PHYS 121 - Week 4 Lecture 2 - Friction & Circular Motion

Summary

In this lecture on friction in circular motion, the speaker explains the concepts of static and kinetic friction, emphasizing their roles in preventing motion. Through examples, such as a block on an inclined plane and a car navigating an unbanked curve, the dynamics of uniform circular motion are explored. The speaker highlights the relationship between friction coefficients and surface interactions, illustrating how static friction is often greater than kinetic friction. A sample problem illustrates how to calculate the coefficient of static friction in practical scenarios, reinforcing the principles of Newton's laws and circular motion.

Takeaways

• 😀 Friction is a force resulting from the interaction of two surfaces in contact, preventing motion.
• 😀 Static friction prevents an object from starting to move, while kinetic friction acts on objects that are already in motion.
• 😀 The static friction force can increase to match the applied force up to a maximum value, after which motion occurs.
• 😀 The coefficient of static friction is generally larger than the coefficient of kinetic friction, meaning it's harder to start moving an object than to keep it moving.
• 😀 The coefficient of static friction (μs) can be calculated using the formula: μs = tan(θ) when the object is on an incline.
• 😀 In uniform circular motion, the net force is directed towards the center of the circle, allowing the object to maintain its circular path.
• 😀 An unbanked curve requires static friction to keep a vehicle on its circular path, while a banked curve utilizes the incline to assist in maintaining motion.
• 😀 A conical pendulum demonstrates uniform circular motion, where tension and friction provide the necessary forces to maintain the circular path.
• 😀 The centripetal acceleration for an object in circular motion can be calculated using the formula: a_c = v² / r.
• 😀 Understanding the dynamics of friction and circular motion is crucial for solving problems related to forces and motion in physics.

Q & A

What is the primary force that stops a rolling ball from continuing to move indefinitely?

The primary force that stops a rolling ball is friction, which arises from the interaction of the surfaces in contact.

What are the two types of friction discussed in the lecture?

The two types of friction discussed are static friction, which prevents an object from moving, and kinetic friction, which acts on moving objects.

How does static friction respond to an applied force?

Static friction increases to counteract the applied force up to its maximum value, beyond which the object begins to move.

What is the relationship between the coefficients of static and kinetic friction?

The coefficient of static friction is typically larger than the coefficient of kinetic friction, meaning it requires more force to initiate movement than to maintain it.

How do you calculate the coefficient of static friction in the example problem presented?

The coefficient of static friction is calculated using the tangent of the angle of inclination at which slipping begins, which in the example was found to be 0.27.

What happens to an object's motion when it is on an inclined plane?

On an inclined plane, the forces acting on the object include gravitational force and normal force, and static friction acts to prevent it from sliding down until the incline is steep enough to overcome this friction.

In uniform circular motion, what force is responsible for keeping the object in a circular path?

In uniform circular motion, the static friction force acts toward the center of the circle, preventing the object from sliding outward.

What distinguishes an unbanked curve from a banked curve?

An unbanked curve has a flat road surface requiring friction to maintain circular motion, while a banked curve has an incline that helps provide the necessary centripetal force without relying solely on friction.

What formula is used to determine the frictional force required for a car to navigate a curve?

The frictional force is determined by the equation: static friction force = mass × centripetal acceleration, where centripetal acceleration is calculated using v^2 / r.

What was the minimum coefficient of static friction calculated for the car in the example problem?

The minimum coefficient of static friction required for the car to navigate the curve safely was calculated to be 0.3.