W8 - Coefficients of friction & restitution

Question 1

What happens to momentum in a perfectly inelastic collisions?

Answer 1

Conserved

Question 2

What happens to maximal kinetic energy in a perfectly inelastic collisions?

Answer 2

Lost

Question 3

What happens to the colliding objects in perfectly inelastic collisions?

Answer 3

They stick together + can be treated as 1 post-collision.

= a result of the maximal KE being lost

Question 4

What happens to the momentum in a perfectly elastic collision?

Answer 4

Conserved

Question 5

What happens to the kinetic energy in perfectly elastic collisions

Answer 5

Conserved

Question 6

Are most collisions perfectly elastic or inelastic?

Answer 6

NO

Question 7

How is coefficient of restitution (e) calculated?

Answer 7

Square root of (KE after collision / KE before collision)

But can be simplified to:

e = (v1-v2) / (u1-u2)

Question 8

What is Newtons law of impact?

Answer 8

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

Question 9

Equation for newtons law of impact

Answer 9

(v1-v2) = e (u1-u2)

e = coefficient of restitution

Question 10

What do collisions with the earth include?

Answer 10

Acceleration due to gravity

Question 11

What equation is used to calculate the experimental determination of coefficient of restitution

Answer 11

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)

Question 12

In the experimental determination of coefficient of restitution, what does the h(little b) stand for?

Answer 12

Height of the bounce

Question 13

In the experimental determination of coefficient of restitution, what does the h(little d) stand for?

Answer 13

Height from which object is dropped.

Question 14

What does the coefficient of restitution refer to?

Answer 14

Ratio of the relative velocity of 2 elastic bodies after rebounding to velocity before impact.

Question 15

Define friction

Answer 15

The force that opposes motion or impending motion when 1 body moves across the surface of another body.

Question 16

What 2 categories can friction be broken down into?

Answer 16

Sliding friction

Rolling friction

Question 17

Define sliding friction

Answer 17

Force opposing the sliding motion between surfaces of 2 objects.

Question 18

Define rolling friction

Answer 18

Frictional force when 1 object rolls over the surface of another

Question 19

Why would gymnasts want to put chalk on their hands, relating to friction?

Answer 19

To ⬆️ friction between their hands + rings to maximise their grip.

Question 20

In a sliding friction model, what does the R, P + F stand for?

Answer 20

R = Reaction force perpendicular to the surface

P = Applied force

F = Frictional force

Question 21

What is the point of limiting friction?

Answer 21

The frictional force that limits movement.

Question 22

What type of value is the coefficient of friction?

Answer 22

Dimensionless scalar value

Question 23

What does the coefficient of friction describe?

Answer 23

Ratio of the force of friction + the normal (perpendicular) force between 2 bodies.

Question 24

How is the coefficient of friction calculated?

Answer 24

F / R

F = Frictional force

R = Reaction force perpendicular to the surface

Question 25

What are the factors influencing friction?

Answer 25

Properties of contacting surfaces

Reaction force

Question 26

What is limiting friction equal to for 2 DRY surfaces?

Answer 26

Normal reaction force (R) x constant (coefficient of limiting friction)

Question 27

Which has to be less than the other? Coefficient of sliding friction or the coefficient of limiting friction?

Answer 27

Coefficient of sliding friction has to be less than our coefficient of limiting friction.

Question 28

Calculating coefficient of friction from a force plate

Answer 28

Fy / Fz

Question 29

When looking at a force plate what does the Fy represent?

Answer 29

The F value

F = Frictional force

Question 30

When looking at a force plate what does the Fz represent?

Answer 30

The R value

R = Reaction force perpendicular to the surface

Question 31

On a force plate, what does the Fy value represent?

Answer 31

Anterior-posterior forces

Question 32

How is the coefficient of friction between different shoes on the same surface determined?

Answer 32

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.

Question 33

Why is the duration of the braking component (Fy) important during turning?

Answer 33

⬆️ 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.

Question 34

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.

Answer 34

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.

Question 35

What were the limitations of the experimental procedures you used to calculate coefficient of restitution?

Answer 35

Didn’t control for the factors mentioned in q2.

Didn’t use familiarisation trials

Didn’t take into account the athletes experience