The Friction Addiction

Is friction an addiction? Sort of. We know that friction is a force that opposes motion. It exists any time two objects come in contact with one another, whether the object is moving or not. You can't escape it! The world as we know it is addicted to friction.

Kinetic Friction: Friction present between two objects during motion.

Ex: A slide and your butt.

Static Friction: Friction that exists between two objects when nothing is moving.

Ex: The book and the desk, assuming the book is at a standstill.

The Challenge: Measure the forces of friction between a block and a smooth surface.

The Tools: The Friction Kit, which includes a spring scale, friction block, three 50g weights and the friction board. When you finish your measurements you'll also need access to a calculator and a laptop.

The Tricks of the Trade: These might be helpful when you start your calculations!

• Force of Kinetic Friction: F_k = m_kN
• Force of Static Friction: F_s = m_sN
• What's N? Weight, in this case. How do you calculate weight?
• Conversion time: 1000g = 1kg

Directions:

Place the friction board on a level surface. Place the block on the board so that the larger of the sides is touching. Hook up the spring scale with the friction block. Hold the spring scale and pull it slowly with increasing force. The block will remain at rest when the force is small, and it will start to move when the force reaches a certain magnitude. Record the force at that moment. That is the F_s , or force of static friction. Pull the spring along the friction board with a steady speed and take a reading of the force. The force is equal to the kinetic friction force, F_k .

Now repeat the above steps after placing one, two and then three weights in the friction block.

Data table:

Object being pulled

Mass (in kg)

N

Fs

Fk

mk

Calculations:

Calculate the "N" for each scenario. Show your work. Add the results to your data chart.

Graph kinetic friction vs. normal force. (Normal force on x axis, friction on y axis)

Calculate m_k . Show your work. Add the results to your data chart.

Questions:

To be answered on a separate piece of paper as an individual for homework. Please use paragraph structure, complete sentences and observe the conventions of writing. This may be hand written or word-processed.

1. Discuss the results of your graph. What does it show? Is it a direct or inverse relationship?

2. m_k is called the coefficient of kinetic friction and should be a constant. Are your values for m_k the same or close? Find the slope of your graph and compare it to m_k. Do you see a relationship?

3. Compare the values of F_s and F_k for the same object. What do you notice? Can you explain why one value is larger than the other?

4. Predict what would happen if the board was propped up on one end so that it was a ramp. If you were pulling the block up the ramp, how might your measurements of friction change?

5. What are some possible sources of error in this experiment? What could you do to reduce error if you did this again?

Extension:

If you have time remaining,

• Try repeating the experiment, putting the smaller side of the friction block in contact with the board as you pull.
• Attempt to graph static friction vs. normal force.
• Investigate question #3 experimentally.