Friction: the coefficient of sliding friction

Objectives

• Measure sliding friction with a spring balance.
• Show that friction is proportional to the normal force.
• Find a coefficient of friction and compare surfaces.

Theory

When a block is pulled at steady speed the pulling force equals the friction force. Sliding friction is proportional to the normal force pressing the surfaces together: F = μN, where N = mg on a level bench and μ is the coefficient of sliding friction for that pair of surfaces. A graph of friction against normal force is a straight line through the origin with gradient μ. Friction depends on the nature of the surfaces but not on the area of contact.

F = μN N = mg

Procedure

1. On the wooden bench, pull the block at steady speed and read the balance.
2. Add masses and repeat for at least five different loads on the wooden bench.
3. Turn the block onto its narrow face and take a reading.
4. Take a reading on sandpaper.
5. Plot pulling force against normal force for the wooden bench and note the gradient.

Precautions

• Pull horizontally and at a steady speed.
• Read the balance while the block is moving, not as it starts.
• Keep the surface clean and dry.

Subject

Physics

Grade

9–11

Chapter

Force and Motion (Nepal National Curriculum (CDC), grade 9)

Difficulty

Easy

Duration

20 min

Safety level

Low

Version

1.0

Apparatus

• Wooden block with hook
• Slotted masses
• Spring balance (0–10 N)
• Bench surfaces