Video Lesson

Simulation

https://phet.colorado.edu/en/simulations/friction

Lesson Objective

Dear Learners,

At the end of this section, you should be able to:

• Define the force of friction;
• Explain the dependence of friction force on the smoothness and roughness of surfaces;
• Explain the advantage and disadvantage of the force of friction.
• Define and describe the scientific meaning of work;
• Describe the quantitative relationship between work, force and displacement;
• Calculate the work done on an object.

Brainstorming Question

Why is it difficult to walk on a smooth surface?

If you carry a sack of teff from your home to the market, how much work do you do?

key terms and concepts

Frictional Force

Video

Frictional force is the resistance force that occurs when two surfaces interact as they move or attempt to move past each other. Forces of friction are very important in our every day lives. They allow us to walk or run and are necessary for the motion of wheeled vehicles. We classify friction forces in to two main types. They are static friction and kinetic friction.

1. Static Friction: Prevents motion between two surfaces that are not moving relative to each other.

2. Kinetic (Sliding) Friction: Acts between surfaces in relative motion.

Properties

• Dependent on the nature of the surfaces and the normal force pressing them together.
• Converts kinetic energy into thermal energy (heat).

Examples:

• Pushing a book across a table.
• Car tires gripping the road.

Concept of Mechanical Work

Mechanical work is done when a force applied to an object causes displacement in the direction of the force. Work is a scalar quantity. The SI unit of work is the Joule (J), where 1 Joule is equivalent to 1 Newton-meter (N·m). Mathematically, work (W) is defined as the product of the force (F) and the displacement (S) in the direction of the force.

• The formula is W=F⋅S.cos(θ),

where θ is the angle between the force and the direction of displacement.

• If the force and displacement are in the same direction, θ=0 and cos(0)=1, so W=F⋅S
• If the force is perpendicular to the direction of displacement, no work is done because (θ=90∘) and cos(90∘)=0 so W=0.

Example:

A person pushes a 10 kg box with a force of 20 N across a floor for a distance of 5 meters in the direction of the force.

Given values:

• Force, F=20N
• Displacement, S =5
• Angle, θ=0∘ (since the force and displacement are in the same direction)

Required values: W=?

Solution:

W = F⋅S⋅cos(θ)

• cos(0∘)=1
• W=20N⋅5m⋅1
• W=100J