AP Physics Lab 5

Tyler Kolby, Gabrielle Murphy, Ryan Partain, Trey Seabrooke 

Friction Lab

Objective:
To determine the graphical and mathematical relationship between normal force, surface texture, and surface area in max friction (static) and constant velocity friction (kinetic)

Apparatus:

Procedures:

1. Measure the mass of the block.

2. Place the block on its side. Connect the spring scale to the hook on the block.

3. Pull scale slowly until block starts moving at a constant velocity, making sure to pull

   horizontally.

4. Watch the force of tension on the scale (record the maximum tension and tension at

   constant velocity).

5. Add weights to the block 10 times, adding about 50g each time.

   6. Repeat steps 14 for each new trial, starting at the same position. Data Table:

   
   

   
   

   Graphs:

   Maximum Force vs Normal Force Graph 

   
   

   
   

   Constant Velocity Force vs Normal Force Graph

   
   

   

   Conclusion:

   Because the block did not move vertically, the normal force the table exerts on the block is equal to the force of gravity the Earth exerts on the block, which is the block’s weight. The normal force can then be calculated by using the Field Constant, 10 N/kg, multiplied by the mass of the block in kg. As more mass was added to the block, the weight of the block also increased.

   The maximum force vs normal force graph shows a direct, linear relationship. As the normal force the table exerted on the block increased, the maximum force applied to the block increased. This graph shows the relationship between the normal force and the friction force when it was not moving, or static. The graph can be described by the equation: F(max) ≤ (0.643)F(N), where F(max) equals the force of friction.

   The block did not move until a force greater than the friction force was applied to it. At the maximum force, the force of tension exerted on the block was equal to the force of friction exerted on the block. We can say that the maximum force applied to the block was a friction force because it was the greatest amount of force that had to be overcome before the block started moving and that the force of tension was equal to the force of friction at that instant. Because the

   The constant velocity force vs the normal force graph also has a direct, linear relationship. As the normal force increased, the force at constant velocity increased. This graph shows the relationship between the normal force and the friction force when the block was moving, or kinetic, and the slope of this graph is smaller than the slope of the graph when the object was static. It can be described by the equation: F(CV)=(0.420)F(N).

   Normal force is compared to the friction force instead of mass because the normal force of the block is a measure of the relationship between the surface of the block and the surface of the table (the magnitude each object “pushes” the other). The friction force also measures the relationship between the surface of the block and the surface of the table (the magnitude each object “slides” against the other). Mass, on the other hand, is the measure of how much matter is in an object. This measurement would not change from surface to surface or place to place (planets with different gravitational pulls). The normal force and the friction force can depend on these changes, which affect the way two objects relate to each other.

   When the experiment was done on a different surface (a smoother floor), the slope of the graphs were smaller than the slope of the graphs on the table surface. There was a smaller force of friction between the floor and the block compared to the force of friction between the table and the block; however, the relationship between the friction force and the normal force could still be described using the same general equation.

   The overall general equation from this experiment is: F(F)=(μ)F(N), where F(F) is the maximum friction force a surface can exert on an object. This relates the maximum force of friction to the normal force using the coefficient of friction, μ, which can vary depending on the surface and the normal force acting on the object. The equation simply allows one to find the force of friction if given the normal force and the coefficient of friction, and vice versa.

   Like in all experiments, there is room for a source of error. We could have pulled the scale too quickly and caused it to be difficult to find an accurate reading of the force applied to the block. We also could have read inaccurately off of the scale and written down the wrong measurements. This applies to both the reading for the maximum force and the constant velocity force. The scale measuring the mass also could have been off by a little, throwing off the calculations later on.