Explain the difference between useful and unwanted friction and describe an example of each.

Useful friction and unwanted friction are two types of friction that occur in our daily lives.

Useful friction is the friction that is necessary and beneficial for certain activities. It is the force that prevents our movements from slipping or sliding. For example, when we walk or run, the friction between our shoes and the ground helps us move forward without slipping. Similarly, the friction between the tires of a vehicle and the road surface allows the vehicle to move without skidding.

On the other hand, unwanted friction is the friction that is not needed and can cause harm or inefficiency. It is the force that opposes motion between two surfaces that are in contact. For example, when you're cycling, the friction between the air and the bicycle slows down the bicycle. This is why cyclists often wear streamlined clothing to reduce this air resistance. Similarly, in machines, unwanted friction between moving parts can cause wear and tear, leading to a decrease in performance and efficiency.

In summary, while useful friction aids in movement and provides grip, unwanted friction opposes movement and can cause damage or inefficiency.