Question

Give Examples 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

(a) Friction on a conveyor belt aids in moving objects. (b) Friction opposes a sliding book on a table.

Step-by-step solution

Understanding Positive Work

In physics, work done by a force is positive when the force and the motion are in the same direction. So, for friction to do positive work, it must aid the motion in the direction of movement.

Providing Example for Positive Work

Consider a conveyor belt moving an object to the right. If friction between the belt and the object is in the same direction (rightward), then friction does positive work as it helps move the object along.

Understanding Negative Work

Work is negative when the force opposes the direction of motion. Friction commonly does negative work by opposing the movement of an object, slowing it down, or bringing it to a stop.

Providing Example for Negative Work

Imagine pushing a book across a table to the right. Friction between the book and the table acts to the left (opposite motion), hence performs negative work by slowing the book down.

Key concepts

Positive Work

Positive work occurs when a force works in the same direction as an object's movement. In such cases, the force contributes to increasing the object's speed or maintaining its movement. This is less common when discussing friction because friction typically opposes motion.
However, one potential scenario where friction does positive work involves a roller coaster ride. When the ride is on a downward slope, and the wheels create friction between the track and the coasters in the same direction, this friction aids the coaster's movement as it speeds up, therefore doing positive work in helping it along its path. This is a rare condition and heavily depends on the mechanical setup.

Negative Work

Negative work occurs when a force acts against the direction of motion. Friction often does negative work because it usually resists movement.
When you slide a box across the floor, the force of friction works against the box's motion. As you push the box forward, the frictional force acts in the opposite direction, effectively reducing the box's speed. This causes the box to slow down and eventually stop.
In this way, frictional force is doing negative work as it helps dissipate energy, transforming kinetic energy into thermal energy, which is evident when the surface feels warmer.

Frictional Force

Frictional force is a resistive force that acts against the relative motion of two surfaces in contact. It plays a crucial role in everyday life and affects almost all movement.
Friction can be static (preventing motion) or kinetic (opposing motion). Static friction keeps objects stationary until the applied force overcomes it and is typically higher than kinetic friction.
Without friction, walking would be impossible as our feet would slide on the ground. Automobiles rely on friction between tires and road surfaces to accelerate, decelerate, and turn safely. Any positive or negative work done by friction is a result of its intrinsic resistance properties.

Physics Examples

To illustrate these concepts, consider examples from everyday life:

• Walking involves friction between your shoes and the ground, where static friction provides the necessary grip to push your body forward, facilitating movement.
• A ball rolling on grass eventually stops due to the negative work done by friction, which gradually reduces its velocity.
• Car brakes apply friction to the wheels, converting kinetic energy into thermal energy, slowing the car down and eventually stopping it, demonstrating negative work.
• On an escalator, friction sometimes assists in the movement of steps and the mechanical components do positive work to carry passengers upwards.

These examples of positive and negative work due to friction help us better understand the balance of forces in motion.