Mechanical Analysis Exam 3 Lecture Review Guides

Force

a push or pull by one body on another (F=ma)

Outside (external) Force

any forces that are external to the system and will change the momentum of a system

Net (effective) Force

the vector sum of all external forces will cause an acceleration of the COM, if forces are unbalanced

Weight

force on a body due to gravity (Fg=MAg)

Propulsive force

a force that is causing a body to speed up (same direction and sign)

Braking force

a force that is causing a body to slow down (different direction and sign)

Contact force

the force created when two bodies are touching each other

Reaction force

as a consequence of Newton’s 3rd law for every force created by body A on body B, there is a force of equal magnitude and opposite direction created by body B acting on body A

Ground reaction force

the equal and opposite force that the ground applies back on a person

Friction

opposes motion between two objects in contact

Impulse

the product of average force and the time that force is applied (equal to the change in momentum)

Rate of force development

beginning of the slop/ positive side, how quickly you can generate power

Free-body diagram

a representation of all forces acting on a body

Newton’s first law of motion

a body will remain at rest or continue to move a constant speed in a straight line unless acted on by an outside force

Newton’s second law of motion

acceleration is directly proportional to force and inversely proportional to mass

Newton’s third law of motion

for every force, there is an equal and opposite force

According to the first law, momentum will not change without an ______. The most common outside force is the ______.

outside force; ground reaction force

Give the alternate form of Newton’s second law (in terms on momentum)

EF = change in L / change in T

Impulse momentum relationship

EF x change in T = change in L

Why does a force not necessarily cause an acceleration?

• other forces may act in opposite direction

• if forces are balanced, net force = 0, acceleration = 0

Your sitting still, your weight is acting on you, so why are you not acceleration downward?

• your weight acts downward

• the chair provides an equal upward reaction/ contact force

The anterior/posterior component of the GRF during walking and running is made up of what two forces? what does the relationship between them determine?

• made up of braking and propulsive forces

• relationship determines walking/running velocity

Weight is a _____ and always acts in the ______ direction

vector; negative vertical

Why do you go down a hill faster when the slope is steeper?

• creates a larger component of gravity

• creates a greater net force/ acceleration down the slope

Friction always acts in what direction relative to the contact surface and the motion?

parallel/tangent to the contact surface and opposite the primary direction of motion

Friction is caused by?

irregularities in the surface between 2 objects

On a force-time graph, impulse is the _________

area under the curve

The upward slope of the force-time curve is the ______, and the downward slope is the _____.

RFD;RFF

Change in momentum depends on average force and time. Average force depends on what two things?

• peak force

• shape of the force-time curve/ RFD and RFF

List the 3 strategies for increasing momentum (impulse)

• increase the magnitude of force

• increase the overall shape of the force-time curve

• change the overall shape of the curve

Why does a hard landing increase the risk of injury?

a body must absorb force over a shorter amount of time, creating a higher force. it places stress on joints, ligaments, tendons, and the muscles cannot absorb as much of the load

To decrease the average force during a landing, braking, or follow-through phase, what can the person change? what can they not change?

you can increase time over which force is applied but cannot change the required changes in momentum

To increase final velocity during a propulsion phase, what should the person increase?

increase average force and time over which force is applied

What is the only constant force experienced by the body?

Gravity

A force is elastic if it follows Hookes law: F(resistive)= ______, where K is ____ and I is ______.

-KIstretch - stiffness - distance stretched

Example of constant resistance

gravity/free weights

Example of elastic resistance

resistance band

What should you always solve first for?

weight

To hold an object still, the vertical force needed is _______

equal to its weight

To lift an object up, the vertical force needed is ___ its weight

greater than

To lower an object, the vertical force needed is ___ its weight

less than

When moving an object striaght out horizontally, what is vertical force? How do you find horizontal force?

Vertical is equal to weight, horizontal = Fx=MAx

When lifting an object up and out at an angle, what do you do first?

• resolve acceleration into x and y components

• solve the horizontal and vertical forces separetly

Torque

the turning effect of a force around an axis

Lever

a rigid body that is used in conjunction with a pivot point to multiply the force or speed applied to another body

Lever arm

distance from the axis of rotation to the point of force application

Moment arm

perpendicular distance from the force vector to the axis of rotation

Mechanical advantage

the ratio of force moment arm to the resistance moment arm

Propulsive torque

a torque that is increasing speed of rotation

Braking torque

a torque that is decreasing the speed of rotation

Net (effective) torque

sum of all torques acting on a body, considering their directions

Angular impulse

torque multiplied by the time it acts, resulting in a change in angular momentum

Accommodating resistance

resistance that increases with the amount of force or torque applied to it

Static equilibrium

both linear and angular accelerations are zero; sum of external forces and sum of external torques are zero

Center of Pressure

location where total GRF acts beneath the feet

Base of Support

area beneath and between the points of contact with the ground that supports the body

Angular version of Newton’s 1st law

angular momentum stays the same unless an outside torque acts on the system

Angular version of Newton’s 2nd law

Net torque equals MOI multiplied by angular acceleration

Angular version of Newton’s 3rd law

for every torque, there is an equal and opposite reaction torque

Why do runners swing their arms in the opposite direction of their legs?

to counteract the angular momentum created by the legs and prevent unwanted rotation of the whole body while running

A clockwise torque on the lower body creates a reactive _______ torque on the upper body.

Counterclockwise

Only the component of the force that is ______ to the rigid body produces torque. The parallel component produces ______

perpendicular, no torque

What are three things that influence torque production?

• amount of force applied

• distance from the axis of rotation

• angle of force application

When using the moment arm, why should you NOT multiply it by the perpendicular component of the force?

moment arm already accounts for the angle of the force

Brittany’s lug nut is stuck, what can she do to produce more torque on the nut?

use a longer wrench to increase the lever arm, apply more force, and push perpendicular to the wrench

In the standard sign convention, a counterclockwise torque is ____ and a clockwise torque is _____

positive, negative

Will torque always act in the same direction as the motion of the body?

No

Two equal forces act on the opposite sides of a seesaw at equal distances from the axis. What is the net torque? What happens if one force moves farther from the axis?

Net torque is 0 since torques are equal and opposite, if one force moves father from the axis it produces more torque

List the 4 functions that a simple machine (lever) can serve

• balance two or more forces

• change the direction of applied force

• favor speed and range of motion

• favor force production

List the 5 components of a lever

• applied force

• force moment arm

• axis of rotation

• resistance force

• resistance moment arm

First class lever

• FAR

• can serve all 4 functions

• example is neck extension

Second class lever

• ARF

• favors force production

• example is toe raise

Third class lever

• RFA

• favors speed and range of motion

• example is knee extension

Which class of lever is most common in the body?

3rd class

Why does Billy circle his lasso several times over his head before throwing it?

to build up angular velocity and angular momentum before releasing the rope, allows him to throw it more effectively

To decrease the average torque during braking, what would be increased?

Braking time, increase time over which the body slows down decreases the average braking torque needed

Give two examples of accommodating resistance

• flywheels

• isokinetic dynamometers

An isokinetic dynamometer maintains a constant ______ no matter how much torque is applied

angular velocity

What are the two conditions for static equilibrium?

sum of external forces and torques = 0

To maintain balance, the COP cannot be located outside the __________

Base of Support

Energy

the state of matter that makes things change or has the potential to make things change

Kinetic energy

energy due to motion

Potential energy

energy due to position or deformation

Gravitational potential energy

potential energy a body has due to its position or height

Strain potential energy

energy a body has due to its deformation

Work

process of changing the amount of energy in a system, only occurs when there is displacement

Mechanical Energy Expenditure (MEE)

the amount of mechanical energy necessary for a prescribed motion

Efficiency

the amount of mechanical energy that can be expended with a given amount of energy, it is an output/input relationship

Economy

the amount of energy required to perform a certain amount of work

Power

the time rate of doing work, or how quickly work is performed

Mechanical energy is divided into what two types? What are two types of each?

• kinetic - linear and angular

• potential - gravitational and strain

In the kinetic energy equations, which is more important: mass or velocity?

velocity because it is squared

State the law of conservation of energy

energy can neither be created or destroyed, it can be transformed from one form to another

While in the air, during ascent, linear kinetic energy is _______ into potential energy. During descent, potential energy is _______ into kinetic energy. The total energy _______.

Transformed, transformed, remains constant

At the apex of a jump, linear kinetic energy is _____ and gravitational potential energy is at its ________

zero, maximum

Work is a ______ quantity, assumes that force is ______, and only occurs if there is a ________.

scalar, constant, displacement

If a force is applied at an angle to the direction of motion, which part of the force does work?

only the component of the force acting in the direction of displacement does work

When force and displacement are in the same direction, the work is _____ and energy _____. When they are in opposite directions, the work is ______ and energy _____.

Positive, increases, Negative, decreases

Net work is done by the _______

effective force

Anne lifts a barbell up during a bench press and lowers it back to the start. What is the net work? Is her MEE also zero?

Net work is zero because they cancel each other out, MEE is not zero because energy is required during both the lifting and lowering phase.