Topic 10 - Friction and Drag (HL) Flashcards
10.1.1. Describe Friction
the force exerted by a surface as an object moves across it or makes an effort to move across it
The interaction between the surface of two materials in contact
two types: static and dynamic
Static Friction
When two surfaces are not moving relative to each other. It must be overcome to start moving the object.
Dynamic/kinetic (Sliding) Friction
Friction that opposes the movement of a body which is already in motion
E.g. Pushing a box across a floor. The floor surface offers resistance to the movement of the box. The floor exerts a friction force upon the box.
10.1.2. Describe the coefficient of friction
- The coefficient of friction is a measure of the amount of friction between two objects
- Its value depends on the materials in contact; e.g., steel on ice has a low COF, while rubber on the ground has a high COF
- Greater molecular interactions between surfaces result in a higher COF
- COF typically ranges from 0 to 1 but can exceed 1, such as 4 for drag racing tires on concrete
Coefficient of friction for a variety of surfaces
Rubber on concrete = 1.0
Ice on ice = 0.1
Waxed wood on snow = 0.14
Copper on steel = 0.53
Coefficient of friction can be calculated from
Ff = μR
Ff: force of friction
μ: coefficient of friction
R: reaction or normal force (mass X gravity)
10.1.3 Distinguish between the coefficient of static friction and dynamic friction
When a force is applied to attempt to move a stationary object over another surface, we consider the coefficient of static friction.
At some point, the force applied is sufficient to overcome the static friction and the object will begin to move. Once the object is in motion, we consider the coefficient of dynamic friction.
The coefficient of dynamic friction is usually lower than the coefficient of static friction.
10.1.4. Explain the influence of friction on sports performance
Running:
- Runner will use friction to decide their choice of footwear as spiked running shoes will create more friction, this ensures that no energy will be lost due to slipping backwards
- Marathon runners may use lubricants on their skin, to reduced the friction between clothing and their skin, prevents rubbing.
Tennis:
- Different courts has different surfaces (grass, clay) resulting in different friction. Can influence players performance e.g. Nadal plays very well on clay courts
Racing:
- Car tyres and track surface friction is very important. F1 cars needs enough friction from their wheels to go quickly
10.1.5. Define drag
Force or forces acting to oppose the motion of an object through a fluid medium such as air or water
Drag is still a type of friction but relates to fluid and not two materials in contact
10.1.6. Outline different types of drag that can be found in a variety of sporting environments
Surface - referring to interaction between body surface and the fluid
Form - refers to resistive forces caused by shape of object or body
Wave - opposing force caused by the body or object making waves in the fluid
Surface Drag (10.1.6)
- As a body moves through a fluid, its surface slows nearby fluid, causing drag.
- Can be minimized by reducing surface interaction e.g. using shark-skin suits or shaving to smooth the swimmer’s body
Form Drag (10.1.6)
- Form Drag, also called profile drag, is influenced by your shape and amount of cross sectional area you present to the water
- Streamlining the body and minimizing the surface area facing the direction of the motion can reduce drag
- Example: Cyclists adopting a low profile position
Wave Drag (or Frontal Resistance) (10.1.6)
- When a body moves along the
surface of a fluid (usually water) some fluid is displaced to form a wave. - These waves cause additional forces that oppose motion. Wave drag can be reduced by avoiding motion at the interface between air and water.
Example: Swimming underwater for as long as is allowed at the start of a race.
Fluid Flow
The amount of drag is related to the flow pattern of the opposing fluid at the boundary layer
Streamlining (10.1.7)
Streamlined bodies incorporate gradual tapering to minimize pressure effect and separation of fluid.
A streamlined shape allows air to flow smoothly in layers, while a non-streamlined helmet creates vortices due to disrupted airflow.
When this happens bits of fluid are randomly flung sideways which causes drag.
The drag is caused by bits of fluid being dragged along with the moving object (the cycle helmet)
Aerodynamics (10.1.7)
The shape of an object affects air flow.
- E.g. If the object is a brick shaped object it causes turbulence because of its shape but if the object was more streamlined, then it creates minimal disruption to the air at its boundary layer.
The nature of the surface of the object also affects air flow.
- E.g. If the surface of the object is smooth and shiny, the flow around the object will be predominantly laminar, if the surface is rough turbulent flow will be produced.
Major impact on sports such as motor riding and cycling.
10.1.8 Annotate a free-body diagram showing the direction of relevant forces acting on an athlete or object in sports
10.1.7 Cycling
CYCLING:
Cyclists adopt a low crouch position (using drop handlebars to reduce their frontal cross-section area) and often wear tight fitting ‘skinsuits’.
Advances in bike design such as oval-shaped frame tubes and disc wheels have helped reduce drag.
Additionally, helmets have been designed to have a more aerodynamic shape.
Turbulent Flow
- Rough multi-directional flow of the fluid that provides greater resistance to the movement of the object.
- Common at rear end of non-streamlined vehicle.
- Often occurs when object is moving quickly through fluid
- Turbulent flow causes more friction than Laminar flow
Laminar Flow
- Smooth flow of the fluid and provides less resistance to the moving object.
- Often occurs when object is moving slowly through fluid
- Turbulent flow causes more friction than Laminar flow
