analysis portion of this activity.

PART II - Peak Static Friction

In this part, you will measure the peak static friction force and the kinetic friction force as a function of the normal force on the block. In each run, you will pull the block as before, but by changing the masses on the block, you will vary the normal force on the block.

7. Remove all masses from the block.

8. Click to begin collecting data and pull as before to gather force vs. time data. 9. Examine the data by clicking Statistics, . The maximum value of the force occurs when the block started to slide. Read this value of the maximum force of static friction from the floating box and record the number in your data table.

RESULTS

PART I - Starting Friction

Force vs Time graph:

PART II - Peak Static Friction

ANALYSIS

1. Inspect your force vs. time graph from Part I. Label the portion of the graph corresponding to the block at rest, the time when the block just started to move, and the time when the block was moving at constant speed.

2. Still using the force vs. time graph you created in Part I, compare the force necessary to keep the block sliding compared to the force necessary to start the slide. How does your answer compare to your answer to Preliminary Question 3?

3. The coefficient of friction is a constant that relates the normal force between two objects (blocks and table) and the force of friction. Based on your graph (Run 1) from Part I, would you expect the coefficient of static friction to be greater than, less than, or the same as the coefficient of kinetic friction?

4. For Part II, calculate the normal force of the table on the block alone and with each combination of added masses. Since the block is on a horizontal surface, the normal force will be equal in magnitude and opposite in direction to the weight of the block and any masses it carries. Fill in the Normal Force entries for both Part II data tables.

5. Plot a graph of the maximum (peak) static friction force (vertical axis) vs. the normal force (horizontal axis). Use excel or openoffice

6. Describe the shape of the line and find the numeric value of the slope, including any units by adding a trendline and displaying equation. Since Fmaximum static = μs N, the slope of this graph is the coefficient of static friction μs.

7. Does the value of μs change with additional mass?

8. What would change the value of μs?