Review the key concepts, formulae, and examples before starting your quiz.
🔑Concepts
Friction is a contact force that opposes the relative motion or the tendency of motion between two surfaces in contact. It always acts in the direction opposite to the direction of applied force.
The force of friction () depends on the nature of the surfaces in contact. Rough surfaces have more irregularities (ridges and grooves), leading to higher friction, while smooth surfaces have fewer irregularities, leading to lower friction.
Friction is directly proportional to the weight of the object or the force pressing the two surfaces together. If weight increases, the interlocking between surfaces becomes stronger, increasing the friction.
There are three main types of friction: Static Friction (acts when an object is at rest), Sliding Friction (acts when one surface slides over another), and Rolling Friction (acts when an object rolls over a surface).
The magnitude of friction follows the order: . This is why it is easier to roll a suitcase than to slide it.
Friction can be reduced using lubricants (oil, grease), polishing surfaces, using ball bearings, or streamlining the shape of objects moving through fluids.
Friction can be increased by making surfaces rougher, such as treading of tires or spikes in the shoes of athletes to provide better grip.
📐Formulae
(where is the Normal Force/Weight pressing the surfaces together)
(Order of Static, Kinetic/Sliding, and Rolling friction)
💡Examples
Problem 1:
A wooden crate of mass is pushed on a floor with a force of . If the force of friction acting against it is , what is the net force acting on the crate?
Solution:
The net force is .
Explanation:
Using the formula , we substitute the values: . The crate will move in the direction of the applied force with this resultant force.
Problem 2:
Why is it easier to pull a trolley fitted with wheels than to pull a wooden box of the same weight?
Solution:
Because rolling friction is much less than sliding friction ().
Explanation:
When wheels are used, the contact area between the surfaces is minimized, and the relative motion is rolling rather than sliding. Since is significantly smaller than , less force is required to overcome the friction.
Problem 3:
If a surface is lubricated with oil, how does it affect the friction?
Solution:
The friction decreases significantly.
Explanation:
Lubricants like oil form a thin layer between the two surfaces. This layer fills the microscopic irregularities (pits and peaks) of the surfaces, preventing them from interlocking closely. As a result, the frictional force () is reduced, allowing smoother motion.