Friction is a Force That Opposes Motion
Friction
A force that opposes the motion between two surfaces in contact, acting parallel to the surfaces and in the opposite direction of motion.
When you push a box across the floor, friction acts against the direction of your push, making it harder to move the box.
Static and Dynamic Friction
- There are two main types of friction:
- Static Friction: The force that prevents two surfaces from sliding past each other when at rest.
- Dynamic Friction: The force that opposes the motion of two surfaces sliding against each other.
Dynamic Friction
Dynamic friction is the force that resists the motion of an object that is already moving. It's also known as kinetic friction.
Static Friction
Static friction is the force that prevents objects from moving, while dynamic friction is the force that opposes the motion of objects that are already moving
Static friction is always greater than dynamic friction. This is why it's harder to start moving a stationary object than to keep it moving.
Key differences between static and dynamic friction:
- Static friction is typically greater than dynamic friction
- Static friction acts before motion begins
- Dynamic friction remains constant during motion
Try sliding a hockey puck across ice:
- More force needed to start the slide (static friction)
- Less force needed to keep it sliding (dynamic friction)
- Smooth ice reduces both types of friction
The Coefficient of Friction
Coefficient of Friction
A number that measures how much friction exists between two surfaces. It's also a measure of how much force is required to move an object across a surface
- The coefficient of friction is a dimensionless value that quantifies how much friction exists between two surfaces.
- It depends on the materials in contact and their surface characteristics.
- The coefficient of friction (μ) determines friction force:
- For static friction: Fs = μs × N
- For dynamic friction: Fk = μk × N
- Where N is the normal force (perpendicular to surface)
- Typical coefficient values:
- Ice on ice: 0.1
- Rubber on concrete: 0.8
- Sport-specific surfaces vary widely
Students often think heavier objects have higher coefficients of friction. Remember: the coefficient (μ) is independent of weight - it only depends on the surfaces in contact.
Factors Affecting the Coefficient of Friction
- Surface characteristics:
- Roughness/smoothness
- Material composition
- Temperature
- Cleanliness
- Environmental conditions:
- Moisture
- Temperature
- Debris
- Air pressure (for air resistance)
- Applied forces:
- Normal force
- Direction of motion
- Speed of movement
How does our understanding of friction in sports challenge the notion that "natural" athletic ability is all that matters? Consider how technology and science have transformed sports equipment and techniques.
NoteDon't confuse the coefficient of friction with the frictional force. The coefficient is a ratio and has no units, while the frictional force is measured in newtons (N).
Modifying Friction in Sports
- Increasing friction for better performance:
- Gymnasts use chalk on hands and apparatus
- Basketball players choose high-traction shoes
- Rock climbers use specialized rubber footwear
- Rugby players wear studded boots
- Decreasing friction for speed:
- Skiers wax their skis
- Swimmers wear streamlined suits
- Bobsled runners are highly polished
- Speed skaters minimize ice contact
In most sports, optimal performance requires managing friction - not just maximising or minimising it. Think about when each type of friction helps or hinders your sport.
