Review the key concepts, formulae, and examples before starting your quiz.
🔑Concepts
Friction is a contact force that acts when two surfaces rub against each other. It always works in the opposite direction to the way an object is moving, or trying to move.
The amount of friction depends on the materials the surfaces are made of. Rough surfaces, like sandpaper, create more friction than smooth surfaces, like ice.
Friction can be useful because it allows us to walk without slipping and helps car brakes stop a vehicle.
Friction can also be a nuisance because it causes wear and tear on moving parts and generates unwanted heat. We can reduce friction using lubricants like oil or grease.
Surface resistance is not just limited to solids; objects moving through air experience 'Air Resistance' and objects moving through water experience 'Water Resistance'. These are both types of friction called 'Drag'.
Force is measured in Newtons () using a device called a force meter or Newton meter.
📐Formulae
💡Examples
Problem 1:
A toy car is pushed across a carpet with a force of . The carpet creates a friction force of acting against the car. What is the resultant force moving the car forward?
Solution:
Explanation:
Because friction acts in the opposite direction to the push, we subtract the friction force from the pushing force to find the total force () moving the car.
Problem 2:
Why does a cyclist crouch down low over the handlebars when racing?
Solution:
To reduce the surface area facing the wind and minimize air resistance ().
Explanation:
By becoming more 'streamlined', the cyclist reduces the amount of air particles hitting them, which decreases the friction caused by the air.
Problem 3:
If you rub your hands together very fast for seconds, what happens to the temperature of your skin?
Solution:
The temperature increases (they get warmer).
Explanation:
Friction causes kinetic energy (movement) to be converted into thermal energy (heat). This is why surfaces get hot when they rub together.