Biomechanics Test Three Notes
How is the moment arm defined?
The perpendicular distance from the line of action to a specified point axis of rotation
Which of the following constitutes a negative angular acceleration
The rotation is slowing down or traveling in a clockwise direction
Which of the following is FALSE regarding Newton's third law of motion?
The torques acting on the two objects are sequential
Which of the following variables is described as a linear acceleration that is a line intersecting the circular path at only ONE point
tangenital acceleration
What is the difference between a relative and an absolute angle of measurement?
A relative angle has two movable segments, whereas, an absolute angle has only one moveable segment
In basketball, which is easier to do, a one-handed dunk or a two-handed dunk? Explain
A two-handed dunk is more difficult because you have to jump higher. Your center of gravity must be higher to complete a two-handed dunk because the distance between your outstretched hands and your center of gravity is shorter than it would be if you extended only one arm.
Why do you feel a centrifugal force if it isn't real
Because your body wants to keep moving in a straight line, as explained by Newtons 1'st law
When the second hand on a clock makes a complete revolution, which of the following is/are true?
Both a and b. Angular displacement is 360 degrees, and angular displacement is 0 degrees (distance and displacement, 0 because it starts and stops in same place so you haven't been displaced at all)
What does an external force not directed through the center of an object (an eccentric force) tends to cause?..a type of force, not a type of muscular contraction, swinging
Both linear and angular motion - 2nd law
Which of the following must be TRUE for an object to be in a state of dynamic equilibrium
Both the external forces and torques must sum to zero
Which of the following varibales is described as a linear variable directed in toward the center of rotation?
Centripetal acceleration
If each dancer is rotating with the same angular momentum about the longitudinal axis, which dancer is spinning the fastest about her longitudinal axis?
Donna, smallest moment of inertia
Which is TRUE regarding eccentric force?
Eccentric forces do not pass through the axis of rotation
Why does a swimmer flex the elbow as the arm comes out of the water during the front crawl?
It decreases swimmers moment of inertia so you can turnover swimmers faster
How does stretching the hip flexors to increase range of motion about the hip relate to overall step length?
It increases step length due to greater angular displacement of the hip. Based on d=0r equation, d is linear displacement, 0 is angular displacement and r is radius
How many revolutions are completed by a top spinning with a constant velocity of 3pi rad/s during a 20 s time interval?
Know: w= 3pi rad/s, t=20s Need: 0 rev/s Use: w= change in 0/t Solve: 3pi rad/s= change in 0/20s 20s (3pi rad/s(1rev/2pi rad) = (0/20s) 20s 20 X 3/2 = 0 = 30 rev
What does an external force directed through the center of an object (a centric force) tend to cause?
Linear motion only - 2nd law
Which lever classification puts the body at a mechanical advantage for speed of movement
3rd!!!
When the elbow is flexed to 90 degrees for a dumbbell lateral fly rather that extended toward 180 degrees, how does this change the moment of inertia about the shoulder joint?
Moment of inertia is decreased about the shoulder joint
How does the moment of inertia change for movements at the shoulder joint but rotate about different axes (ie abduction/add vs internal/external rotation)
Moment of inertia is greater during ab/adduction than internal/external rotation
What happens to the angular acceleration as the rate of rotation is constant?
The angular acceleration becomes 0
What happens to the angular acceleration as the rate of rotation increases?
The angular acceleration increases
How does angular momentum change if the body's angular velocity is doubled but moment of inertia stays constant
The angular momentum is twice as great
Which exercise is easier to do, a straight-arm pullover with a 45 lb barbell or a bent-arm pullover with a 45 lb barbell? Why?
The bent-arm pullover is easier to do because the moment arm of the barbell around the axis of the shoulder joint is smaller. This smaller moment arm creates a smaller torque around the shoulder joint, so the torque created around the shoulder extensor muscles is smaller as well.
What is the average angular acceleration of a tennis racket when the racket is swung from zero at the point of changing from the backswing to forward and is moving at a rate of 573 deg/s in just .25 seconds?
573/.25 = 2.292 deg/s
. Explain how larger handles on doors, valves, and so on are helpful to a person with limited dexterity and hand strength.
The larger handles increase the moment arms and thus decrease the force needed to turn the handles.
What would happen to the linear speed of a soccer ball if one could extend the knee 2x faster?
The linear speed would be 2x faster b/c equation V=wr, if we extend x2 as fast, we are cutting time in half, and are talking about angular velocity....saying omega is 2X as fast means angular velocity is 2X as fast.
Which of the following is true for an object experiencing a positive acceleration?
The object is speeding up or traveling in a counterclockwise direction
Which of the following is true for an object speeding up in a counterclockwise direction?
The objects angular acceleration is positive
How would you measure a moment arm, to determine its length
The perpendicular distance from the axis of rotation to the force's line of action
An object may have more than one moment of inertia because an object may rotate about more than one axis of rotation
True
Angular displacement is the change in absolute angular position experienced by a rotating line, ie clockwise and counterclockwise confusing
True
Instantaneous velocity at ball contact determines how fast and how far the ball will go. Average angular velocity determines where they will complete a certain number of twists, or somersaults before landing or entering the water
True
Newtons first law does not require that the angular velocity be constant, but rather that the product of the moment of inertia and the angular velocity be constant if no external torques act
True
The design goal of some exercise machines on the market is to provide a constant resistance torque throughout the range of motion of the exercise.
True
The net torque acting on an object must be zero to ensure that no changes occur in the angular motion of an object
True
The quality that describes angular inertia is called moment of inertia
True
A diver attempts a 1 1/2 front flip in a tuck position off the 1 m board, but over rotates 1 3/4 times (ouch!). Which of the following suggestions is NOT appropriate for the diver?
Tuck sooner and looser, creates less toque and less rotations...
During a golf drive, the angular velocity off a driver is 20 rad/s just before the impact with the golf ball. If the distance from the club head to the axis of rotation is 2.0m, what is the LINEAR velocity of the club head?
V=wr so 20X2 = 40 m/s
At approximately what angle at the elbow joint is the moment arm of the biceps brachii muscle about the elbow joint at its largest?
90 degrees
A personal trainer asks a client to perform a 90 degree squat slowly. The client should take a full four seconds to complete the downward phase. What is the average angular velocity of the clients downward phase of the squat exercise?
90/4 = 22.5 deg/s
Which of the following is true regarding the unit measure, radians?
A radian is equal to 57.3 degrees and are unitless
Maximizing tangential acceleration is important in which of the following situations?
punting a football for distance, spiking a volleyball
Harvey has skied over a mogul and become airborne. He has some angular momentum that is rotating him forward (clockwise). If he continues to rotate, he will probably fall on his face in the snow when he lands. In which direction should Harvey rotate his arms to try to prevent a fall?
rotate clockwise in same direction as fall
Which of the following is FALSE regarding moment of inertia
Moment of inertia is a linear kinetic variable
Angular acceleration occurs when something spins faster and faster or slower and slower, or when the spinning object's axis of spin changes direction
true
Angular motion occurs when all points on an object move in circular paths about the same fixed axis. Most human movements are the result of angular motions of limbs and joints
true
The average linear speed of a point on a rotating object is equal to the average angular velocity of the object times the radius
true
The most stable stance or position in which an object or person can be is the one that minimizes potential energy
true
The muscles are at a mechanical disadvantage for producing torque
true
What is the angular speed of a diver that makes 3 revolutions/flips in .5 seconds?
3/.5 = 6 rev/s
The relative angle at the elbow changes from 0 to 85 during the elbow flexion phase of a push-up. If 10 complete push-ups are performed, what is the total angular distance and displacement undergone at the elbow?
1700 and 0 degrees (b/c you have to go down and UP so 85 on flexion phase and 85 on extension phase, 85 X2 = __170_ X 10
Which classification of lever has the axis of rotation between the resistance moment arm and the motive moment arm
1st
If the centripetal force on an object moving in a circle suddenly turned off, what would happen to the object
It would move in a straight line, tangent to the circle
A bicycle tire rolls through a strip of fresh paint on the road making marks with each rotation of the tire. How many rotations does the tire make if 35 paint marks are observed? What is the total linear distance traveled by the cyclist if the bicycle tire radius is 40cm?
Know: r=40cm, 0=35 rotations Need: d (m) Use: d=0r Solve: Convert 35 rotations to radians first using chart b/c using lower equation 35rot (6.28rad)= 219.8 radians 40cm converted to m = .4m d=219.8radians(.4m) d= 87.92 m
The sweet spot on a baseball bat is 120cm from the axis of rotation during the swing of the bat. If this spot on the bat moves 40m/s, how fast is the angular velocity of the bat?
Know: radius= 1.2m, Vt=40m/s Need: w (rad/s) Use: V=wr Solve: 40m/s= w(1.2m) 40/1.2 w= 33.3 rad/s
What is the radial acceleration of a child riding a bike, traveling at a constant 10.0m/s around a flat circular track of radius 50m?
Know: radius=50m, Linear V=10m/s Need: Ar/c (m/s^2) Use: Ar=Vt^2/r Solve: Ar= 10m/s^2/(50m) Ar= 100/50 Ar= 2 m/s^2
A kicker's extended leg is swung for 0.4s in a counterclockwise direction while accelerating at 200deg/s^2. What is the angular velocity of the leg at the instant of contact with the ball?
Know: t= 0.4s, a=200deg/s^2, vi=0m/s? Need: w (deg/s) Use: a= change in w/t Solve: 200 deg/s^2 = change in w/0.4s 200(.4s) = 80 deg/s
A hammer thrower spins with an angular velocity of 1700 degrees/s. The distance from his axis of rotation to the hammer head is 1.2m. What is the linear velocity of the hammer head? What is the centripetal acceleration?
Know: w= 1700 degree/s, r=1.2m Need: V (m/s), Ac (m/s^2) Use: V=wr, Ac=w^2r Solve: Convert 1700 degrees to radians first using chart so 1700/57.3 degrees = 29.67 radians V=29.67radians(1.2m) V= 35.604 m/s Ac= 29.67^2(1.2) Ac=880.31(1.2) = 1056.3m/s^2
A hockey puck rotating on frictionless ice, will maintain a state of constant angular motion unless acted on by an external torque refers to which of Newton's laws
Newton's 1st law
Stability may be increased by doing which of the following?
No idea... leaning into an oncoming force or widening the base of support perpendicular to an oncoming force...
What is the advantage of a banked running track over a flat running track?
The banked track allows some component of a runner's normal contact force to create the change in direction necessary to negotiate the turn, so the runner does not have to rely solely on friction force to cause the change in direction.
A baseball delivers two pitches with exactly the same technique. The first pitch is thrown twice as fast as the second. During which pitch does the athlete experience greater centripetal/radial acceleration at the shoulder joint?
The first pitch, use equation W^2r b/c there is a PULL on shoulder joint, and plug in for w and same unit for r..four times as great??
John jumps up in the air to block a shot in basketball. After he is off the ground, John raises both hands over his head. Explain how this action affects the flight path of his center of gravity
The flight path of the center of gravity is unaffected by John's actions in the air. While John is in the air, his center of gravity is a projectile, and its trajectory is governed by the equations of projectile motion. The only force acting on him is gravity. This is the only force affecting the motion of his center of gravity
According to Newton's second law of motion, what is the relationship between torque and angular acceleration?
The greater the torque acting at the axis of rotation, the greater the angular acceleration of a given body
If angular momentum is conserved, what must be true about an object's initial angular momentum and final angular momentum?
The initial angular momentum is equal to the final angular momentum
A bike wheel .55m in diameter rotates a total 30 radians. What is the linear displacement of a point on the edge of the wheel?
d=0r so .55(30) = 16.5m
What advantages does a longer-limbed individual have in throwing and striking activities?
has a longer swing radius, which may cause a faster linear velocity at the end of the limb according to the relationship v = ωr
When performing a hammer throw in track and field, if the athlete's rotational rate is the same, which one of the following would increase the tangenital velocity of the hammer at release?
lengthening the hammer cable
What must be required to observe a centripetal acceleration for an object moving around a curved path?
the object must be changing direction
Implemenets enable us to impart faster linear velocities to the ball
true
The prolonging of the torque application by increasing the moment of inertia to slow down the spin results in a larger impulse and thus a larger change in momentum ie discus thrower
true
Draw (equation) to explain why the longest club is the driver and the shortest club is the pitching wedge.
v=wr, The driver is longer so that the ball can receive more speed and the radius is longer. The pitching wedge is shorter so that the ball doesn't go as far when hit, and is curved to help get out of rough grass or sand and will create a bigger angle
Sadie jumps up in the air to spike the volleyball. She leaves the ground without any angular momentum. As Sadie swings her arm forward to spike the ball, what happens to her legs if her trunk and nonhitting arm do not rotate?
. Her legs rotate in the opposite angular direction
Jessica is performing her routine on the balance beam. She is standing on her left leg and facing forward along the long axis of the beam. Her right leg is flexed 90° at the hip and the knee is extended. Her left arm is fully abducted 180° at the shoulder, and her right arm is abducted 120° at the right shoulder. She begins to fall to the right. What should Jessica do with her arms to help regain her balance?
. Jessica should swing her arms in the same angular direction of her fall (in a clockwise direction as viewed from the front)—her left arm should adduct at the shoulder and her right arm should further abduct at the shoulder.
Why is it more difficult to get up out of a chair whose seat is lower to the ground than if the seat is higher?
. Normally, the seat height of a chair should put you in a position in which your feet are flat on the floor as you sit in the chair with a 90° angle at your knee joint. Rising from this position into a stable standing position requires that you move your center of gravity forward over your feet. Leaning forward at the waist while extending at the knees and hips accomplishes this fairly easily. Using your arms as well will make it even easier. If the chair seat is quite low to the ground, this task is more difficult. Your feet are now resting on the floor well in front of your center of gravity. Rising out the chair with the low seat is more difficult because you have to move your center of gravity farther forward to position it over your feet. You also have to raise your center of gravity more in order to stand up
What is Lebron James' angular displacement when performing one full revolution about the longitudinal axis before dunking the ball?
0 degrees (explained above, it starts and stops in same place so you haven't been displaced at all)
What is the angular velocity of an elbow that flexes through 90 degrees in 5 seconds
90/5 = 180 deg/s
Which of the following is true regarding relative angles
A relative angle has TWO movable segments
A freestyle aerial skier, Liesl, completes a helicopter 1080 in which she rotates three times around her longitudinal axis while in the air. While performing the trick, Liesl adducts her arms and keeps her poles close to her sides. Just before landing, she abducts her arms. How do Liesl's actions—adducting her arms during the trick and abducting her arms just before landing—contribute to the successful execution of the trick?
Adducting her arms during the helicopter 1080 trick reduces Liesl's moment of inertia about the longitudinal (twist) axis. This will speed up her rate of spin, since she is airborne and angular momentum is conserved. Abducting her arms just before landing increases her moment of inertia about the longitudinal (twist) axis. This will slow down her rate of spin and improve her chances of not falling when she lands
Which of the following is FALSE for an object to be in a state of static equilibrium
All external masses must sum to zero
Looking at the range of motion at the knee joint during the downward phase of a squat is a measurement of what?
Angular displacement?
Which of the following is FALSE regarding angular impulse
Angular impulse can use the notation of "Ft"
What does it mean that the angular momentum of someone spinning on a bar stool is conserved?
As the person's arms and legs are stretched out and then pulled in, the angular velocity decreases then increases
Which of the following best exemplifies how we increase tangential velocity of an object
Either increase the radius of rotation or angular velocity while the other remains constant
What happens to your COG when you raise your arms over your head to reach up toward the ceiling to change a lightbulb
The COG raises, toward the direction of reach :)
If each dancer is rotating with the same angular velocity about the longitudinal axis, which dancer has the greatest angular momentum about the longitudinal axis?
Catherine, she also had greatest moment of inertia...(moment of inertia order largest to smallest: catherine, anna, brynn, donna)
What is the center-directed force of a spinning object (a ball on a string) called?
Centripetal force
Which force on an object directed to the center of a circular path keeps that object on the path?
Centripetal force
In which situation can angular momentum conservation be observed?
Considering the number of rotations a person makes during a dive
Ann performs three different dives and rotates through the same number of revolutions in the same amount of time in all three dives. In all the dives, Ann's positions at takeoff and just before water entry are identical layout positions. In dive 1, she is in a layout position in midflight. In dive 2, she is in a tuck position in midflight. In dive 3, she is in a pike position in midflight
Dive 1 (largest angular momentum spread out), 2 (smallest ""), 1 (rotating fastest), 2 (rotating slowest)
Which sprinting technique is more effective—flexing the knee of the recovery leg (the leg that is swinging forward) more during the swing-through or flexing the knee of the recovery leg less during the swing-through? Why?
Flexing the recovery leg more during the swing-through is more effective because it reduces the moment of inertia of the leg about the transverse axis through the hip joint, thus speeding up the swing-through.
Where would you grip a tennis racket for a more effective ball velocity? (all other factors would remain constant)
Further from the racket head b/c equation V=wr, further away more speed b/c radius is longer
Which rotating body has the smallest moment of inertia?
Ice skater spinning with arms folded on chest, b/c of axis of rotation closest to body
When a body is completely motionless, what is it considered to be?
In a state of static equilibrium
Describe the best position to be in when you compete in a tug-of-war.
Place your feet slightly wider than shoulder-width apart and aligned with the rope; that is, the rope should pass above both feet. Lean back so that most of your weight is over your rear foot—your center of gravity should be over your rear foot. Flex slightly at the knees and hips to lower your center of gravity further and so that you can quickly extend and pull on the rope when the signal to begin the competition is given. This stance increases the distance from the line of gravity to the front edge of the base of support and lowers the center of gravity as well.
Two runners are running around the curve of a track. Which runner would have the greater centripetal (radial) acceleration?
Runner in the inside lane, not as gradual and will require more friction, will have to slow down. It is more difficult to run fast in lane 1 because your centripetal acceleration is greater due to the smaller radius of lane 1 compared to lane 8. This means that you must generate greater frictional forces under your feet in the radial direction to cause your centripetal acceleration
Why do runners lean when running around a curve?
The medial-lateral friction force that acts under a runner's feet to cause him to change direction also causes a moment of force (a torque) that tends to tip him over away from the center of the curve. Leaning allows the normal contact force to produce a torque in the opposite direction and balance the torques created by the friction force.
Which of the following is true regarding a relative angle
The relative angle for the knee joint should be measured as the intersection between the femur and tibia
Why are most gymnasts small in stature?
Their smaller stature gives them smaller moments of inertia—an advantage in spinning, somersaulting, and tumbling maneuvers
Stand with your back and heels against a wall. Now try to lean forward and pick something up off the floor while still keeping your heels against the wall. Can you do this without falling over? Why is this so difficult or impossible to do?
To maintain your stability when you reach forward to pick something up off the floor, you must keep your line of gravity within your base of support—the area beneath and between your shoes. As your trunk leans forward, your center of gravity moves forward as well, and the line of gravity soon gets close to your toes. To keep the center of gravity over the base of support, you normally move your hips and legs back as you lean forward. But with the wall behind you, you can't do this. So, as you keep leaning forward, your center of gravity keeps moving forward until the line of gravity moves forward of your toes and outside your base of support, and you fall forward.
Doubling the mass of an object would double its moment of inertia, but doubling its radius of gyration would quadruple its moment of inertia
True
Torques cause changes in angular motion, the movements of our limbs at joints are controlled by torques produced by muscles, and muscles create torques about joints. These cause and control movements of limbs and body. Torques are important even if not moving b/c of equilibrium and balance. Also can be called a moment of force. Can be positive or negative, complicated stuff, wrench, tools. Sports golf, wrestling, baseball in club or bat to SWING, initiate turns, spins, muscles, free weights, weight lifting machines, dumbbells, etc
True
One of your little leaguers can't get his bat around fast enough. What would you suggest he do?
Use a lighter bat, and choke up on bat, based on force of moment of inertia equation, if you choke up center of gravity is closer to axis of rotation and your hands
During a discus throw, Anthony's arm rotates 20 rad/s, and the discus is 1.5 m from the axis of rotation when releases it. How fast does the discus travel when released
V=wr so 20X1.5 = 30 m/s
Does a point on a rotating object experience any linear acceleration if the object spins at a constant angular velocity with no angular acceleration? (ie centrip acceleration)
Yes, linear acceleration occurs as object speeds up/dwn or changes direction, becuase point follows circular path it is always changing direction
Can a pole vaulter successfully clear a crossbar without having his center of gravity pass above it?
Yes, they must drape herself over the bar in an inverted U position when she is at her peak height. In this position, her center of gravity may be outside of her body—below the peak of the inverted U—and the crossbar may be above her center of gravity but below that part of her body that forms the peak of the inverted U.
Most skeletal muscles in our limbs attach close to the joints. Advantages? Disadvantages?
advantages: increased range of motion of the limbs and speed. Our muscles attach close to the joint, the axis of rotation, while end of the limb is far from the axis of rotation. Speed: The distal ends of our limbs can move with fast linear velocity because of the relationship of linear velocity to angular velocity and radius. V=wr. There is a limit to the speed of shortening of a muscle, but the long length of our limbs allows a slow muscle contraction to produce a much faster linear velocity at the end of the limb. b. The disadvantage: the moment arm of most muscles is relatively short; thus muscles must produce very large forces to create relatively modest torques about a joint.