Question

Give two examples of (a) positive work done by a frictional force (if possible) and (b) negative work done by a frictional force.

Short answer

Positive work by friction occurs on a moving conveyor belt and an accelerating car; negative work occurs when an object slides to a stop or a moving box slows on a rough surface.

Step-by-step solution

Understanding Work Done by Friction

Work by friction is determined by the force of friction and the displacement of the object. If friction acts in the direction of displacement, it does positive work; if it acts opposite to displacement, it does negative work.

Analyze Positive Work by Friction

While it is rare, friction can do positive work. One example is when an object is on a conveyor belt: the belt's motion causes the frictional force to act in the same direction as the object's displacement, making the work done by friction positive.

Consider Another Scenario for Positive Work

A less common example is during the acceleration of a car on a road. The friction between the tires and road propels the car forward, doing positive work as the car moves in the direction of the frictional force.

Define Negative Work by Friction

Negative work by friction commonly occurs when friction opposes the direction of motion. This is evident when a sliding object slows down due to the friction between its surface and the ground.

Provide Another Example of Negative Work

Another instance is when a box is pushed across a rough surface and comes to a stop. The frictional force acts opposite to its motion, performing negative work until the box halts.

Key concepts

Positive Work

Positive work occurs when the force applied, in this case friction, acts in the same direction as the object's displacement. While not a common phenomenon with friction, it can happen under specific circumstances. One such example is a conveyor belt moving an object. Here, the frictional force between the belt and the object acts in the direction the belt is moving, thereby performing positive work. Similarly, when a car accelerates, the friction between the tires and the road surface acts in the same direction as the car's movement—forward. Thus, friction contributes positively to the work done, assisting in the car’s acceleration. In situations of positive work, energy is transferred to the object, increasing its kinetic energy.

Negative Work

Negative work is more commonly associated with friction as it generally works against motion. This occurs when the frictional force and the object’s displacement are in opposite directions. A classic example is an object sliding to a stop, like a sled coming to rest on snow. Here, friction between the sled and snow opposes the sled's movement, doing negative work until the sled stops moving. Similarly, when a box is pushed across a rough floor and eventually stops, the force of friction acts opposite to the direction of the push, performing negative work and reducing the object’s kinetic energy. Negative work typically results in the energy from the object being dissipated, often as thermal energy due to friction.

Frictional Force

Frictional force is the resistance that one surface or object encounters when moving over another. It is a force that acts parallel to the surfaces in contact and can do both positive and negative work. The direction and nature of work done by friction depend on how it interacts with the motion of objects. Key characteristics of frictional force include:

• It always acts parallel to the contact surfaces.
• Can be classified as static (preventing motion) or kinetic (opposing motion).
• Involves interaction at a microscopic level, such as surface roughness and material properties.
• Can dissipate energy, often as heat, when doing negative work.

Understanding frictional force is crucial in explaining why objects start, stop, or continue moving.

Physics Concepts

Understanding the physics concepts of work and friction enhances our comprehension of motion in everyday life. Work in physics is defined as the product of force and displacement along the direction of the force. Additionally, friction is a force that opposes or aids in the the movement of objects across surfaces. It plays a significant role in both facilitating and opposing motion. When discussing work done by friction, we consider both direction and energy transfer:

• Positive work signifies that energy is transferred to an object, increasing its kinetic energy.
• Negative work indicates energy is being dissipated from the object, often as heat or sound.

By exploring these concepts, students gain insight into how forces affect motion, energy efficiency, and the design of mechanisms in vehicles and machinery.