Friction

When an object lies on or moves along a surface, there is a contact force. It is convenient to break that contact force into two components: one "normal" to the surface, the other "tangential" to the surface. In fact, the two components are often discussed as if they are two separate forces. Of course, as we saw previously in the force lab(s), it makes no difference to its effect whether it is two forces or one force (addition of vectors).

The force normal to the surface is called the normal force. The normal force prevents the object from breaking through the surface. Its magnitude exactly balances all other forces in the normal (perpendicular to the surface) direction.

The force along the tangential direction is called the frictional force. Friction behaves slightly differently depending on whether or not the object is moving relative to the surface.

• Kinetic: If the object is moving (has a non-zero velocity) relative to the surface, then friction opposes that motion, i.e., its direction is opposite to that of the velocity.
  
• Static: If an object is not moving relative to the surface, then the friction opposes any would-be motion, i.e. its magnitude is equal to and its direction opposite to the sum of the other tangential forces. There is a limit to the force that static friction can successfully oppose. When that limit is exceeded, motion results.

The frictional force is ultimately due to the interaction of the object atoms with the surface atoms and vice versa. The strength (magnitude) of the frictional force might depend on the following:

1. the number of atoms in contact (microscopic) or the surface area of contact (macroscopic)
   
2. the distance between interacting atoms (microscopic) or the pressure (force per area) exterted at the surface (macroscopic); the greater the pressure, the closer the atoms are squeezed together
   
3. the kind of atoms (microscopic) or the materials from which the block and surface are made 
   

In the first part of this course, we usually approximate objects by point particles. To keep consistent with this approximation, we assume that the area dependences of the first two considerations above exactly cancel. One is then left with the frictional force being proportional to the force exterted perpendicular to the surface, that is, the normal force. Any material dependence from the third consideration will be encoded in the proportionality constant called the coefficient of friction.

Friction: Finding the coefficient of static friction

In a static situation, the magnitude of the friction force is whatever it has to be to balance other tangential forces up to some limit, that is,

F_f < m_s N,

where N is the magnitude of the normal force and m_s is called the coefficient of static friction. The expression m_s N is the largest the static friction can be. If the opposing tangential forces exceed it, motion results. The direction of the frictional force is opposite to the direction of motion the object would have in the absence of friction.

Measurements

• Set up a track as shown below

  

• Record the length of the track and masses of the various blocks.
  
  

  Length of track (   )
  Mass of friction block (   )
  Mass of heavy metal block (   )

  
  
  
• Make sure your track is clean. Sticky parts on the track will lead to nonuniform coefficients of friction. Why?
• Place a friction block (two views of which are shown below) with the wide wooden surface against the track. Starting at small angles, increase the angle until the block first begins to move. Record the height, then use trigonometry to find the corresponding angle. (The block just beginning to moves means we are at the upper limit for static friction.)
  
  
  
  
  
• Repeat this measurement two more times for a total of three trials for the wide wooden surface of the friction block.
• Repeat the measurements for the narrow wooden side, the wide felt side, and the narrow felt side.
  
  

  	Height trial 1 ( )	Height  trial 2 ( )	Height  trial 3 ( )	Angle  trial 1  ( )	Angle  trial 2  ( )	Angle  trial 3  ( )	ms  trial 1	ms  trial 2	ms  trial 3	ms  Ave	ms  St. dev.
  Felt Wide
  Felt Narrow
  Wood Wide
  Wood Narow

  
  
  
• Choosing either Felt Wide or Wood Wide, place one of the heavy metal blocks on top of the friction block and repeat the measurements (three trials). You may need to tape or some other way to secure the weights onto the friction blocks. Place a second metal block on the friction block and take another set of measurements.
  
  

  	Height trial 1 ( )	Height  trial 2 ( )	Height  trial 3 ( )	Angle  trial 1  ( )	Angle  trial 2  ( )	Angle  trial 3  ( )	ms  trial 1	ms  trial 2	ms  trial 3	ms  Ave	ms  St. dev.
  Friction Block Alone
  With One Heavy Block
  With Two Heavy Blocks

Analysis

• Once the block starts moving, does it seem to travel down the track at a constant velocity? Or does it appear to be accelerating?
• In your write-up, draw the set-up above. Draw all of the forces acting on the block (the weight, the normal force and the frictional force).
• Break the forces into their components in the coordinate system shown above.
• At an angle just below the one at which the block began to move, the forces were in equilibrium. Thus the net force was zero. This should provide you with two equations (one for the x components, one for the y components).
• Using these equations and the form above for the maximal static friction force, find an expression for m_s. (It should depend only on the angle.)
• Use this expression to find m_s for your three runs above. Find the standard deviation as well.
• What are the dimensions of m_s?
• Do you observe any dependence on the cofficient of friction you found on the area of contact?
• Do you observe any dependence on the material?
• Do you observe any differences when you add the heavy block on top?

Friction: Finding the coefficient of kinetic friction

When the object is moving along the surface, we have what is called kinetic friction, and in our approximation the formula is

F_f = m_k N,

(In the case of kinetic friction we have an equation rather than an inequality; the equation applies provided the object is moving.)

Measurements

• Obtain the various masses: friction block, metal block.
  
  

  Mass of friction block (   )
  Mass on hanger including hanger (   )
  Mass of metal blocks (   )

  
  
  
• Using a universal clamp, secure a Smart Pulley at the edge of the lab bench.
  
  
  
  
  
  
• Plug the Smart Pulley into the Digital Channel 1 of the Data Studio Interface.
• Start Data Studio, click on the digital plug icon into the Digital Channel 1 icon, and select Smart Pulley from the menu. (It is also possible to make the measurements using a Motion Sensor on the other end of the track.) Remember to set the Sampling Rate (Frequency) high enough, e.g. 20 or 50 Hz. (50 Hz provides more data but is sometimes problematic.)
• Tie one end of a string to the friction block and the other end to a 50-g hanger.
• Place the two heavy metal blocks on top of the friction block. Add enough mass to the hanger so that it sets the friction block into motion. (We want an acceleration here, but a small one.)
• Note the distance between the hanger and the floor so you can stop the friction block before it travels that distance.
• Click Start and the release the friction block.
• Copy the position vs time data and use Excel to graph it.
• Fit it a polynomial of order two and extract the acceleration. (Remember to leave out any data from the start or end where the "physics changed", e.g. if it hit the floor. If the data is not reasonably well fit by a polynomial of order 2, consider retaking it.)
• Repeat the process for each of the two wide surfaces of the friction block being in contact with the track.

Analysis

Looking at the forces acting on the block and the hanger and breaking the block forces into components yields the following three equations:

where the subscript b indicates block and the subscript h indicates hanger. If the string and pulley are considered massless, then T_h=T_b. Furthermore, if the string does not stretch or break then a_h=a_b. Solve the equations above for m_k. Enter the value you find for the coefficient of kinetic friction in the table.

Do you observe any dependence on the material? How do the kinetic coefficents compare to the static coefficients?