Shoe Friction

Purpose

To discover the relationship between the weight of an object and the force of friction on the object. Also the examine if this relationship changes when the object is placed on different surfaces.

Prelab

Make a prediction about how the frictional force will compare in each of the four situations given below. You must justify your prediction using four free body diagrams. Your free body diagrams should accurately display your understanding of the relative size and direction of the forces on the shoe, both in each situation, and across situations. For example, when the shoe contains more mass, it should have a stronger gravitational force (weight), so the downward force of weight should be larger in that case. Remember in each case the shoe is not being accelerated, so the FORCES MUST BE BALANCED, that does not mean that all four must be equal.

Each diagram should include the following forces: Weight, Normal, Applied, Friction

Experimental

Materials

• Shoe
• Spring Scale
• 3 Masses
• Two surfaces

Procedure

1. Suspend your shoe from the spring scale and record its weight.

2. Set your shoe on the first surface and pull with a slowly increasing force until the shoe lurches forward. Record the highest force reached as the applied force for this trial.

3. Add one mass to your shoe and repeat steps 1 and 2.

4. Repeats steps 1-3 for all masses and both surfaces until the table below is completely filled out.

Data

Put your data into the table below:

Analysis

Prepare a scatter plot of of the applied force (y-axis) vs. the weight of the shoe (x-axis). You can either have two separate graphs or one graph with two labeled plots. Determine the slope of the best fit line through the data points.

Discussion

1. We measured the applied force on the shoe, how does this force relate to the amount of friction on the shoe. How do you know?

2. Would this same relationship, between applied force and friction, hold true if you were pulling the shoe along at a constant speed (very hard to do)? Explain.

3. According to your data is there a relationship between the force of friction and the weight of the shoe? What is it?

4. How does the relationship change on a rough vs. a smooth surface?

5. The force of friction is actually better related to the normal force on the surface and not the weight of the object. Why do you think this is more accurate? Why does it not matter that we used the weight in this activity?

6. Evaluate how well your diagrams matched your lab data, in terms of the relative size of the forces between situations.