Static and Kinetic Friction

Introduction

If you try to slide a heavy box resting on the floor, you may find it difficult to get the box moving. Static friction is the force that is counters your force on the box. If you apply a light horizontal push that does not move the box, the static friction force is also small and directly opposite to your push. If you push harder, the friction force increases to match the magnitude of your push. There is a limit to the magnitude of static friction, so eventually you may be able to apply a force larger than the maximum static force, and the box will move. The maximum static friction force is sometimes referred to as starting friction. We model static friction, F_static, with the inequality F_static ≤ μ_s N where μ_s is the coefficient of static friction and N the normal force exerted by a surface on the object. The normal force is defined as the perpendicular component of the force exerted by the surface. In this case, the normal force is equal to the weight of the object.

Objectives

• Use a Dual-Range Force Sensor to measure the force of static friction.
• Determine the relationship between force of static friction and the weight of an object.
• Measure the coefficients of static and kinetic friction for a particular block and track.
• Use a Motion Detector to independently measure the coefficient of kinetic friction and compare it to the previously measured value.
• Determine if the coefficient of kinetic friction depends on weight.