Friction

As you do this experiment and you are asked for values, please specify the units. Numbers without units are meaningless unless they are truly dimensionless numbers. All answers are worth 5 pts.

Go to: http://phet.colorado.edu/en/simulation/forces-and-motion

1. Click the Forces and Motion link above. Click Run Now.

2. Click on the Friction Tab across the top of the window. Select Show Free Body Diagram and check Sum of Forces.

3. Set the values of the Static and Kinetic Friction equal to zero

4. Calculate the force Fg = _____________ and the normal force FN = _____________

What does the force Fg represent? [Answer]

What is the sum of these two forces? ____________

5. Set the coefficient of Static Friction to 0.3 and the Value of Kinetic Friction to 0.2

6. Note that the object mass is given in the lower part of the screen. Type in the Applied Force, FA, window a force equal to the normal force. Start the animation and then stop it quickly.

· What does the force FN represent? [Answer]

· What does the force Ff represent? [Answer]

· What does the force Fa represent? [Answer]

· What does the force Fsum represent? [Answer]

· Why does the for Fsum only act in the horizontal direction? [Answer]

7. How does the applied force compare to the force of friction?

[Answer]

8. Click on the Play Button. How does the Free Body Diagram change when the box hits the wall? [Answer]

· What can you say about the sum of the forces in the x-direction? [Answer]

· What can you say about the sum of the forces in the y-direction? [Answer]

9. Reset All! Show the free body diagram and the sum of the forces again.

· Set the initial position as far left as you can. X Position ______________

10. Now you want to determine the force necessary to start the box in motion. The static coefficient of friction should now be 0.5 and the kinetic coefficient 0.3.

11. Set the applied force to 100 N. Start the animation. If the box does not accelerate then increase the force by 100 N and recheck if the box will move. When the box starts to move, stop the animation and reset the applied force to the previous value of the applied force plus 10 N. Continue 10 N to the applied force until the box starts to move. Tthen reset the applied force to the previous value plus 1 N. Continue doing this until you have determined the applied force necessary to start the box moving to the nearest 1 N. Record the value below.

· Force to Start the Box Moving = _________________

12. If the applied force is unchanged the box will accelerate. Determine the force necessary to keep the box moving without accelerating. This force is the value of the normal force time the coefficient of kinetic friction, FA = µkN. Calculate the force necessary to keep the box moving without acceleration and record below.

· Force necessary to Keep the box Moving without accelerating = _________________

13. Set the applied force to the value of 500 N.

Start the animation and quickly stop it.

14. Set the applied force to Force necessary to Keep the box Moving without accelerating. Restart the animation. Just before the box hits the wall, stop the animation. What can you tell me about relative magnitudes of the frictional force and the applied force.

[Answer]

15. Does an object have to have a net force on it to be moving?

[Answer]

16. If there is a net force acting on an object, what does the acceleration equal?

[Answer]

17. Tell me what you have learned about how the force of friction works.

[Answer]

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