W8 - Coefficients of friction & restitution Flashcards
What happens to momentum in a perfectly inelastic collisions?
Conserved
What happens to maximal kinetic energy in a perfectly inelastic collisions?
Lost
What happens to the colliding objects in perfectly inelastic collisions?
They stick together + can be treated as 1 post-collision.
= a result of the maximal KE being lost
What happens to the momentum in a perfectly elastic collision?
Conserved
What happens to the kinetic energy in perfectly elastic collisions
Conserved
Are most collisions perfectly elastic or inelastic?
NO
How is coefficient of restitution (e) calculated?
Square root of (KE after collision / KE before collision)
But can be simplified to:
e = (v1-v2) / (u1-u2)
What is Newtons law of impact?
If 2 bodies travel towards each other along a straight line, the difference between their velocities after impact bears a constant relationship to the diff at the moment of impact
Equation for newtons law of impact
(v1-v2) = e (u1-u2)
e = coefficient of restitution
What do collisions with the earth include?
Acceleration due to gravity
What equation is used to calculate the experimental determination of coefficient of restitution
u1^2 = u^2 + 2as = velocity pre collision
v1^2 = v1^2 + 2as = velocity post collision
Then:
e = v1 / u1
To:
e = square root (hb / hd)
In the experimental determination of coefficient of restitution, what does the h(little b) stand for?
Height of the bounce
In the experimental determination of coefficient of restitution, what does the h(little d) stand for?
Height from which object is dropped.
What does the coefficient of restitution refer to?
Ratio of the relative velocity of 2 elastic bodies after rebounding to velocity before impact.
Define friction
The force that opposes motion or impending motion when 1 body moves across the surface of another body.
What 2 categories can friction be broken down into?
Sliding friction
Rolling friction
Define sliding friction
Force opposing the sliding motion between surfaces of 2 objects.
Define rolling friction
Frictional force when 1 object rolls over the surface of another
Why would gymnasts want to put chalk on their hands, relating to friction?
To ⬆️ friction between their hands + rings to maximise their grip.
In a sliding friction model, what does the R, P + F stand for?
R = Reaction force perpendicular to the surface
P = Applied force
F = Frictional force
What is the point of limiting friction?
The frictional force that limits movement.
What type of value is the coefficient of friction?
Dimensionless scalar value
What does the coefficient of friction describe?
Ratio of the force of friction + the normal (perpendicular) force between 2 bodies.
How is the coefficient of friction calculated?
F / R
F = Frictional force
R = Reaction force perpendicular to the surface
What are the factors influencing friction?
Properties of contacting surfaces
Reaction force
What is limiting friction equal to for 2 DRY surfaces?
Normal reaction force (R) x constant (coefficient of limiting friction)
Which has to be less than the other? Coefficient of sliding friction or the coefficient of limiting friction?
Coefficient of sliding friction has to be less than our coefficient of limiting friction.
Calculating coefficient of friction from a force plate
Fy / Fz
When looking at a force plate what does the Fy represent?
The F value
F = Frictional force
When looking at a force plate what does the Fz represent?
The R value
R = Reaction force perpendicular to the surface
On a force plate, what does the Fy value represent?
Anterior-posterior forces
How is the coefficient of friction between different shoes on the same surface determined?
Shoe is placed on a board.
Goniometer is used to measure the angle between the board + the horizontal at the point of which the shoe starts to slide.
Why is the duration of the braking component (Fy) important during turning?
⬆️ duration might ⬇️ the peak forces experienced i.e impulse area under curved ‘dragged’ out.
Impulse = Change in momentum so an ⬆️ duration may influence the velocity of the ‘return’ movement.
Consider the influence of friction in the context of injury risk. Describe one example where high friction would increase the risk, and another where low friction would increase the risk of a specific injury.
High friction – rolling ankle if shoe grips ground too much instead of sliding or if boot studs get caught in the grass.
Low friction – sliding too much on clay court or when walking on ice + falling i.e breaking knee.
What were the limitations of the experimental procedures you used to calculate coefficient of restitution?
Didn’t control for the factors mentioned in q2.
Didn’t use familiarisation trials
Didn’t take into account the athletes experience
