To Study the Variation in Limiting Friction with Mass and the Nature of Surfaces in Contact

# Objective

To study the variation in limiting friction with mass and the nature of surfaces in contact.

# Theory

Reason of Friction

Why do balls stop motion after some time? Why do you fall on smooth surfaces? The reason for these situations is friction.

When we zoom in on the interface between two surfaces, we can see there are a lot of irregularities. Irregularities of two surfaces interlock each other, which opposes the relative motion of two surfaces and produces the force of friction.

Figure 1: Surface irregularities

Definition of friction

Friction is a force that opposes relative motion between systems in contact.

Characteristics of frictional force

• The frictional force is a contact force. Frictional forces come into play only when two surfaces are in contact.
• Frictional force opposes the relative motion or sliding between two surfaces in contact.
• The frictional force acts in the direction opposite to the applied force.
• The frictional force is parallel to the contact surface.

If we want to move the object, we want to apply some force to overcome frictional force.

Different types of friction

• Static friction- When two objects are in contact and stationary relative to each other, friction between these two objects is called static friction.
• Kinetic friction- When two objects are in contact and moving relative to each other, friction between these objects is known as kinetic friction.
• Limiting friction- When the applied force increases, static friction also increases. There is a maximum value of static friction known as limiting friction. If the applied force is greater than the limiting friction, the object will initiate motion.

Which is easier to move—an object that is stationary or an object that is in motion?

• When moving an object already in motion, it doesn't have sufficient time to interlock with another surface.
• Therefore, kinetic friction is always less than static friction.
• Hence, moving an object that is already in motion is easier than moving a stationary object.

Factors affecting limiting friction

• As the weight of the object sliding on a given surface increases, limiting friction increases. The harder the surfaces are pushed together, the larger the limiting friction.
• The limiting friction also depends on the nature of the surfaces in contact. A rough surface exhibits more irregularities, resulting in a higher limiting friction. A smooth surface has fewer irregularities, leading to a lower limiting friction.

Friction has both pros and cons simultaneously

Pros

• The friction between the soles of our shoes and the ground helps us walk without slipping.
• The friction between pen and paper helps to write.
• The friction helps to light the matchstick.

Cons

• Friction between moving parts in machines causes wear and tear.
• Friction wears out the soles of shoes.

# Learning Outcomes

• Students understand the concept of friction.
• Students understand how limiting friction changes with the mass of an object.
• Students understand how limiting friction changes in different types of surfaces.