Quiz 5

Question 1

Newton’s 1st law of motion

Answer 1

a body will maintain a state of rest or constant velocity unless acted on by an external force

Question 2

example used to show Newton’s first law of motion

Answer 2

• pushing a stalled car from rest
  -> changing the static to dynamic inertia
• slowing an opposing player down
  -> retarding force is needed to slow down mass that is set in motion & moving at a constant velocity

Question 3

Newton’s 2nd law of motion

Answer 3

the acceleration of a body is proportional to the applied force and takes place in the direction in which the force is applied. inversely proportional to the mass of body.

Question 4

example used to show how 2nd law of motion is applied

Answer 4

pushing a car vs pushing a cart
- will accelerate in the direction that you push it
- need more force to push car than cart because it has more mass

Question 5

Newton’s 3rd law of motion

Answer 5

for every action there is an equal and opposite attraction

Question 6

example used to show how 3rd law of motion is applied

Question 7

explain GRF

Answer 7

• ground reaction force
• reactive force experienced when pushing against the ground during all forms of gait

Question 8

applications of GRF

Answer 8

• high jump: object is to convert horizontal velocities at the time of take off by using GRF to change direction of component force
• GRF increases with the height from which a person steps down or jumps
  -> increased step height = higher impact forces = joint stress & injury risk

Question 9

friction formula

Answer 9

frictional force = mew x R
- mew = coefficient of friction and is dependent on nature of surfaces in contact with each other
- R = perpendicular or normal (N) reaction force
- mew of static friction is always greater than mew of kinetic fraction

Question 10

pulling vs pushing

Question 11

explain friction and stride length

Answer 11

• changes in stride length are dependent on frictional forces
• if there is sufficient frictional force: increase in stride length = increase in horizontal vector (pushing backwards), increasing retarding frictional force
• if there is not enough frictional force: greater horizontal vector will result in loss of traction and the foot will slide forward

Question 12

explain rolling friction

Answer 12

resistive force that slows down the motion of a rolling ball or wheel

Question 13

example of rolling friction

Answer 13

under-inflated tires = more surface area in contact with road = increased rolling friction that is resisting the tire’s rolling motion

Question 14

use of rosin on dance floors

Answer 14

• static friction: prevents slipping when a dancer is stationary or initiating movement
• kinetic fraction: acts during movement across the floor
• rosin increases the coefficient of static friction and kinetic fraction
  > SF = more grip when needed, controlled traction, joint protection, enhanced performance
  > KF = better traction for controlled sliding, reduced slipping, more control, more effort needed to slide, safer stopping

Question 15

use of artificial turf

Answer 15

• SF: keeps from slipping when pivoting, pushing off
• KF: helps slow down or stop when sliding
• increases SF & KF

Question 16

formula for conversation of momentum, as they relate to colliding bodies

Answer 16

• when colliding, the bodies can either be moving in opposite directions or in same direction
• total M before collision = M after collision
• for 2 bodies that collide & then separate: m1v1 + m2v2 = m1v3 + m2v4
• for 2 bodies that collide & stick together after collision: m1v1 + m2v2 = (m1 + m2)v3

Question 17

impulse formula

Answer 17

impulse = force x time

Question 18

how does impulse formula apply in sports

Answer 18

jumping: applying force over prolonged time is counterproductive
- applying forces over long period of time is distinct advantage (wrestling, throwing sports like baseball)
- small force applied for long time
- large force applied for brief or moderate time

Question 19

explain elasticity

Question 20

how is “e” the coefficient of restitution applied

Answer 20

• index of elasticity for colliding bodies
• capability of an object to rebound after being deformed at impact
• between 0 and 1
• ensuring balls used in different sports conform to specific values of e
  -> determines how much the ball bounces of ground or racket or …

Question 21

explain dissipation of kinetic energy

Question 22

explain instant axis of rotation

Answer 22

calculating the instant center of rotation of a joint during dynamic movements for a given joint angle
- imaginary point or line around which a body rotates at a given moment in time, even if axis is constantly moving

Question 23

explain screw-home mechanism

Answer 23

tibia externally rotates with extension

Question 24

explain how angular velocity relates to linear velocity

Answer 24

the linear velocity = angular velocity x radius of the circle
- V = rw

Question 25

radian

Answer 25

- defined as size of subtended angle whose length on the circle = radius of circle - radian = degrees x (pi/180 deg)

Question 26

if angular velocity (w) = 1...

Answer 26

V = 1 x r - V is directly proportional to r - serving in tennis: r is increased by extending arm & stretching body out maximally

Question 27

when velocity = 1...

Answer 27

- 1 = r x w --> w = 1/r - w is inversely proportional to r --> we can change the rate of spin (w) when the radius of rotation is changed (smaller radius = faster spin) - difference in mechanics when vaulting with summersaults in gymnastics