10. Friction and Drag Flashcards
What is friction?
A force that acts parallel to the interface of two surfaces that are in contact, and opposes their relative motion.
How is friction calculated?
Ff = μR
Where μ is the coefficient of friction and R is the normal reaction force.
What is the coefficient of friction?
The coefficient of friction (COF, μ) is a dimensionless scalar quantity which is the ratio of the force of friction
(The magnitude of the coefficient of friction depends on the materials in contact: steel on ice (in ice skating) has a low coefficient of friction)
What is static friction?
when the surfaces of two objects are at rest relative to one another.
E.g. A book sitting on a table.
What is dynamic friction
the friction of an object moving or sliding across a surface.
E.g. Wheels of moving vehicles,
Distinguish between the coefficient of static friction and dynamic friction.
The coefficient of static friction is when a force is applied to attempt to move a stationary object over another surface. At some point, the force applied is sufficient to overcome the static friction and the object will begin to move causing kinetic friction. Once the object is in motion, this is the coefficient of dynamic friction (aka kinetic friction). The coefficient of dynamic friction is usually lower than the coefficient of static friction because when the object is in motion relative to not, kinetic friction takes over and overall resistance is reduced.
Explain the influence of friction on sports performance.
In sports, different equipment is used to maximise or minimise the friction between an object and surface in order to enhance performance within a specific sport.
For example:
- In sports, shoes like cleats or basketball shoes allow for greater traction/ grip on playing surfaces to increase friction.
- In Winter sports like skiing and ice skating, a in decrease friction is required to allow the people to go faster and slide more easily. For ice skates, the steel on ice creates o low coefficient of friction.
- In Golf, a glove is used to increase the grip and friction between the contact of hand and club, making swing more powerful and easier to control when hitting.
Define drag
Drag is the force or forces acting to oppose the motion of an object through a fluid medium such as air or water. This is also known as aerodynamic friction.
what is surface drag
Surface drag is caused by the interaction between the surface of the object and the fluid molecules. As an object moves through a fluid, its outer surface catches a layer of the fluid nearby, slowing it down compared to the fluid further away, ultimately causing drag. This can be minimised by changing the surface of the object to reduce the interaction between surface and fluid.
Example: The use of shark-skin suits in swimming or shaving the swimmer’s body to make it smooth.
How can surface drag be affected
- The velocity of the object relative to the fluid
- The surface of the object (smooth or rough)
- The density of the fluid.
what is form drag
It’s caused by the shape of the object. As a body pushes against air or a fluid, the fluid pushes back and this is called action and reaction. By streamlining the body and minimising the surface area facing the direction of the motion, form drag is reduced.
Example: Cyclists lying down flat moving down a hill to reduce form drga and make them go faster. A bike frame design also being slimmer and aerodynamic reduces this drag because the body is more streamlined.
How can form drag be affected
- The frontal area of the object
- The shape of the object
- The relative velocity of the fluid (low wind speeds will have lower form drag on an object compared to high wind speeds because the air is pushing back on it faster/harder.
what is Wave drag (or Frontal Resistance)
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 but if the object is moving fast through the fluid, wave drag is increased.
Example: Swimming underwater for as long as is allowed at the start of a race to reduce wave drag.
- Wave drag is particularly important in motion through water in swimming, canoeing, rowing or sailing.
what Is laminar and turbulent Flow
Laminar flow is the smooth flow of the fluid on the boundary layer of an object and provides less resistance to the movement of it meaning it can move faster through the fluid.
Turbulent flow is the rough multi-directional flow of the fluid that provides greater resistance to the movement of the object. It is common at the rear end of non-streamlined bodys which causes more friction than Laminar flow.
What is streamlining
Streamlined bodies incorporate gradual tapering to minimize pressure effect and separation of fluid. The point of a streamlined shape is that the air moves past it in layers which allow laminar flow to occur to glide through a fluid with maximum efficiency, overall decreasing friction.
E.g. a streamlined helmet is pointy at the back to make the person more aerodynamic, whereas a non-streamlined helmet causes more drag because the air behind the object is jumbled up where the fluid does not flow smoothly.
Annotate a free-body diagram showing the direction of relevant forces acting on an athlete or object in sports
