Physics 2A

Coefficient of Friction Lab

Objectives: To investigate the nature of the frictional forces between a smooth metal block and a smooth flat wooden board.

Equipment: Wooden inclined plane, pulley, slotted mass and mass holder, protractor

Inclined Plane: Whenever two bodies interact by direct contact (touching) of their surfaces, we call the interaction forces contact forces. The force that opposes the sliding of the bodies is known as the friction force : the friction force is parallel to the contact surface and always directed against the applied force. Normal (or support) force n is perpendicular to the contact surface and is equal to the perpendicular component of the weight of the body.

There are two different kinds of friction - static and kinetic. Static friction f_s arises when an attempt is made to slide a body at rest. As the applied force is increased to move the body, the force of static friction also increases. At some point the applied force becomes greater than the maximum static friction the surface can exert and the body breaks loose.

(1)

Here m_s is the coefficient of static friction and n is the normal force.

Now consider the following inclined plane whose inclination angle q can be adjusted. If a block is placed on the plane and, and the angle is slowly increased, a point will be reached at which the block begins to slide.

At the point when the block just slips, the maximum frictional force is exerted and we get:

(2)

A component of the weight of the block, mgcosq, is supported by the normal force n.

(3)

Combining equations (2) and (3) we get

The same inclined plane can be used to determine the coefficient of kinetic friction. Decrease the angle and give the block a slight push so that it slides down the plane at constant velocity. The static frictional force is greater than the kinetic frictional force.

Coefficient of Static Friction: Procedure

1. Incline the plane until the mass starts slipping.
2. Note the angle of the incline.
3. Calculate the coefficient of friction.

Table 1

	1	2	3	4
qs
ms
qk
mk

2. Horizontal Plane: Place the board in the horizontal position as given in the following figure. The following forces describe the situation:

(4)

Here T is the tension in the string. From Eq (4) we can derive:

(5)

Measure mass m₁. Add mass m₂ to the holder until the system moves. Record the masses in Table 2. Add mass to m₁ and repeat the procedure. Calculate the coefficient of static friction for each of the cases. Follow the procedure for Expt. 1 to calculate the coefficient of kinetic friction.

Table 2

Added Mass	0 kg	0.2 kg	0.4 kg
m1
m2 trial 1
m2 trial 2
m2 trial 3
Avg. m2
ms
mk

Conclusion:

1) Why is m_s > m_k ?

2) Does your data confirm that the coefficients of friction are independent of normal forces.

3) State in your own words what was accomplished in today’s laboratory.