Forces Opposing Motion

Question 1

forces opposing motion

Answer 1

high velocity in air
movement through water
gravity

Question 2

friction

Answer 2

force that occurs when an object moves while in
contact with an object
with two fluids

Question 3

types of friction

Answer 3

sliding
rolling
static

Question 4

sliding friction

Answer 4

2 surfaces in contact slide past one another

when walking

Question 5

limiting friction

Answer 5

friction force required to overcome inertia

equals normal reaction x constant (depending on surface)

Question 6

F = μ x R

Answer 6

F = friction force
μ = coefficient of friction (depends on hardness and roughness)
R = reaction force (normal) (perpendicular to surface of contact
(doesn’t say anything about surface area)

Question 7

contact area

Answer 7

pressure,

Question 8

friction in the human body

Answer 8

increased heat production, damage and wear to tissues
synovial fluid
smooth articular surfaces
bursae, blisters

Question 9

rolling friction

Answer 9

resistive force, spherical object rolls

depends on diameter, surfaces, normal force

Question 10

ranking of friction forces

Answer 10

static > sliding > rolling

Question 11

applying brakes

Answer 11

want to maximise static friction as it has limiting friction, therefore when applying brakes you want to pump them and don’t let them lock up otherwise sliding friction is being used and that’s not wanted.

Question 12

physical properties of striking equipment

Answer 12

rotational inertia, centre of percussion

Question 13

COP

Answer 13

below COG
point where the translation of the forward = + opposite to the rotation of the bat backwards?
no oscillation or vibration
maximum distance, all of the bats velocity is transferred to the ball
therefore we want to make the COP as large as possible

Question 14

variants of the COP

Answer 14

more mass toward the bottom of the bat = COP lower

Question 15

elasticity

Answer 15

regain original shape

Question 16

falling ball hits ground (elasticity)

Answer 16

GRF compresses ball, until COG stops its downward motion, deformed, elastic energy
ball recoil causes bottom surface to push against ground, generating GRF that moves ball upwards

Question 17

coefficient of restitution

Answer 17

measure of elasticity when striking a surface + ability to reform
larger the coefficient = more elasticity

Question 18

factors affecting coefficient of restitution, in relation to more temp, P and softness (ground)

Answer 18

more P = More elasticity
more softness = less rebound (ground)
more temp = More elasticity

Question 19

velocity before impact, formula

Answer 19

u1 = (2gH d) square root
u1 = velocity before impact
g = gravity
H = height dropped

Question 20

direct + oblique impact

Answer 20

either head on or perpendicular

Question 21

Angle of approach + angle of rebound

Answer 21

horizontal (AR)

Question 22

vertical

Answer 22

angle of incidence

angle of reflection

Question 23

Rebound- topspin

Answer 23

grips (accelerates) and stays lower
V(L) is high + V(R) is low, decreased friction effect, doesnt lose that much forward speed
V(L) is low + V(R) is high, gain speed on bounce

Question 24

Rebound- backspin

Answer 24

higher and slows down on surface, GRF is greater, therefore greater friction, slows down