Mechanics of Human Movement Test #2

Can external forces act on an object and not cause acceleration? Explain.

Yes, but only if the net external forces sum to zero.

The moment arm is defined as what?

The perpendicular distance from the line of action of a force to a specified axis of rotation.

For any given muscle, the moment arm is the largest when the angle of muscle pull on the bone is closest to what?

90 degrees.

What is a cardinal plane?

A cardinal plane is a plane that passes through the midpoint or center of gravity of the body. The center of gravity is the point on which the body would balance if it were supported at only one point. The cardinal sagittal plane (midsagittal or median plane) is the plane that divides the body into equal right and left halves.

Each anatomical plane has what?

A corresponding anatomical axis that passes perpendicularly through the plane. The planes are useful is describing relative movements of body parts, with the axes used to describe the line around which these motions occur.

What is a frontal plane?

A frontal plane, also called a coronal or lateral plane, runs side to side and superior to inferior, dividing the body into anterior and posterior parts.

What is a longitudinal axis?

A longitudinal axis (vertical, frontal-sagittal, or twist axis) is an imaginary line running from top to bottom and perpendicular to the transverse planes. A longitudinal axis is defined by the intersection of a frontal plane with a sagittal plane, so it may also be called a front-sagittal axis.

What is a force couple?

A pair of forces that are equal in size but opposite in direction and noncolinear.

What is a plane?

A plane is a flat two-dimensional surface.

What is a sagittal plane?

A sagittal plane, also called an anteroposterior plane, is an imaginary plane running anterior (front) to posterior (back) and superior (top) to inferior (bottom), dividing the body into right and left parts.

What is a transverse axis?

A transverse axis (lateral, frontal, mediolateral, frontal-transverse, or somersault axis) is an imaginary line running from left to right and perpendicular to the sagittal planes. A transverse axis is defined by the intersection of a transverse plane with a frontal plane, so it may also be called a frontal-transverse axis.

What is a transverse plane?

A transverse plane, or horizontal plane, runs from side to side and anterior to posterior, dividing the body into superior and inferior parts.

What joint action occurs at the shoulder joint and what are the plane and axis of motion during the pulling up phase of a wide-grip pull-up?

Adduction in the frontal plane around the AP axis.

Describe which axes are perpendicular to each other.

All AP axes are perpendicular to all transverse axes, which are perpendicular to all longitudinal axes, which is perpendicular to all AP axes.

Describe which planes are perpendicular to each other.

All sagittal planes are perpendicular to all frontal planes, which are perpendicular to all transverse planes, which are perpendicular to all sagittal planes.

The weight of the average NFL football player increased by 10% in 20 years. Describe the mechanical advantages that a heavier player has over a lighter player.

American football is a game of collisions. Players collide with one another to try to prevent or change the motion of the opposing player. These collisions are close to inelastic. If momentum is conserved in the collisions, then the heavier player's larger mass gives him an advantage. Newton's second law could also be applied here. To change the motion of a heavier, more massive player requires more force.

What is an angle?

An angle is formed by the intersection of two lines, two planes, or a line and a plane. The term angle refers to the orientation of these lines or planes to each other. The Greek letter theta is used to represent the angle formed by the intersections of the lines and planes.

What is an anteroposterior axis?

An anteroposterior axis (sagittal, sagittal-transverse, or cartwheel axis) is an imaginary line running from anterior to posterior and perpendicular to frontal planes. The anteroposterior axis is often abbreviated as AP axis. An AP axis is defined by the intersection of a transverse plane with a sagittal plane, so it may also be called a sagittal-transverse axis.

What is a centric force?

An external force directed through the center of gravity of an object.

What is an eccentric force?

An external force not directed through the center of gravity of an object.

Which of the following is an incorrect representation of Newton's first law?

An object with no unbalanced forces applied to it will naturally come to rest.

What is angular displacement?

Angular displacement is the angular analog of linear displacement. It is the change in absolute angular position experience by a rotating line. Angular displacement is thus the angle formed between the final position and the initial position of a rotating line.

Describe angular motion.

Angular motion occurs when all points on an object move in a circular path about the same fixed axis.

What is angular position?

Angular position refers to the orientation of a line with another line or plane.

A seatbelt is an important safety mechanism in cars. This is because the seat belt:

Applies a force to your body so that it does not continue to move forward once the car comes to a stop.

Changes in impact time are accompanied by inversely proportional changes in what?

As long as the change in momentum is the same for both activities, change in impact time are accompanied by inversely proportional changes in average impact force.

Movements in the transverse plane occur around which axis?

Longitudinal

How is angular direction described?

Clockwise and counterclockwise are common terms used to describe the direction of rotation.

Newton's second law of motion

Commonly referred to as the law of acceleration. "The change of motion of an object is proportional to the force impressed; and is made in the direction of the straight line in which the force is impressed."

Newton's third law of motion

Commonly referred to as the law of action-reaction. "To every action there is always opposed an equal reaction: or the mutual actions of two bodies upon each other are always equal and directed to contrary parts."

Newton's first law of motion

Commonly referred to as the law of inertia. This law states, "Every body continues in its state of rest, or of uniform motion in a straight line, unless it is compelled to change that state by forces impressed upon it."

You are doing abdominal crunches on a bench whose incline can be adjusted. As you increase the incline of the bench, your feet get higher than your hips. What effect does the increase in the incline of the bench have on the abdominal crunch exercise?

During an abdominal crunch exercise, the abdominal muscles exert their greatest force when the moment arm of the upper body about the axis of motion is greatest. During a crunch that begins on the floor, this position occurs at the start of the crunch when you first lift your upper body off the floor. When you do crunches on an incline bench, this position of greater torque still occurs when your body is farthest horizontally from the axis of motion; but as the incline increases, the position occurs with increasing trunk flexion.

What must occur in order to change the moment of an object?

Either its mass or velocity must change. In sports and human movement, most objects we deal with have a constant mass. A change in momentum thus implies a change in velocity.

When you spike a volleyball, what joint action occurs at the elbow joint of your hitting arm?

Extension.

Newton's law of universal gravitation

First, all objects attract each other with a gravitational force that is inversely proportional to the square of the distance between objects. Second, this force of gravity was proportional to the mass of each of the two bodies being attracted to each other.

If a force is directed through the center of gravity of an object, the object will do what?

Move in a straight line.

Explain the cause-and-effect relationship expressed by Newton's second law.

Forces cause acceleration. Acceleration is the effect of forces. If a net external force acts on an object, the object accelerates. If an object accelerates, a net external force must be acting to cause the acceleration.

When you turn a corner while walking, what force causes you to change directions and where does this force act on you?

Friction is the force that cause your change in direction. It acts on the sole of your shoes.

What is the external horizontal force that causes you to change direction as you run around a curve?

Friction.

The cardinal frontal plane divides the body into equal what?

Front and back halves.

Hip abduction occurs in which plane?

Frontal.

When you push against a wall why do you not accelerate as a result of the force the wall exerts against you?

Gravity pulls down on you with a force equal to your weight. The floor pushes up against the soles of your shoes or your feet. And... a frictional force from the floor acts against your feet. This frictional force opposes the pushing force from the wall and prevents you from accelerating as a result of the wall pushing against you.

When you swing a baseball bat, what is the plane of motion for the action occurring at your leading shoulder? What is the axis of motion? What joint action occurs at the leading shoulder during the swing?

Horizontal abduction (or horizontal extension) in the transverse plane around a longitudinal axis.

What is a relative angular position?

If the other line or plane is capable of moving, the angular position is a relative angular position.

What is an absolute angular position?

If the other line or plane is fixed and immovable relative to the earth, the angular position is an absolute angular position.

What is the right-hand thumb rule?

If you situate the thumb of your right hand so that it points in the positive direction along the axis of rotation, the direction in which your fingers curl is the positive direction of rotation.

What is impulse?

Impulse is the product of force and the time during which the force acts. If the force is not constant, impulse is the average force times the duration of the average force.

How do the actions at the shoulder joint differentiations between a wide-grip pull-up and a narrow-grip pull-up?

In a narrow-grip pull-up, extension and flexion occur at the shoulder joint in the sagittal plane around a transverse axis. In a wide-grip pull-up, adduction and abduction occur at the shoulder joint in the frontal plane around the AP axis.

For throwing a light and heavy object, which is more important for success - the force or the time?

In both instances, maximizing both quantities, the average net force and time of its application, will result in the fastest throw. But in throwing a lighter object, technique (duration of force application) is more important for success, whereas in throwing heavy objects, the force applied is more important.

The impulse produced by a net force acting over some duration of time causes a change in what?

In momentum of the object upon which the net force acts.

During the delivery phase of a baseball pitch, what joint actions occur at the shoulder and elbow joints of the throwing arm?

Internal rotation, adduction, and extension at the shoulder joint; extension at the elbow.

What plane does your forearm lie in when you flex your elbow? What axis is perpendicular to a sagittal plane?

It is moving within a sagittal plane. A transverse axis is perpendicular to a sagittal plane. The movement of your forearm and hand is in a sagittal plane and around a transverse axis.

Why is it recommended that you flex your knees and hips (squat down) when lifting a heavy load off the floor, rather than just flexing your hips and back (leaning over)?

Leaning over to lift something creates a large moment arm between the object being lifted and the lumbar spine. When the lift begins, this moment arm creates a large torque about the lumbosacral joint, requires large forces in the back extensor muscles, and creates large stresses on the intervertebral discs in the lower back. The muscles and structures of the lower back are thus susceptible to injury when this technique is used to lift heavy objects.

How can a plane be useful for describing movement?

Movements of most limbs occur as rotations of the limbs. Rotations occur around specific axes of rotation and within specific planes of movement. Descriptions of the limb movements relative to each other and thus facilitated by identification of the axis of rotation around which the limb moves and the plane within which the limb moves.

Which of the following is a correct unit of torque?

N*m

Show how Newton's first law of motion can be derived from Newton's second law of motion.

Newton's first law of motion is just a special case of Newton's second law of motion where the external forces sum to zero, so acceleration must be zero.

What happens to an object if no external forces act on it or the next external force (the resultant of all the external forces acting on it) is zero?

Newton's first law says that if no net external force acts on an object, that object will not move (it will remain it its state of rest) if it wasn't moving to begin with, or it will continue moving at constant speed in a straight line (it will remain in its state of uniform motion in a straight line) if it was already moving.

If the opposing force exerted on you by an object is exactly equal to the force that you exert on the object, what are the effects of these forces?

Newton's second law of motion tells us that the effect of a force on a body is dependent on the mass of the body and the other forces acting on the body. The larger the mass, the smaller the effect. The smaller the mass, the larger the effect.

What happens if a net external force acts on an object?

Newton's second law says that if a net external force is exerted on an object, the object will accelerate in the direction of the net external force, and its acceleration will be directly proportional to the net external force and inversely proportional to its mass.

What is the origin of the external forces required to change motion?

Newton's third law says that if an object (A) exerts a force on another object (B), the other object (B) exerts the same force on the first object (A) but in the opposite direction. So forces exist in mirrored pairs. The effects of these forces are not canceled by each other because they act on different objects. Another important point is that it is the forces that are equal but opposite, not the effects of the forces.

Can an object change its direction of motion if no external forces act on it?

No. A change in direction of motion is an acceleration, and an external force is required to cause acceleration.

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 90º angle at your knee joint. Rising from this position into a stable 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. 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 of 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.

The formula for torque includes force and what other variable?

Perpendicular distance from the axis of rotation.

The axis of rotation is always what?

Perpendicular to the plane in which the motion occurs.

What is the plane of motion for most of the joint actions that occur during sprint running? What is the corresponding axis of motion for these joint actions?

Sagittal plane and transverse axis

Elbow extension takes place in what plane

Sagittal.

Which of Newton's laws is described by the equation F=ma?

Second.

Which of the following is NOT an example of a sagittal plane movement?

Shoulder abduction.

Interpret the mathematical equation of the impulse-momentum relationship.

The average net force acting over some interval of time will cause a change in momentum of an object. We can interpret change in momentum to mean change in velocity, because most objects have constant mass.

A pitcher is being evaluated for shoulder joint range of motion. The measurement begins with the pitcher's arm at his side. The pitcher then raises his arm away from his side as far as he can (he abducts his shoulder). What is the angular displacement of his arm?

The axis of rotation is the anteroposterior axis, a line through the shoulder joint with the positive direction pointing out of the page toward us. The plane of motion is the frontal plane - the plane formed by his arms, legs, and trunk. If the initial position of the arm is 5º from the vertical and its final position is 170º from the vertical, the angular displacement is +165º.

Which exercise is easier to do, a straight-arm pullover with a 45 Ib barbell or a bent-arm pullover with a 45 Ib barbell?

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.

Explain how the average impact forces between landing a jump from a chair stiff-legged and landing a jump from a chair with flexed legs differ.

The change in momentum is the same for the two landing techniques, which means that the impulse, the left side of the impulse-momentum equation, must also be the same for the two techniques. This doesn't mean that the average impact force must be the same for both conditions, or that the impact time must be the same. It means only that the product of the two, impact force times impact time, must be the same for the two landing techniques. If the impact time is short, the average impact force must be large. If the impact time is long, the average impact force must be small.

When a dancer pushes off from the floor, the observed acceleration of the dancer is greater than that of the floor because:

The dancer is less massive than the floor.

How is the axis of rotation of a limb twisting about its length defined?

The direction of the line running the length of the limb from the proximal to distal end.

What is the moment arm of force?

The distance between the line of action of the force and the line of action of the force parallel to it that passes through the axis of rotation.

According to Newton's third law, when two bodies collide,

The effects of the force applied to each body will be the same only if the mass of each body is approximately the same.

Newton's third law states that for every action there is an equal but opposite reactions. When you are standing on the floor, the force of gravity pulls down on you. What is the equal but opposite reaction to the force of gravity?

The equal but opposite force is a force (equal to your weight) exerted upward on the earth. This is so small compared to the mass of the earth that it has no effect on the earth.

The change in momentum of an object is proportional to what?

The force impressed.

If you want to change the velocity of an object in a specific direction you must do what?

The force you apply, or some component of that force, must be in that specific direction.

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 does the anatomical axes correspond to?

The lines that are perpendicular to the anatomical planes.

What is anatomical position?

The most commonly used reference position of the human body is called anatomical position. The body is in anatomical position when it is standing erect, facing forward, with the feet aligned parallel to each other, toes forward, arms and hands hanging straight below the shoulders at the sides, fingers extended, and palms facing forward. Anatomical position is the standard reference position for the body when we describe locations, positions, or movements of limbs or other anatomical structures.

If you abduct your shoulder what plane are you moving in and about what axis?

The movement occurs within the frontal plane and around the AP axis.

The earth's gravitational force acting on an object is what?

The object's weight.

What is reaction force?

The term reaction force refers to the force that one object exerts on another. In Newton's third law the words action and reaction mean force.

What is a moment of force?

The torque produced by a force.

What is torque?

The turning effect produced by a force.

Two teams are in a tug-of-war contest. Team A pulled harder initially, and now Team B, the rope, and Team A are all moving with constant velocity in the direction of the pulling force from Team A. Which team is pulling with greater force on the rope at this instant?

The two teams are pulling with the same magnitude of force but in opposite directions. Since velocity is constant, acceleration must be zero, and thus the net force action on the rope is zero.

When you are lifting a weight upward, the weight often feels "heavier" near the beginning of the lift and "lighter" near the end of the lift, especially if the lift is done quickly. Why?

The weight feels "heavier" at the beginning of the lift because it is accelerating upward. It is speeding up in the upward direction. To accelerate something upward, the net force acting on the object must be upward. The net force acting on the weight is the force you exert minus the weight itself. To achieve a net upward force, your upward force must be larger than the weight. Near the end of the lift, the weight feels "lighter" because it is accelerating downward. It is slowing down in the upward direction, and this is a downward acceleration. To accelerate something downward, the net force acting on the object must be downward. The net force acting on the weight is the force you exert minus the weight itself. To achieve a net downward force, your upward force must be smaller than the weight.

Flexion, extension, and hyperextension are joint actions occurring at what joints?

The wrist, elbow, shoulder, hip, knee, and intervertebral joints.

What happens if the only force acting on a projectile is the downward force of gravity?

Then the acceleration of the projectile will also be downward and proportional to the force. The force of gravity is the object's weight (W).

How do you define a plane in space?

Three noncolinear points or two intersecting lines define a plane is space.

Why is so much more force required to accelerate an object (ex. ball) upward than to accelerate an object sideways?

To accelerate the ball upward, you must exert an upward force larger than the weight of the ball. In general, causing something to accelerate horizontally requires much less force (and thus less effort) than does causing something to accelerate upward.

How do you measure angular displacement of an object when the object is not a line?

To measure such a displacement, choose any two points on the object. Imagine a line connecting these two points. If the object is rigid, the angular displacement of this line segment is identical to the angular displacement of the object.

How do we increase the momentum of an object if the initial velocity of the object is zero, and the final velocity is fast?

We accomplish this by exerting a large force against the object for as long a time as possible (by exerting a large impulse). Techniques in sport activities such as throwing or jumping are largely based on increasing the time of force application to obtain a large impulse.

How do we decrease the momentum of an object if the initial velocity of the object is fast and we want to decrease this velocity to a slow or zero final velocity?

We accomplish this by increasing the impact time in order to decrease the average impact force.

If we want to change the velocity of an object, we can do what?

We can produce a larger change in velocity by having a larger average net force act on the object or by increasing the time during which the net force acts.

Net external forces that cause an object to accelerate change with what?

With the exception of gravity, most external forces that contribute to a net external force change with time.

Can an object be in motion if no external forces act on it?

Yes, if it was moving to begin with - but it cannot change its state of motion (accelerate). That is it cannot speed up, slow down, or change direction since an external force is required to cause acceleration.

Ex. In order to reduce the impact force landing from a jump you flex your knees, ankles, and hips. This increased the impact time - the time it took you to slow down. What happens if you increase impact time (delta t) in the impulse-momentum equation?

You decrease the average impact force since the change in momentum would be the same whether you flexed your legs or not.

Explain why the right side of the impulse-momentum equation remains the same whether you land from a jump from a chair stiff-legged or with flexed legs.

Your mass does not change. Your final velocity is the same for both conditions. This is your velocity at the end of the landing, which will be zero. Your initial velocity, is also the same for both conditions if you jump form the same height. This is your velocity when your feet first make contact with the ground. Your change in velocity is the same for both conditions. So your change in momentum, the right side of the impulse-momentum equation, is the same whether you land stiff-legged or flex your legs.

Kinetics is the...

branch of dynamics concerned with the forces that cause motion.

Dynamics is the...

branch of mechanics concerned with the mechanics of moving objects.