4-2 Lab 4-Frictional Forces PDF

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4-2 Lab 4-Frictional Forces of Algebra-based Physics I Lab...

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Miami Dade College/North Campus PHY1004L, PHY2053L, and PHY2048L

Lab 6 “Investigating Static and Kinetic Frictions.” Whenever two surfaces that are in contact with each other try to move pass one another there is a force that resists the motion. In some case you could push on an object trying to move it and the object doesn’t move. The force of static friction fs balances your force. This is because in order to move an object you need to apply a force larger or equal to the maximum possible static friction, fsmax .Once moving the object still offers some resistance and if you stop pushing the object will immediately start to slow down to a stop, the force responsible for this behavior is the force of kinetic friction, fk. In this experiment, you will use a Force Sensor to study static friction and kinetic friction on a tray. Purpose: To find the graphical and mathematical relationship between the forces of maximum static and kinetic frictions and the normal force on the objects, as to determine the coefficients of static and kinetic frictions for the surface being studied. Materials: ● Computer with Logger Pro;

● Mass sets;

● Lab Pro with dual range force sensor;

● String;

● Ramp;

● Trays.

Preliminary questions: 1. 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? Explain your answer. __________________________________________________________________________________________ __________________________________________________________________________________________ __________________________________________________________________________________________ 2. How do you think the force of friction is related to the weight of the box? Explain. __________________________________________________________________________________________ __________________________________________________________________________________________ 1

Miami Dade College/North Campus PHY1004L, PHY2053L, and PHY2048L

__________________________________________________________________________________________ 3. Create a force or free body diagram for the case when you are pushing on the box without moving it and when it slides across the floor at a constant velocity.

Procedure: Part I Practice Pulling. 1. Measure the mass of the tray and record it in the data table. 2. Connect the Dual-Range Force Sensor to Channel 1 of the interface. Set the range switch on the Force Sensor to 505N. 3. Hold the Force Sensor in position as ready to pull the block, but without the string. Click Force Sensor to zero.

to set the

4. Tie string on the tray to the hook on the Force Sensor. Place a total of 200g mass on top of the block, fastened so the masses cannot shift. Practice pulling the block and masses with the Force Sensor using this straight-line motion: Slowly and gently pull horizontally with a small force. Very gradually, increase the force until the block starts to slide, and then keep the block moving at a constant speed. 5. 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. 6. Copy the force vs. time graph for the force you felt on your hand. 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.

Part II Measuring Maximum Static Friction and Kinetic Friction In this section, 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.

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Miami Dade College/North Campus PHY1004L, PHY2053L, and PHY2048L

9. Examine the data by clicking the Statistics button, . 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. 10. Drag across the region of the graph corresponding to the block moving at constant velocity. Click on the Statistics button again and read the average (or mean) force during the time interval. This force is the magnitude of the kinetic frictional force. 11. Repeat Steps 8-10 for two more measurements and average the results to determine the reliability of your measurements. Record the values in the data table. 12. Add masses totaling 200 g to the block. Repeat Steps 9 – 12, recording values in the data table. 13. Repeat for additional masses of 400, 600, 800, 1000, and 1200 g. Record values in your data table. DATA:

Mass of block

590 kg

Table One: Maximum Static Friction. Peak static friction Total mass (m)

Normal force (N)

Average

Trial 1

Trial 2

Trial 3

590g

1.161

1.161

0.8920

1090

2.063

2.082

1.919

1590

2.583

2.846

3.128

2090

3.867

3.729

3.329

2590

4.481

5.571

4.187

3090

5.646

4.945

5.245

peak static friction (N)

Table Two: Kinetic Friction.

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Miami Dade College/North Campus PHY1004L, PHY2053L, and PHY2048L Kinetic friction Total mass (m)

Normal force (N)

Average

Trial 1

Trial 2

Trial 3

590

0.7819

0.7646

.6976

1090

1.331

1.273

1.268

1590

1.958

1.892

1.950

2090

2.551

2.445

2.513

2590

3.157

2.979

2.959

3090

3.786

3.850

3.596

kinetic friction (N)

Analysis: 1. Inspect the printout or sketch of 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 question 1 in the Preliminary Questions section? 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. Use logger Pro to graph of the maximum static friction force (vertical axis) vs. the normal force (horizontal axis). Comment on the relationship and write the corresponding equation. Explain the meaning of the slope and the y-intercept. 6. Repeat step five for the relationship between the force of kinetic friction and normal force. Please graph them of the same graph. 4

Miami Dade College/North Campus PHY1004L, PHY2053L, and PHY2048L

7.

Does the force of kinetic friction depend on the weight of the block? Explain.

8. Does the coefficient of kinetic friction depend on the weight of the block? Extension for PHY2048. 1. Examine the force of static friction for an object on an incline. Find the angle that causes a tray to start to slide. Calculate the coefficient of friction and compare it to the value you obtain when the angle of the incline is 0°.

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