3. A Simple Test of Static Friction

In this section, you will attempt to measure the forces of static and kinetic friction directly. You will determine what physical characteristics of the system contribute to the frictional force.

3.1: In sections 3 and 4, you will use counterweight with known mass (shown at right). With the Brakes On, drag the spool of string to the left connection on the cart and it will snap in place. Drag the mass hanger to the loop on the end of the string. It will also snap into place. Turn on the friction pad and set µKS to # assigned by your TA.

3.2: Consider this system when the cart and hanger are stationary and derive expressions for the tension in the string and the friction force in terms of measurable quantities.

• Draw free-body diagrams for both the cart and the hanger.

• From your free-body diagrams, write Newton’s 2nd law equations for both the cart and the hanger.

• Solve this system of equations to get expressions for the frictional force in terms of the mass of the hanger, the mass of the cart, and the acceleration of the system.

Q: What does this tell you about the nature of static friction? Does it always equal µsN (where us is the coefficient of static friction and N is the normal force between the cart and track)?

3.3: You can use the following procedure to measure the maximum value of the static frictional force acting on the cart.

• Drag the cart fully to the right of the track. Add enough mass to the cart to make its total mass 1kg. Set Brakes Off.

• The mass of the hanger provides the pulling force on the cart. You will record data in an excel document with Hanger Mass in one column and then another with your calculation of Force pulling the cart.

• Increase the pulling force slowly, 10g increments, until the cart starts to move.

Q: Even though you are measuring the pulling force rather than the frictional force, the two have equal magnitude when the cart is not moving. Explain why this is (and draw pictures/diagrams to help).